Galaxy

Summary

A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity.[1][2] The word is derived from the Greek galaxias (γαλαξίας), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. Galaxies, averaging an estimated 100 million stars,[3] range in size from dwarfs with less than a thousand stars,[4] to the largest galaxies knownsupergiants with one hundred trillion stars, each orbiting its galaxy's center of mass. Most of the mass in a typical galaxy is in the form of dark matter, with only a few percent of that mass visible in the form of stars and nebulae. Supermassive black holes are a common feature at the centres of galaxies.

NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 55,000 light-years in diameter and approximately 60 million light-years from Earth.

Galaxies are categorized according to their visual morphology as elliptical,[5] spiral, or irregular.[6] The Milky Way is an example of a spiral galaxy. It is estimated that there are between 200 billion[7] (2×1011) to 2 trillion[8] galaxies in the observable universe. Most galaxies are 1,000 to 100,000 parsecs in diameter (approximately 3,000 to 300,000 light years) and are separated by distances on the order of millions of parsecs (or megaparsecs). For comparison, the Milky Way has a diameter of at least 26,800 parsecs (87,400 ly)[9][a] and is separated from the Andromeda Galaxy, its nearest large neighbor, by just over 750,000 parsecs (2.5 million ly.)[12]

The space between galaxies is filled with a tenuous gas (the intergalactic medium) with an average density of less than one atom per cubic meter. Most galaxies are gravitationally organized into groups, clusters and superclusters. The Milky Way is part of the Local Group, which it dominates along with the Andromeda Galaxy. The group is part of the Virgo Supercluster. At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense voids.[13] Both the Local Group and the Virgo Supercluster are contained in a much larger cosmic structure named Laniakea.[14]

Etymology edit

The word galaxy was borrowed via French and Medieval Latin from the Greek term for the Milky Way, galaxías (kúklos) γαλαξίας (κύκλος)[15][16] 'milky (circle)', named after its appearance as a milky band of light in the sky. In Greek mythology, Zeus places his son born by a mortal woman, the infant Heracles, on Hera's breast while she is asleep so the baby will drink her divine milk and thus become immortal. Hera wakes up while breastfeeding and then realizes she is nursing an unknown baby: she pushes the baby away, some of her milk spills, and it produces the band of light known as the Milky Way.[17][18]

In the astronomical literature, the capitalized word "Galaxy" is often used to refer to the Milky Way galaxy, to distinguish it from the other galaxies in the observable universe. The English term Milky Way can be traced back to a story by Geoffrey Chaucer c. 1380:

See yonder, lo, the Galaxyë
 Which men clepeth the Milky Wey,
 For hit is whyt.

— Geoffrey Chaucer, The House of Fame[16]

Galaxies were initially discovered telescopically and were known as spiral nebulae. Most 18th- to 19th-century astronomers considered them as either unresolved star clusters or anagalactic nebulae, and were just thought of as a part of the Milky Way, but their true composition and natures remained a mystery. Observations using larger telescopes of a few nearby bright galaxies, like the Andromeda Galaxy, began resolving them into huge conglomerations of stars, but based simply on the apparent faintness and sheer population of stars, the true distances of these objects placed them well beyond the Milky Way. For this reason they were popularly called island universes, but this term quickly fell into disuse, as the word universe implied the entirety of existence. Instead, they became known simply as galaxies.[19]

Nomenclature edit

 
Galaxy cluster SDSS J1152+3313. SDSS stands for Sloan Digital Sky Survey, J for Julian epoch, and 1152+3313 for right ascension and declination respectively.

Millions of galaxies have been catalogued, but only a few have well-established names, such as the Andromeda Galaxy, the Magellanic Clouds, the Whirlpool Galaxy, and the Sombrero Galaxy. Astronomers work with numbers from certain catalogues, such as the Messier catalogue, the NGC (New General Catalogue), the IC (Index Catalogue), the CGCG (Catalogue of Galaxies and of Clusters of Galaxies), the MCG (Morphological Catalogue of Galaxies), the UGC (Uppsala General Catalogue of Galaxies), and the PGC (Catalogue of Principal Galaxies, also known as LEDA). All the well-known galaxies appear in one or more of these catalogs but each time under a different number. For example, Messier 109 (or "M109") is a spiral galaxy having the number 109 in the catalog of Messier. It also has the designations NGC 3992, UGC 6937, CGCG 269–023, MCG +09-20-044, and PGC 37617 (or LEDA 37617), among others.[20] Millions of fainter galaxies are known by their identifiers in sky surveys such as the Sloan Digital Sky Survey.[21]

Observation history edit

Milky Way edit

Greek philosopher Democritus (450–370 BCE) proposed that the bright band on the night sky known as the Milky Way might consist of distant stars.[22] Aristotle (384–322 BCE), however, believed the Milky Way was caused by "the ignition of the fiery exhalation of some stars that were large, numerous and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the World that is continuous with the heavenly motions."[23] Neoplatonist philosopher Olympiodorus the Younger (c. 495–570 CE) was critical of this view, arguing that if the Milky Way was sublunary (situated between Earth and the Moon) it should appear different at different times and places on Earth, and that it should have parallax, which it did not. In his view, the Milky Way was celestial.[24]

According to Mohani Mohamed, Arabian astronomer Ibn al-Haytham (965–1037) made the first attempt at observing and measuring the Milky Way's parallax,[25] and he thus "determined that because the Milky Way had no parallax, it must be remote from the Earth, not belonging to the atmosphere."[26] Persian astronomer al-Biruni (973–1048) proposed the Milky Way galaxy was "a collection of countless fragments of the nature of nebulous stars."[27] Andalusian astronomer Avempace (d. 1138) proposed that it was composed of many stars that almost touched one another, and appeared to be a continuous image due to the effect of refraction from sublunary material,[23][28] citing his observation of the conjunction of Jupiter and Mars as evidence of this occurring when two objects were near.[23] In the 14th century, Syrian-born Ibn Qayyim al-Jawziyya proposed the Milky Way galaxy was "a myriad of tiny stars packed together in the sphere of the fixed stars."[29]

Actual proof of the Milky Way consisting of many stars came in 1610 when the Italian astronomer Galileo Galilei used a telescope to study it and discovered it was composed of a huge number of faint stars.[30][31] In 1750, English astronomer Thomas Wright, in his An Original Theory or New Hypothesis of the Universe, correctly speculated that it might be a rotating body of a huge number of stars held together by gravitational forces, akin to the Solar System but on a much larger scale, and that the resulting disk of stars could be seen as a band on the sky from a perspective inside it.[b][33][34] In his 1755 treatise, Immanuel Kant elaborated on Wright's idea about the Milky Way's structure.[35]

 
The shape of the Milky Way as estimated from star counts by William Herschel in 1785; the Solar System was assumed to be near the center.

The first project to describe the shape of the Milky Way and the position of the Sun was undertaken by William Herschel in 1785 by counting the number of stars in different regions of the sky. He produced a diagram of the shape of the galaxy with the Solar System close to the center.[36][37] Using a refined approach, Kapteyn in 1920 arrived at the picture of a small (diameter about 15 kiloparsecs) ellipsoid galaxy with the Sun close to the center. A different method by Harlow Shapley based on the cataloguing of globular clusters led to a radically different picture: a flat disk with diameter approximately 70 kiloparsecs and the Sun far from the center.[34] Both analyses failed to take into account the absorption of light by interstellar dust present in the galactic plane; but after Robert Julius Trumpler quantified this effect in 1930 by studying open clusters, the present picture of the Milky Way galaxy emerged.[38]

Distinction from other nebulae edit

A few galaxies outside the Milky Way are visible on a dark night to the unaided eye, including the Andromeda Galaxy, Large Magellanic Cloud, Small Magellanic Cloud, and the Triangulum Galaxy. In the 10th century, Persian astronomer Abd al-Rahman al-Sufi made the earliest recorded identification of the Andromeda Galaxy, describing it as a "small cloud".[39] In 964, he probably mentioned the Large Magellanic Cloud in his Book of Fixed Stars, referring to "Al Bakr of the southern Arabs",[40] since at a declination of about 70° south it was not visible where he lived. It was not well known to Europeans until Magellan's voyage in the 16th century.[41][40] The Andromeda Galaxy was later independently noted by Simon Marius in 1612.[39]

In 1734, philosopher Emanuel Swedenborg in his Principia speculated that there might be other galaxies outside that were formed into galactic clusters that were minuscule parts of the universe that extended far beyond what could be seen. These views "are remarkably close to the present-day views of the cosmos."[42] In 1745, Pierre Louis Maupertuis conjectured that some nebula-like objects were collections of stars with unique properties, including a glow exceeding the light its stars produced on their own, and repeated Johannes Hevelius's view that the bright spots were massive and flattened due to their rotation.[35] In 1750, Thomas Wright correctly speculated that the Milky Way was a flattened disk of stars, and that some of the nebulae visible in the night sky might be separate Milky Ways.[34][43]

 
Photograph of the "Great Andromeda Nebula" by Isaac Roberts, 1899, later identified as the Andromeda Galaxy

Toward the end of the 18th century, Charles Messier compiled a catalog containing the 109 brightest celestial objects having nebulous appearance. Subsequently, William Herschel assembled a catalog of 5,000 nebulae.[34] In 1845, Lord Rosse examined the nebulae catalogued by Herschel and observed the spiral structure of Messier object M51, now known as the Whirlpool Galaxy.[44][45]

In 1912, Vesto M. Slipher made spectrographic studies of the brightest spiral nebulae to determine their composition. Slipher discovered that the spiral nebulae have high Doppler shifts, indicating that they are moving at a rate exceeding the velocity of the stars he had measured. He found that the majority of these nebulae are moving away from us.[46][47]

In 1917, Heber Doust Curtis observed nova S Andromedae within the "Great Andromeda Nebula", as the Andromeda Galaxy, Messier object M31, was then known. Searching the photographic record, he found 11 more novae. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within this galaxy. As a result, he was able to come up with a distance estimate of 150,000 parsecs. He became a proponent of the so-called "island universes" hypothesis, which holds that spiral nebulae are actually independent galaxies.[48]

In 1920 a debate took place between Harlow Shapley and Heber Curtis, the Great Debate, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim that the Great Andromeda Nebula is an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift.[49]

In 1922, the Estonian astronomer Ernst Öpik gave a distance determination that supported the theory that the Andromeda Nebula is indeed a distant extra-galactic object.[50] Using the new 100-inch Mt. Wilson telescope, Edwin Hubble was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables, thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way.[51] In 1926 Hubble produced a classification of galactic morphology that is used to this day.[52][53]

Modern research edit

 
Rotation curve of spiral galaxy Messier 33 (yellow and blue points with error bars), and a predicted one from distribution of the visible matter (gray line). The discrepancy between the two curves can be accounted for by adding a dark matter halo surrounding the galaxy.[54]

In 1944, Hendrik van de Hulst predicted that microwave radiation with wavelength of 21 cm would be detectable from interstellar atomic hydrogen gas;[55] and in 1951 it was observed. This radiation is not affected by dust absorption, and so its Doppler shift can be used to map the motion of the gas in this galaxy. These observations led to the hypothesis of a rotating bar structure in the center of this galaxy.[56] With improved radio telescopes, hydrogen gas could also be traced in other galaxies. In the 1970s, Vera Rubin uncovered a discrepancy between observed galactic rotation speed and that predicted by the visible mass of stars and gas. Today, the galaxy rotation problem is thought to be explained by the presence of large quantities of unseen dark matter.[57][58]

Beginning in the 1990s, the Hubble Space Telescope yielded improved observations. Among other things, its data helped establish that the missing dark matter in this galaxy could not consist solely of inherently faint and small stars.[59] The Hubble Deep Field, an extremely long exposure of a relatively empty part of the sky, provided evidence that there are about 125 billion (1.25×1011) galaxies in the observable universe.[60] Improved technology in detecting the spectra invisible to humans (radio telescopes, infrared cameras, and x-ray telescopes) allows detection of other galaxies that are not detected by Hubble. Particularly, surveys in the Zone of Avoidance (the region of sky blocked at visible-light wavelengths by the Milky Way) have revealed a number of new galaxies.[61]

A 2016 study published in The Astrophysical Journal, led by Christopher Conselice of the University of Nottingham, used 20 years of Hubble images to estimate that the observable universe contained at least two trillion (2×1012) galaxies.[62][63] However, later observations with the New Horizons space probe from outside the zodiacal light reduced this to roughly 200 billion (2×1011).[64][65]

Types and morphology edit

 
Types of galaxies according to the Hubble classification scheme : an E indicates a type of elliptical galaxy; an S is a spiral; and SB is a barred spiral galaxy

Galaxies come in three main types: ellipticals, spirals, and irregulars. A slightly more extensive description of galaxy types based on their appearance is given by the Hubble sequence. Since the Hubble sequence is entirely based upon visual morphological type (shape), it may miss certain important characteristics of galaxies such as star formation rate in starburst galaxies and activity in the cores of active galaxies.[6]

Many galaxies are thought to contain a supermassive black hole at their center. This includes the Milky Way, whose core region is called the Galactic Center.[66]

Ellipticals edit

The Hubble classification system rates elliptical galaxies on the basis of their ellipticity, ranging from E0, being nearly spherical, up to E7, which is highly elongated. These galaxies have an ellipsoidal profile, giving them an elliptical appearance regardless of the viewing angle. Their appearance shows little structure and they typically have relatively little interstellar matter. Consequently, these galaxies also have a low portion of open clusters and a reduced rate of new star formation. Instead, they are dominated by generally older, more evolved stars that are orbiting the common center of gravity in random directions. The stars contain low abundances of heavy elements because star formation ceases after the initial burst. In this sense they have some similarity to the much smaller globular clusters.[67]

Type-cD galaxies edit

 
The galaxy cluster Abell 1413 is dominated by this cD elliptical galaxy designated Abell 1413 BCG. It has an isophotal diameter of over 800,000 light-years across. Note the gravitational lensing.

The largest galaxies are the type-cD galaxies. First described in 1964 by a paper by Thomas A. Matthews and others,[68] they are a subtype of the more general class of D galaxies, which are giant elliptical galaxies, except that they are much larger. They are popularly known as the supergiant elliptical galaxies and constitute the largest and most luminous galaxies known. These galaxies feature a central elliptical nucleus with an extensive, faint halo of stars extending to megaparsec scales.[69] The profile of their surface brightnesses as a function of their radius (or distance from their cores) falls off more slowly than their smaller counterparts.[70]

The formation of these cD galaxies remains an active area of research, but the leading model is that they are the result of the mergers of smaller galaxies in the environments of dense clusters, or even those outside of clusters with random overdensities.[71] These processes are the mechanisms that drive the formation of fossil groups or fossil clusters, where a large, relatively isolated, supergiant elliptical resides in the middle of the cluster and are surrounded by an extensive cloud of X-rays as the residue of these galactic collisions. Another older model posits the phenomenon of cooling flow, where the heated gases in clusters collapses towards their centers as they cool, forming stars in the process,[72] a phenomenon observed in clusters such as Perseus,[73] and more recently in the Phoenix Cluster.[74]

Shell galaxy edit

 
NGC 3923 Elliptical Shell Galaxy (Hubble photograph)

A shell galaxy is a type of elliptical galaxy where the stars in its halo are arranged in concentric shells. About one-tenth of elliptical galaxies have a shell-like structure, which has never been observed in spiral galaxies. These structures are thought to develop when a larger galaxy absorbs a smaller companion galaxy—that as the two galaxy centers approach, they start to oscillate around a center point, and the oscillation creates gravitational ripples forming the shells of stars, similar to ripples spreading on water. For example, galaxy NGC 3923 has over 20 shells.[75]

Spirals edit

 
The Pinwheel Galaxy, NGC 5457

Spiral galaxies resemble spiraling pinwheels. Though the stars and other visible material contained in such a galaxy lie mostly on a plane, the majority of mass in spiral galaxies exists in a roughly spherical halo of dark matter which extends beyond the visible component, as demonstrated by the universal rotation curve concept.[76]

Spiral galaxies consist of a rotating disk of stars and interstellar medium, along with a central bulge of generally older stars. Extending outward from the bulge are relatively bright arms. In the Hubble classification scheme, spiral galaxies are listed as type S, followed by a letter (a, b, or c) which indicates the degree of tightness of the spiral arms and the size of the central bulge. An Sa galaxy has tightly wound, poorly defined arms and possesses a relatively large core region. At the other extreme, an Sc galaxy has open, well-defined arms and a small core region.[77] A galaxy with poorly defined arms is sometimes referred to as a flocculent spiral galaxy; in contrast to the grand design spiral galaxy that has prominent and well-defined spiral arms.[78] The speed in which a galaxy rotates is thought to correlate with the flatness of the disc as some spiral galaxies have thick bulges, while others are thin and dense.[79][80]

 
NGC 1300, an example of a barred spiral galaxy

In spiral galaxies, the spiral arms do have the shape of approximate logarithmic spirals, a pattern that can be theoretically shown to result from a disturbance in a uniformly rotating mass of stars. Like the stars, the spiral arms rotate around the center, but they do so with constant angular velocity. The spiral arms are thought to be areas of high-density matter, or "density waves".[81] As stars move through an arm, the space velocity of each stellar system is modified by the gravitational force of the higher density. (The velocity returns to normal after the stars depart on the other side of the arm.) This effect is akin to a "wave" of slowdowns moving along a highway full of moving cars. The arms are visible because the high density facilitates star formation, and therefore they harbor many bright and young stars.[82]

 
Hoag's Object, an example of a ring galaxy

Barred spiral galaxy edit

A majority of spiral galaxies, including the Milky Way galaxy, have a linear, bar-shaped band of stars that extends outward to either side of the core, then merges into the spiral arm structure.[83] In the Hubble classification scheme, these are designated by an SB, followed by a lower-case letter (a, b or c) which indicates the form of the spiral arms (in the same manner as the categorization of normal spiral galaxies). Bars are thought to be temporary structures that can occur as a result of a density wave radiating outward from the core, or else due to a tidal interaction with another galaxy.[84] Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms.[85]

Our own galaxy, the Milky Way, is a large disk-shaped barred-spiral galaxy[86] about 30 kiloparsecs in diameter and a kiloparsec thick. It contains about two hundred billion (2×1011)[87] stars and has a total mass of about six hundred billion (6×1011) times the mass of the Sun.[88]

Super-luminous spiral edit

Recently, researchers described galaxies called super-luminous spirals. They are very large with an upward diameter of 437,000 light-years (compared to the Milky Way's 87,400 light-year diameter). With a mass of 340 billion solar masses, they generate a significant amount of ultraviolet and mid-infrared light. They are thought to have an increased star formation rate around 30 times faster than the Milky Way.[89][90]

Other morphologies edit

  • Peculiar galaxies are galactic formations that develop unusual properties due to tidal interactions with other galaxies.
    • A ring galaxy has a ring-like structure of stars and interstellar medium surrounding a bare core. A ring galaxy is thought to occur when a smaller galaxy passes through the core of a spiral galaxy.[91] Such an event may have affected the Andromeda Galaxy, as it displays a multi-ring-like structure when viewed in infrared radiation.[92]
  • A lenticular galaxy is an intermediate form that has properties of both elliptical and spiral galaxies. These are categorized as Hubble type S0, and they possess ill-defined spiral arms with an elliptical halo of stars[93] (barred lenticular galaxies receive Hubble classification SB0).
  • Irregular galaxies are galaxies that can not be readily classified into an elliptical or spiral morphology.
    • An Irr-I galaxy has some structure but does not align cleanly with the Hubble classification scheme.
    • Irr-II galaxies do not possess any structure that resembles a Hubble classification, and may have been disrupted.[94] Nearby examples of (dwarf) irregular galaxies include the Magellanic Clouds.[95]
  • A dark or "ultra diffuse" galaxy is an extremely-low-luminosity galaxy. It may be the same size as the Milky Way, but have a visible star count only one percent of the Milky Way's. Multiple mechanisms for producing this type of galaxy have been proposed, and it is possible that different dark galaxies formed by different means.[96] One candidate explanation for the low luminosity is that the galaxy lost its star-forming gas at an early stage, resulting in old stellar populations.[97][98]

Dwarfs edit

Despite the prominence of large elliptical and spiral galaxies, most galaxies are dwarf galaxies.[99] They are relatively small when compared with other galactic formations, being about one hundredth the size of the Milky Way, with only a few billion stars. Ultra-compact dwarf galaxies have recently been discovered that are only 100 parsecs across.[100]

Many dwarf galaxies may orbit a single larger galaxy; the Milky Way has at least a dozen such satellites, with an estimated 300–500 yet to be discovered.[101] Most of the information we have about dwarf galaxies come from observations of the local group, containing two spiral galaxies, the Milky Way and Andromeda, and many dwarf galaxies. These dwarf galaxies are classified as either irregular or dwarf elliptical/dwarf spheroidal galaxies.[99]

A study of 27 Milky Way neighbors found that in all dwarf galaxies, the central mass is approximately 10 million solar masses, regardless of whether it has thousands or millions of stars. This suggests that galaxies are largely formed by dark matter, and that the minimum size may indicate a form of warm dark matter incapable of gravitational coalescence on a smaller scale.[102]

Variants edit

Interacting edit

 
The Antennae Galaxies are undergoing a collision that will result in their eventual merger.

Interactions between galaxies are relatively frequent, and they can play an important role in galactic evolution. Near misses between galaxies result in warping distortions due to tidal interactions, and may cause some exchange of gas and dust.[103][104] Collisions occur when two galaxies pass directly through each other and have sufficient relative momentum not to merge. The stars of interacting galaxies usually do not collide, but the gas and dust within the two forms interacts, sometimes triggering star formation. A collision can severely distort the galaxies' shapes, forming bars, rings or tail-like structures.[103][104]

At the extreme of interactions are galactic mergers, where the galaxies' relative momentums are insufficient to allow them to pass through each other. Instead, they gradually merge to form a single, larger galaxy. Mergers can result in significant changes to the galaxies' original morphology. If one of the galaxies is much more massive than the other, the result is known as cannibalism, where the more massive larger galaxy remains relatively undisturbed, and the smaller one is torn apart. The Milky Way galaxy is currently in the process of cannibalizing the Sagittarius Dwarf Elliptical Galaxy and the Canis Major Dwarf Galaxy.[103][104]

Starburst edit

 
M82, a starburst galaxy that has ten times the star formation of a "normal" galaxy[105]

Stars are created within galaxies from a reserve of cold gas that forms giant molecular clouds. Some galaxies have been observed to form stars at an exceptional rate, which is known as a starburst. If they continue to do so, they would consume their reserve of gas in a time span less than the galaxy's lifespan. Hence starburst activity usually lasts only about ten million years, a relatively brief period in a galaxy's history. Starburst galaxies were more common during the universe's early history,[106] but still contribute an estimated 15% to total star production.[107]

Starburst galaxies are characterized by dusty concentrations of gas and the appearance of newly formed stars, including massive stars that ionize the surrounding clouds to create H II regions.[108] These stars produce supernova explosions, creating expanding remnants that interact powerfully with the surrounding gas. These outbursts trigger a chain reaction of star-building that spreads throughout the gaseous region. Only when the available gas is nearly consumed or dispersed does the activity end.[106]

Starbursts are often associated with merging or interacting galaxies. The prototype example of such a starburst-forming interaction is M82, which experienced a close encounter with the larger M81. Irregular galaxies often exhibit spaced knots of starburst activity.[109]

Radio galaxy edit

 
Hercules A, supergiant elliptical radio galaxy

A radio galaxy is a galaxy with giant regions of radio emission extending well beyond its visible structure. These energetic radio lobes are powered by jets from its active galactic nucleus.[110] Radio galaxies are classified according to their Fanaroff–Riley classification. The FR I class have lower radio luminosity and exhibit structures which are more elongated; the FR II class are higher radio luminosity. The correlation of radio luminosity and structure suggests that the sources in these two types of galaxies may differ.[111]

Radio galaxies can also be classified as giant radio galaxies (GRGs), whose radio emissions can extend to scales of megaparsecs (3.26 million light-years). Alcyoneus is an FR II class low-excitation radio galaxy which has the largest observed radio emission, with lobed structures spanning 5 megaparsecs (16×106 ly). For comparison, another similarly sized giant radio galaxy is 3C 236, with lobes 15 million light-years across. It should however be noted that radio emissions are not always considered part of the main galaxy itself.[112]

A giant radio galaxy is a special class of objects characterized by the presence of radio lobes generated by relativistic jets powered by the central galaxy's supermassive black hole. Giant radio galaxies are different from ordinary radio galaxies in that they can extend to much larger scales, reaching upwards to several megaparsecs across, far larger than the diameters of their host galaxies.[113]

A "normal" radio galaxy do not have a source that is a supermassive black hole or monster neutron star; instead the source is synchrotron radiation from relativistic electrons accelerated by supernova. These sources are comparatively short lived, making the radio spectrum from normal radio galaxies an especially good way to study star formation. [114]

Active galaxy edit

 
A jet of particles is being emitted from the core of the elliptical radio galaxy M87.

Some observable galaxies are classified as "active" if they contain an active galactic nucleus (AGN).[115] A significant portion of the galaxy's total energy output is emitted by the active nucleus instead of its stars, dust and interstellar medium. There are multiple classification and naming schemes for AGNs, but those in the lower ranges of luminosity are called Seyfert galaxies, while those with luminosities much greater than that of the host galaxy are known as quasi-stellar objects or quasars. Models of AGNs suggest that a significant fraction of their light is shifted to far-infrared frequencies because optical and UV emission in the nucleus is absorbed and remitted by dust and gas surrounding it.[116]

The standard model for an active galactic nucleus is based on an accretion disc that forms around a supermassive black hole (SMBH) at the galaxy's core region. The radiation from an active galactic nucleus results from the gravitational energy of matter as it falls toward the black hole from the disc.[117][118] The AGN's luminosity depends on the SMBH's mass and the rate at which matter falls onto it. In about 10% of these galaxies, a diametrically opposed pair of energetic jets ejects particles from the galaxy core at velocities close to the speed of light. The mechanism for producing these jets is not well understood.[119]

Seyfert galaxy edit

Seyfert galaxies are one of the two largest groups of active galaxies, along with quasars. They have quasar-like nuclei (very luminous, distant and bright sources of electromagnetic radiation) with very high surface brightnesses; but unlike quasars, their host galaxies are clearly detectable.[120] Seen through a telescope, a Seyfert galaxy appears like an ordinary galaxy with a bright star superimposed atop the core. Seyfert galaxies are divided into two principal subtypes based on the frequencies observed in their spectra.[121]

Quasar edit

Quasars are the most energetic and distant members of active galactic nuclei. Extremely luminous, they were first identified as high redshift sources of electromagnetic energy, including radio waves and visible light, that appeared more similar to stars than to extended sources similar to galaxies. Their luminosity can be 100 times that of the Milky Way.[122] The nearest known quasar, Markarian 231, is about 581 million light-years from Earth,[123] while others have been discovered as far away as UHZ1, roughly 13.2 billion light-years distant.[124][125] Quasars are noteworthy for providing the first demonstration of the phenomenon that gravity can act as a lens for light.[126]

Other AGNs edit

Blazars are believed to be active galaxies with a relativistic jet pointed in the direction of Earth. A radio galaxy emits radio frequencies from relativistic jets. A unified model of these types of active galaxies explains their differences based on the observer's position.[119]

Possibly related to active galactic nuclei (as well as starburst regions) are low-ionization nuclear emission-line regions (LINERs). The emission from LINER-type galaxies is dominated by weakly ionized elements. The excitation sources for the weakly ionized lines include post-AGB stars, AGN, and shocks.[127] Approximately one-third of nearby galaxies are classified as containing LINER nuclei.[118][127][128]

Luminous infrared galaxy edit

Luminous infrared galaxies (LIRGs) are galaxies with luminosities—the measurement of electromagnetic power output—above 1011 L☉ (solar luminosities). In most cases, most of their energy comes from large numbers of young stars which heat surrounding dust, which reradiates the energy in the infrared. Luminosity high enough to be a LIRG requires a star formation rate of at least 18 M☉ yr−1. Ultra-luminous infrared galaxies (ULIRGs) are at least ten times more luminous still and form stars at rates >180 M☉ yr−1. Many LIRGs also emit radiation from an AGN.[129][130] Infrared galaxies emit more energy in the infrared than all other wavelengths combined, with peak emission typically at wavelengths of 60 to 100 microns. LIRGs are believed to be created from the strong interaction and merger of spiral galaxies.[131] While uncommon in the local universe, LIRGs and ULIRGS were more prevalent when the universe was younger.[130]

Physical diameters edit

Galaxies do not have a definite boundary by their nature, and are characterized by a gradually decreasing stellar density as a function of increasing distance from their center, making measurements of their true extents difficult. Nevertheless, astronomers over the past few decades have made several criteria in defining the sizes of galaxies.

Angular diameter edit

As early as the time of Edwin Hubble in 1936, there have been attempts to characterize the diameters of galaxies. The earliest efforts were based on the observed angle subtended by the galaxy and its estimated distance, leading to an angular diameter (also called "metric diameter").[132] This type of measurement is subject to two significant issues, namely that the estimated distance to the galaxy must be corrected for the redshift-related space expansion and that collections of angular-diameter data are subject to selection bias as more distant observations preferentially select the most luminous objects.[133]

Isophotal diameter edit

The isophotal diameter is introduced as a conventional way of measuring a galaxy's size based on its apparent surface brightness.[134] Isophotes are curves in a diagram - such as a picture of a galaxy - that adjoins points of equal brightnesses, and are useful in defining the extent of the galaxy. The apparent brightness flux of a galaxy is measured in units of magnitudes per square arcsecond (mag/arcsec2; sometimes expressed as mag arcsec−2), which defines the brightness depth of the isophote. To illustrate how this unit works, a typical galaxy has a brightness flux of 18 mag/arcsec2 at its central region. This brightness is equivalent to the light of an 18th magnitude hypothetical point object (like a star) being spread out evenly in a one square arcsecond area of the sky.[135] The isophotal diameter is typically defined as the region enclosing all the light down to 25 mag/arcsec2 in the blue B-band,[136] which is then referred to as the D25 standard.[137]

Examples of isophotal diameters (25.0 B-mag/arcsec2 isophote)
galaxy diameter reference
Large Magellanic Cloud 9.96 kiloparsecs (32,500 light-years) [138]
Milky Way 26.8 kiloparsecs (87,400 light-years) [9]
Messier 87 40.55 kiloparsecs (132,000 light-years)

[139]

Andromeda Galaxy 46.58 kiloparsecs (152,000 light-years) [140]

Effective radius (half-light) and its variations edit

The half-light radius (also known as effective radius; Re) is a measure that is based on the galaxy's overall brightness flux. This is the radius upon which half, or 50%, of the total brightness flux of the galaxy was emitted. This was first proposed by Gérard de Vaucouleurs in 1948.[141] The choice of using 50% was arbitrary, but proved to be useful in further works by R. A. Fish in 1963,[142] where he established a luminosity concentration law that relates the brightnesses of elliptical galaxies and their respective Re, and by José Luis Sérsic in 1968[143] that defined a mass-radius relation in galaxies.[134]

In defining Re, it is necessary that the overall brightness flux galaxy should be captured, with a method employed by Bershady in 2000 suggesting to measure twice the size where the brightness flux of an arbitrarily chosen radius, defined as the local flux, divided by the overall average flux equals to 0.2.[144] Using half-light radius allows a rough estimate of a galaxy's size, but is not particularly helpful in determining its morphology.[145]

Variations of this method exist. In particular, in the ESO-Uppsala Catalogue of Galaxies values of 50%, 70%, and 90% of the total blue light (the light detected through a B-band specific filter) had been used to calculate a galaxy's diameter.[146]

Petrosian magnitude edit

First described by Vahe Petrosian in 1976,[147] a modified version of this method has been used by the Sloan Digital Sky Survey (SDSS). This method employs a mathematical model on a galaxy whose radius is determined by the azimuthally (horizontal) averaged profile of its brightness flux. In particular, the SDSS employed the Petrosian magnitude in the R-band (658 nm, in the red part of the visible spectrum) to ensure that the brightness flux of a galaxy would be captured as much as possible while counteracting the effects of background noise. For a galaxy whose brightness profile is exponential, it is expected to capture all of its brightness flux, and 80% for galaxies that follow a profile that follows de Vaucouleurs's law.[148]

Petrosian magnitudes have the advantage of being redshift and distance independent, allowing the measurement of the galaxy's apparent size since the Petrosian radius is defined in terms of the galaxy's overall luminous flux.[149]

A critique of an earlier version of this method has been issued by the Infrared Processing and Analysis Center,[150] with the method causing a magnitude of error (upwards to 10%) of the values than using isophotal diameter. The use of Petrosian magnitudes also have the disadvantage of missing most of the light outside the Petrosian aperture, which is defined relative to the galaxy's overall brightness profile, especially for elliptical galaxies, with higher signal-to-noise ratios on higher distances and redshifts.[151] A correction for this method has been issued by Graham et al. in 2005, based on the assumption that galaxies follow Sérsic's law.[149]

Near-infrared method edit

This method has been used by 2MASS as an adaptation from the previously used methods of isophotal measurement. Since 2MASS operates in the near infrared, which has the advantage of being able to recognize dimmer, cooler, and older stars, it has a different form of approach compared to other methods that normally use B-filter. The detail of the method used by 2MASS has been described thoroughly in a document by Jarrett et al., with the survey measuring several parameters.[152]

The standard aperture ellipse (area of detection) is defined by the infrared isophote at the Ks band (roughly 2.2 μm wavelength) of 20 mag/arcsec2. Gathering the overall luminous flux of the galaxy has been employed by at least four methods: the first being a circular aperture extending 7 arcseconds from the center, an isophote at 20 mag/arcsec2, a "total" aperture defined by the radial light distribution that covers the supposed extent of the galaxy, and the Kron aperture (defined as 2.5 times the first-moment radius, an integration of the flux of the "total" aperture).[152]

Magnetic fields edit

Galaxies have magnetic fields of their own. A galaxy's magnetic field influences its dynamics in multiple ways, including affecting the formation of spiral arms and transporting angular momentum in gas clouds. The latter effect is particularly important, as it is a necessary factor for the gravitational collapse of those clouds, and thus for star formation.[153]

The typical average equipartition strength for spiral galaxies is about 10 μG (microgauss) or 1 nT (nanotesla). By comparison, the Earth's magnetic field has an average strength of about 0.3 G (Gauss) or 30 μT (microtesla). Radio-faint galaxies like M 31 and M33, the Milky Way's neighbors, have weaker fields (about 5 μG), while gas-rich galaxies with high star-formation rates, like M 51, M 83 and NGC 6946, have 15 μG on average. In prominent spiral arms, the field strength can be up to 25 μG, in regions where cold gas and dust are also concentrated. The strongest total equipartition fields (50–100 μG) were found in starburst galaxies—for example, in M 82 and the Antennae; and in nuclear starburst regions, such as the centers of NGC 1097 and other barred galaxies.[153]

Formation and evolution edit

Formation edit

 
Artist's impression of a protocluster forming in the early universe[154]

Current models of the formation of galaxies in the early universe are based on the ΛCDM model. About 300,000 years after the Big Bang, atoms of hydrogen and helium began to form, in an event called recombination. Nearly all the hydrogen was neutral (non-ionized) and readily absorbed light, and no stars had yet formed. As a result, this period has been called the "dark ages". It was from density fluctuations (or anisotropic irregularities) in this primordial matter that larger structures began to appear. As a result, masses of baryonic matter started to condense within cold dark matter halos.[155][156] These primordial structures eventually became the galaxies we see today.[157]

 
Artist's impression of a young galaxy accreting material

Early galaxy formation edit

Evidence for the appearance of galaxies very early in the Universe's history was found in 2006, when it was discovered that the galaxy IOK-1 has an unusually high redshift of 6.96, corresponding to just 750 million years after the Big Bang and making it the most distant and earliest-to-form galaxy seen at that time.[158] While some scientists have claimed other objects, such as Abell 1835 IR1916, have higher redshifts, and therefore are seen in an earlier stage of the universe's evolution, IOK-1's age and composition have been more reliably established. In December 2012, astronomers reported UDFj-39546284 to be the most distant object known at the time, with a redshift value of 11.9[159][160] which was later confirmed to be 11.58 by the Near-Infrared Spectrograph (NIRSpec) instrument on the James Webb Space Telescope (JWST).[161]

In September 2022, the JWST discovered JADES-GS-z13-0, a galaxy with a redshift value of 13.2, establishing it as the oldest and most distant spectroscopically-confirmed astronomical object known as of 2024.[161][162] JADES-GS-z13-0, estimated to have existed only 350 million years after the Big Bang, has a light-travel distance of 13.4 billion years.[c][163] Although thought to be a galaxy, a 2023 study suggested that this object is actually a dark star,[164] which would make UNCOVER-z13 the most distant galaxy instead.[165][166]

The Near Infrared Camera (NIRCam) on the JWST has imaged numerous other very early galaxies.[167][168][169] In May 2023, a study in the journal Nature identified an ultra-faint galaxy named JD1.[170] Galaxy JD1 was observed by the JWST using the NIRSpec instrument and was found to have a redshift value of 9.79.[170] This means that JD1 was observed at 480 million years after the Big Bang when the universe was only about 4% of its present age.[170][171] Observations of this ultra-faint galaxy were aided by the effect of a gravitational lens in the galaxy cluster Abell 2744 which helped make the image of JD1 larger and 13 times brighter than it otherwise would be.[170][171] This effect and the use of the JWST's NIRCam showed JD1's structure to be three starforming clumps of dust and gas.[171] In late 2023, scientists also reported results suggesting that newborn galaxies in the very early universe were "banana"-shaped.[172][173]

 
Different components of near-infrared background light detected by the Hubble Space Telescope in deep-sky surveys[174]

The existence of galaxies so soon after the Big Bang suggests that protogalaxies must have grown in the so-called "dark ages".[155] The detailed process by which the earliest galaxies formed is an open question in astrophysics. Theories can be divided into two categories: top-down and bottom-up. In top-down correlations (such as the Eggen–Lynden-Bell–Sandage [ELS] model), protogalaxies form in a large-scale simultaneous collapse lasting about one hundred million years.[175] In bottom-up theories (such as the Searle-Zinn [SZ] model), small structures such as globular clusters form first, and then a number of such bodies accrete to form a larger galaxy.[176]

Once protogalaxies began to form and contract, the first halo stars, called Population III stars, appeared within them. These were composed almost entirely of hydrogen and helium and may have been more massive than 100 times the Sun's mass. If so, these huge stars would have quickly consumed their supply of fuel and became supernovae, releasing heavy elements into the interstellar medium.[177] This first generation of stars re-ionized the surrounding neutral hydrogen, creating expanding bubbles of space through which light could readily travel.[178]

In June 2015, astronomers reported evidence for Population III stars in the Cosmos Redshift 7 galaxy at z = 6.60. Such stars are likely to have existed in the very early universe (i.e., at high redshift), and may have started the production of chemical elements heavier than hydrogen that are needed for the later formation of planets and life as we know it.[179][180]

Evolution edit

Within a billion years of a galaxy's formation, key structures begin to appear.[181] Globular clusters, the central supermassive black hole, and a galactic bulge of metal-poor Population II stars form. The creation of a supermassive black hole appears to play a key role in actively regulating the growth of galaxies by limiting the total amount of additional matter added.[182] During this early epoch, galaxies undergo a major burst of star formation.[183]

During the following two billion years, the accumulated matter settles into a galactic disc.[184] A galaxy will continue to absorb infalling material from high-velocity clouds and dwarf galaxies throughout its life.[185] This matter is mostly hydrogen and helium. The cycle of stellar birth and death slowly increases the abundance of heavy elements, eventually allowing the formation of planets.[186]

 
XDF view field compared to the angular size of the Moon. Several thousand galaxies, each consisting of billions of stars, are in this small view.
 
XDF (2012) view: Each light speck is a galaxy, some of which are as old as 13.2 billion years[187] – the observable universe is estimated to contain 200 billion to two trillion galaxies.
 
XDF image shows (from left) fully mature galaxies, nearly mature galaxies (from five to nine billion years ago), and protogalaxies, blazing with young stars (beyond nine billion years).

Star formation rates in galaxies depend upon their local environment. Isolated 'void' galaxies have highest rate per stellar mass, with 'field' galaxies associated with spiral galaxies having lower rates and galaxies in dense cluster having the lowest rates.[188]

The evolution of galaxies can be significantly affected by interactions and collisions. Mergers of galaxies were common during the early epoch, and the majority of galaxies were peculiar in morphology.[189] Given the distances between the stars, the great majority of stellar systems in colliding galaxies will be unaffected. However, gravitational stripping of the interstellar gas and dust that makes up the spiral arms produces a long train of stars known as tidal tails. Examples of these formations can be seen in NGC 4676[190] or the Antennae Galaxies.[191]

The Milky Way galaxy and the nearby Andromeda Galaxy are moving toward each other at about 130 km/s, and—depending upon the lateral movements—the two might collide in about five to six billion years. Although the Milky Way has never collided with a galaxy as large as Andromeda before, it has collided and merged with other galaxies in the past.[192] Cosmological simulations indicate that, 11 billion years ago, it merged with a particularly large galaxy that has been labeled the Kraken.[193][194]

Such large-scale interactions are rare. As time passes, mergers of two systems of equal size become less common. Most bright galaxies have remained fundamentally unchanged for the last few billion years, and the net rate of star formation probably also peaked about ten billion years ago.[195]

Future trends edit

Spiral galaxies, like the Milky Way, produce new generations of stars as long as they have dense molecular clouds of interstellar hydrogen in their spiral arms.[196] Elliptical galaxies are largely devoid of this gas, and so form few new stars.[197] The supply of star-forming material is finite; once stars have converted the available supply of hydrogen into heavier elements, new star formation will come to an end.[198][199]

The current era of star formation is expected to continue for up to one hundred billion years, and then the "stellar age" will wind down after about ten trillion to one hundred trillion years (1013–1014 years), as the smallest, longest-lived stars in the visible universe, tiny red dwarfs, begin to fade. At the end of the stellar age, galaxies will be composed of compact objects: brown dwarfs, white dwarfs that are cooling or cold ("black dwarfs"), neutron stars, and black holes. Eventually, as a result of gravitational relaxation, all stars will either fall into central supermassive black holes or be flung into intergalactic space as a result of collisions.[198][200]

Larger-scale structures edit

 
Seyfert's Sextet is an example of a compact galaxy group.

Deep-sky surveys show that galaxies are often found in groups and clusters. Solitary galaxies that have not significantly interacted with other galaxies of comparable mass in the past few billion years are relatively scarce.[201] Only about 5% of the galaxies surveyed are isolated in this sense.[202][203] However, they may have interacted and even merged with other galaxies in the past,[204] and may still be orbited by smaller satellite galaxies.[205]

On the largest scale, the universe is continually expanding, resulting in an average increase in the separation between individual galaxies (see Hubble's law). Associations of galaxies can overcome this expansion on a local scale through their mutual gravitational attraction. These associations formed early, as clumps of dark matter pulled their respective galaxies together. Nearby groups later merged to form larger-scale clusters. This ongoing merging process, as well as an influx of infalling gas, heats the intergalactic gas in a cluster to very high temperatures of 30–100 megakelvins.[206] About 70–80% of a cluster's mass is in the form of dark matter, with 10–30% consisting of this heated gas and the remaining few percent in the form of galaxies.[207]

Most galaxies are gravitationally bound to a number of other galaxies. These form a fractal-like hierarchical distribution of clustered structures, with the smallest such associations being termed groups. A group of galaxies is the most common type of galactic cluster; these formations contain the majority of galaxies (as well as most of the baryonic mass) in the universe.[208][209] To remain gravitationally bound to such a group, each member galaxy must have a sufficiently low velocity to prevent it from escaping (see Virial theorem). If there is insufficient kinetic energy, however, the group may evolve into a smaller number of galaxies through mergers.[210]

Clusters of galaxies consist of hundreds to thousands of galaxies bound together by gravity.[211] Clusters of galaxies are often dominated by a single giant elliptical galaxy, known as the brightest cluster galaxy, which, over time, tidally destroys its satellite galaxies and adds their mass to its own.[212]

 
Southern plane of the Milky Way from submillimeter wavelengths[213]

Superclusters contain tens of thousands of galaxies, which are found in clusters, groups and sometimes individually. At the supercluster scale, galaxies are arranged into sheets and filaments surrounding vast empty voids.[214] Above this scale, the universe appears to be the same in all directions (isotropic and homogeneous),[215] though this notion has been challenged in recent years by numerous findings of large-scale structures that appear to be exceeding this scale. The Hercules–Corona Borealis Great Wall, currently the largest structure in the universe found so far, is 10 billion light-years (three gigaparsecs) in length.[216][217][218]

The Milky Way galaxy is a member of an association named the Local Group, a relatively small group of galaxies that has a diameter of approximately one megaparsec. The Milky Way and the Andromeda Galaxy are the two brightest galaxies within the group; many of the other member galaxies are dwarf companions of these two.[219] The Local Group itself is a part of a cloud-like structure within the Virgo Supercluster, a large, extended structure of groups and clusters of galaxies centered on the Virgo Cluster.[220] In turn, the Virgo Supercluster is a portion of the Laniakea Supercluster.[221]

Multi-wavelength observation edit

 
This ultraviolet image of Andromeda shows blue regions containing young, massive stars.

Advances in astronomy have always been driven by technology. After centuries of success in optical astronomy, recent decades have seen major progress in other regions of the electromagnetic spectrum.[222]

The dust present in the interstellar medium is opaque to visual light. It is more transparent to far-infrared, which can be used to observe the interior regions of giant molecular clouds and galactic cores in great detail.[223] Infrared is also used to observe distant, red-shifted galaxies that were formed much earlier. Water vapor and carbon dioxide absorb a number of useful portions of the infrared spectrum, so high-altitude or space-based telescopes are used for infrared astronomy.[224]

The first non-visual study of galaxies, particularly active galaxies, was made using radio frequencies. The Earth's atmosphere is nearly transparent to radio between 5 MHz and 30 GHz. The ionosphere blocks signals below this range.[225] Large radio interferometers have been used to map the active jets emitted from active nuclei. Radio telescopes can also be used to observe neutral hydrogen (via 21 cm radiation), including, potentially, the non-ionized matter in the early universe that later collapsed to form galaxies.[226]

Ultraviolet and X-ray telescopes can observe highly energetic galactic phenomena. Ultraviolet flares are sometimes observed when a star in a distant galaxy is torn apart from the tidal forces of a nearby black hole.[227] The distribution of hot gas in galactic clusters can be mapped by X-rays. The existence of supermassive black holes at the cores of galaxies was confirmed through X-ray astronomy.[228]

Gallery edit

See also edit

Notes edit

  1. ^ This is the diameter measured using the D25 standard. A 2018 study suggested that there is a presence of disk stars beyond this diameter, although it is not clear how much of this influences the surface brightness profile.[10][11]
  2. ^ Wright called the Milky Way the Vortex Magnus (Great Whirlpool) and estimated its diameter to be 8.64×1012 miles (13.9×1012 km).[32]
  3. ^ That is, the light from this galaxy has taken 13.4 billion years to reach Earth.

References edit

  1. ^ Sparke & Gallagher 2000, p. i
  2. ^ Hupp, Erica; Roy, Steve; Watzke, Megan (August 12, 2006). "NASA Finds Direct Proof of Dark Matter". NASA. Archived from the original on March 28, 2020. Retrieved April 17, 2007.
  3. ^ Howell, Elizabeth; Harvey, Ailsa (February 11, 2022). "How many stars are in the universe? - Can we estimate the total number of stars?". Space.com. Retrieved July 16, 2022.
  4. ^ Strigari, Louis E.; Bullock, James S.; Kaplinghat, Manoj; Simon, Joshua D.; Geha, Marla; Willman, Beth; Walker, Matthew G. (2008). "A common mass scale for satellite galaxies of the Milky Way". Nature. 454 (7208): 1096–1097. arXiv:0808.3772. Bibcode:2008Natur.454.1096S. doi:10.1038/nature07222. ISSN 0028-0836. PMID 18756252.
  5. ^ Hoover, Aaron (June 16, 2003). "UF Astronomers: Universe Slightly Simpler Than Expected" (Press release). University of Florida. Archived from the original on July 20, 2011. Retrieved March 4, 2011.
    • Based on: Graham, Alister W.; Guzmán, Rafael (2003). "HST Photometry of Dwarf Elliptical Galaxies in Coma, and an Explanation for the Alleged Structural Dichotomy between Dwarf and Bright Elliptical Galaxies". The Astronomical Journal. 125 (6): 2936–2950. arXiv:astro-ph/0303391. Bibcode:2003AJ....125.2936G. doi:10.1086/374992. S2CID 13284968.
  6. ^ a b Jarrett, T. H. "Near-Infrared Galaxy Morphology Atlas". Infrared Processing and Analysis Center. California Institute of Technology. Archived from the original on November 20, 2023. Retrieved January 9, 2007.
  7. ^ Chemla, Sarah (January 14, 2021). "Astronomers were wrong about the number of galaxies in universe". The Jerusalem Post. Archived from the original on January 14, 2021. Retrieved January 14, 2021.
  8. ^ Saunders, Toby (July 25, 2023). "How many galaxies are in the Universe? A lot more than you'd think". BBC Science Focus. Retrieved January 9, 2024.
  9. ^ a b Goodwin, Simon P.; Gribbin, John; Hendry, Martin A. (1998). "The relative size of the Milky Way" (PDF). The Observatory. 118: 201–208. Bibcode:1998Obs...118..201G.
  10. ^ López-Corredoira, M.; Allende Prieto, C.; Garzón, F.; Wang, H.; Liu, C.; Deng, L. (April 9, 2018). "Disk stars in the Milky Way detected beyond 25 kpc from its center". Astronomy & Astrophysics. 612: L8. arXiv:1804.03064. Bibcode:2018A&A...612L...8L. doi:10.1051/0004-6361/201832880. S2CID 59933365.
  11. ^ Freeman, David (May 25, 2018). "The Milky Way galaxy may be much bigger than we thought" (Press release). CNBC. Archived from the original on August 13, 2018. Retrieved August 13, 2018.
  12. ^ Riess, Adam G.; Fliri, Jürgen; Valls-Gabaud, David (2012). "Cepheid Period-Luminosity Relations in the Near-Infrared and the Distance to M31 from the Hubble Space Telescope Wide Field Camera 3". The Astrophysical Journal. 745 (2): 156. arXiv:1110.3769. Bibcode:2012ApJ...745..156R. doi:10.1088/0004-637X/745/2/156. S2CID 119113794.
  13. ^ "Galaxy Clusters and Large-Scale Structure". University of Cambridge. Archived from the original on April 26, 2012. Retrieved January 15, 2007.
  14. ^ Gibney, Elizabeth (2014). "Earth's new address: 'Solar System, Milky Way, Laniakea'". Nature. doi:10.1038/nature.2014.15819. S2CID 124323774.
  15. ^ Onions, Charles T.; Friedrichsen, G.W.S.; Burchfield, Robert W., eds. (1966). The Oxford Dictionary of English Etymology. Oxford: Clarendon Press. p. 385.
  16. ^ a b Harper, Douglas. "galaxy". Online Etymology Dictionary. Archived from the original on November 17, 2011. Retrieved November 11, 2011.
  17. ^ Waller & Hodge 2003, p. 91
  18. ^ Konečný, Lubomír. "Emblematics, Agriculture, and Mythography in The Origin of the Milky Way" (PDF). Academy of Sciences of the Czech Republic. Archived from the original (PDF) on July 20, 2006. Retrieved January 5, 2007.
  19. ^ Rao, Joe (September 2, 2005). "Explore the Archer's Realm". Space.com. Archived from the original on October 31, 2010. Retrieved January 3, 2007.
  20. ^ "Results for object MESSIER 109 (M109)". NASA/IPAC Extragalactic Database. California Institute of Technology. Retrieved January 13, 2023.
  21. ^ "What is the Sloan Digital Sky Survey?". Sloan Digital Sky Survey. Archived from the original on January 12, 2024. Retrieved March 3, 2024.
  22. ^ Plutarch (2006). The Complete Works Volume 3: Essays and Miscellanies. Echo Library. p. 66. ISBN 978-1-4068-3224-2. Archived from the original on March 24, 2021. Retrieved July 25, 2018.
  23. ^ a b c Montada, Josep Puig (September 28, 2007). "Ibn Bâjja". Stanford Encyclopedia of Philosophy. Archived from the original on March 16, 2020. Retrieved July 11, 2008.
  24. ^ Heidarzadeh 2008, pp. 23–25
  25. ^ Mohamed 2000, pp. 49–50
  26. ^ Bouali, Hamid-Eddine; Zghal, Mourad; Lakhdar, Zohra B. (2005). Flory, François (ed.). "Popularisation of Optical Phenomena: Establishing the First Ibn Al-Haytham Workshop on Photography" (PDF). Proceedings of SPIE. Ninth International Topical Meeting on Education and Training in Optics and Photonics. 9664 (Ninth International Topical Meeting on Education and Training in Optics and Photonics). Bibcode:2005SPIE.9664E..22B. doi:10.1117/12.2207764.
  27. ^ O'Connor, John J.; Robertson, Edmund F., "Abu Arrayhan Muhammad ibn Ahmad al-Biruni", MacTutor History of Mathematics Archive, University of St Andrews
  28. ^ Heidarzadeh 2008, p. 25, Table 2.1
  29. ^ Livingston, John W. (1971). "Ibn Qayyim al-Jawziyyah: A Fourteenth Century Defense against Astrological Divination and Alchemical Transmutation". Journal of the American Oriental Society. 91 (1): 96–103. doi:10.2307/600445. JSTOR 600445.
  30. ^ Galilei, Galileo (1610). Sidereus Nuncius. Venice: Thomas Baglioni. pp. 15–16. Retrieved January 10, 2024.
    • English translation: Galilei, Galileo; Kepler, Johannes (1880). The Sidereal Messenger of Galileo Galilei: and a part of the preface to Kepler's Dioptrics containing the original account of Galileo's astronomical discoveries. Translated by Carlos, Edward Stafford. London: Rivingtons. pp. 42–43. Retrieved January 10, 2024.
  31. ^ O'Connor, John J.; Robertson, Edmund F. (November 2002). "Galileo Galilei". MacTutor History of Mathematics Archive. University of St. Andrews. Archived from the original on March 3, 2016. Retrieved January 8, 2007.
  32. ^ Wright 1750, p. 73.
  33. ^ Wright 1750, pp. 48–.
  34. ^ a b c d Evans, J. C. (November 24, 1998). "Our Galaxy". George Mason University. Archived from the original on September 24, 2003. Retrieved January 4, 2007.
  35. ^ a b Kant, Immanuel (November 20, 2016) [1755]. Allgemeine Naturgeschichte und Theorie des Himmels [Universal Natural History and Theory of the Heavens]. Königsberg and Leipzig: Johann Friederich Petersen. ISBN 978-3-89131-002-1. Archived from the original on November 20, 2016. English translation by Ian Johnstone available at the Wayback Machine (archived August 29, 2014)
  36. ^ Herschel, William (1785). "XII. On the Construction of the Heavens". Giving Some Accounts of the Present Undertakings, Studies, and Labours, of the Ingenious, in Many Considerable Parts of the World. Philosophical Transactions of the Royal Society of London. Vol. 75. London. pp. 213–266. doi:10.1098/rstl.1785.0012. ISSN 0261-0523. S2CID 186213203. Archived from the original on November 20, 2016. Retrieved January 27, 2016. Herschel's diagram of the galaxy appears immediately after p. 266.
  37. ^ Paul 1993, pp. 16–18
  38. ^ Trimble, V. (1999). "Robert Trumpler and the (Non)transparency of Space". Bulletin of the American Astronomical Society. 31 (31): 1479. Bibcode:1999AAS...195.7409T.
  39. ^ a b Kepple & Sanner 1998, p. 18
  40. ^ a b Frommert, Hartmut; Kronberg, Christine (March 11, 2004). "The Large Magellanic Cloud, LMC". Observatoire de Paris. Archived from the original on June 22, 2017.
  41. ^ Frommert, Hartmut; Kronberg, Christine. "Abd-al-Rahman Al Sufi (December 7, 903 – May 25, 986 A.D.)". Observatoire de Paris. Archived from the original on April 16, 2007. Retrieved April 19, 2007.
  42. ^ Gordon, Kurtiss J. (1969). "History of our Understanding of a Spiral Galaxy: Messier 33" (PDF). Quarterly Journal of the Royal Astronomical Society. 10: 293–307. Archived (PDF) from the original on April 26, 2021.
  43. ^ See text quoted from Wright's An Original Theory or New Hypothesis of the Universe in Dyson, Freeman J. (1979). Disturbing the Universe. Pan Books. p. 245. ISBN 978-0-330-26324-5. Archived from the original on March 24, 2021. Retrieved July 25, 2018.
  44. ^ Payne-Gaposchkin, Cecilia H. (1953). "Why Do Galaxies Have a Spiral Form?". Scientific American. Vol. 189, no. 3. pp. 89–99. ISSN 0036-8733. JSTOR 24944338.
  45. ^ Steinicke, Wolfgang (2012). "The M51 mystery: Lord Rosse, Robinson, South and the discovery of spiral structure in 1845". Journal of Astronomical History and Heritage. 15 (1): 19–29. Bibcode:2012JAHH...15...19S. doi:10.3724/SP.J.1440-2807.2012.01.03.
  46. ^ Slipher, Vesto. M. (1913). "The radial velocity of the Andromeda Nebula". Lowell Observatory Bulletin. 1 (8): 56–57. Bibcode:1913LowOB...2...56S.
  47. ^ Slipher, Vesto M. (1915). "Spectrographic Observations of Nebulae". Popular Astronomy. Vol. 23. pp. 21–24. Bibcode:1915PA.....23...21S.
  48. ^ Curtis, Heber D. (1988). "Novae in Spiral Nebulae and the Island Universe Theory". Publications of the Astronomical Society of the Pacific. 100: 6. Bibcode:1988PASP..100....6C. doi:10.1086/132128.
  49. ^ Weaver, Harold. F. "Robert Julius Trumpler". US National Academy of Sciences. Archived from the original on December 24, 2013. Retrieved January 5, 2007.
  50. ^ Öpik, Ernst (1922). "An estimate of the distance of the Andromeda Nebula". The Astrophysical Journal. 55: 406. Bibcode:1922ApJ....55..406O. doi:10.1086/142680.
  51. ^ Hubble, Edwin P. (1929). "A spiral nebula as a stellar system, Messier 31". The Astrophysical Journal. 69: 103–158. Bibcode:1929ApJ....69..103H. doi:10.1086/143167.
  52. ^ Hubble, Edwin P. (1926). "No. 324. Extra-galactic nebulae". Contributions from the Mount Wilson Observatory. 324. Carnegie Institution of Washington: 1–49. Bibcode:1926CMWCI.324....1H.
  53. ^ Sandage, Allan R. (1989). "Edwin Hubble, 1889–1953". Journal of the Royal Astronomical Society of Canada. 83 (6): 351–362. Bibcode:1989JRASC..83..351S. Archived from the original on February 5, 2024. Retrieved April 21, 2024.
  54. ^ Corbelli, E.; Salucci, P. (2000). "The extended rotation curve and the dark matter halo of M33". Monthly Notices of the Royal Astronomical Society. 311 (2): 441–447. arXiv:astro-ph/9909252. Bibcode:2000MNRAS.311..441C. doi:10.1046/j.1365-8711.2000.03075.x.
  55. ^ Tenn, Joe. "Hendrik Christoffel van de Hulst". The Bruce Medalists. Sonoma State University. Archived from the original on January 14, 2012. Retrieved January 5, 2007.
  56. ^ López-Corredoira, M.; et al. (2001). "Searching for the in-plane Galactic bar and ring in DENIS". Astronomy & Astrophysics. 373 (1): 139–152. arXiv:astro-ph/0104307. Bibcode:2001A&A...373..139L. doi:10.1051/0004-6361:20010560. S2CID 18399375.
  57. ^ Rubin, Vera C. (1983). "Dark matter in spiral galaxies". Scientific American. Vol. 248, no. 6. pp. 96–106. Bibcode:1983SciAm.248f..96R. doi:10.1038/scientificamerican0683-96.
  58. ^ Rubin, Vera C. (2000). "One Hundred Years of Rotating Galaxies". Publications of the Astronomical Society of the Pacific. 112 (772): 747–750. Bibcode:2000PASP..112..747R. doi:10.1086/316573. S2CID 122927800.
  59. ^ "Hubble Rules Out a Leading Explanation for Dark Matter". Hubble News Desk (Press release). NASA. October 17, 1994. Archived from the original on November 27, 2012. Retrieved January 8, 2007.
  60. ^ Mattson, Barbara (November 27, 2002). Gibb, Meredith (ed.). "How many galaxies are there?". Imagine the Universe!. NASA. Archived from the original on July 28, 2012. Retrieved January 8, 2007.
  61. ^ Kraan-Korteweg, R. C.; Juraszek, S. (2000). "Mapping the hidden Universe: The galaxy distribution in the Zone of Avoidance". Publications of the Astronomical Society of Australia. 17 (1): 6–12. arXiv:astro-ph/9910572. Bibcode:2000PASA...17....6K. doi:10.1071/AS00006. S2CID 17900483.
  62. ^ Conselice, Christopher J.; et al. (2016). "The Evolution of Galaxy Number Density at z < 8 and its Implications". The Astrophysical Journal. 830 (2): 83. arXiv:1607.03909. Bibcode:2016ApJ...830...83C. doi:10.3847/0004-637X/830/2/83. S2CID 17424588.
  63. ^ Fountain, Henry (October 17, 2016). "Two Trillion Galaxies, at the Very Least". The New York Times. Archived from the original on December 31, 2019. Retrieved October 17, 2016.
  64. ^ Lauer, Tod R.; et al. (January 11, 2021). "New Horizons Observations of the Cosmic Optical Background". The Astrophysical Journal. 906 (2): 77. arXiv:2011.03052. Bibcode:2021ApJ...906...77L. doi:10.3847/1538-4357/abc881. hdl:1721.1/133770. ISSN 1538-4357. S2CID 226277978.
  65. ^ "New Horizons spacecraft answers the question: How dark is space?". Phys.org. Archived from the original on January 15, 2021. Retrieved January 15, 2021.
  66. ^ Fraknoi et al. 2023, pp. 851–856, 907–915.
  67. ^ Barstow, M. A. (2005). "Elliptical Galaxies". Leicester University Physics Department. Archived from the original on July 29, 2012. Retrieved June 8, 2006.
  68. ^ Matthews, Thomas A.; Morgan, William W.; Schmidt, Maarten (1964). "A Discussion of Galaxies Indentified [sic] with Radio Sources". The Astrophysical Journal. 140: 35. Bibcode:1964ApJ...140...35M. doi:10.1086/147890.
  69. ^ "Tracing the Growth of Galaxies". NASA. February 6, 2017. Retrieved April 20, 2024.
  70. ^ Tonry, John L. (1987). "Properties of cD Galaxies". Structure and Dynamics of Elliptical Galaxies. Vol. 127. p. 89. Bibcode:1987IAUS..127...89T. doi:10.1007/978-94-009-3971-4_7. ISBN 978-90-277-2586-8. S2CID 117980521.
  71. ^ Tovmassian, Hrant M.; Andernach, Heinz (December 11, 2012). "On the formation of cD galaxies and their parent clusters: Formation of cD galaxies and parent clusters". Monthly Notices of the Royal Astronomical Society. 427 (3): 2047–2056. arXiv:1212.0238. doi:10.1111/j.1365-2966.2012.22044.x.
  72. ^ Bender, Ralf; Kormendy, John; Cornell, Mark E.; Fisher, David B. (June 30, 2015). "Structure and Formation of cD Galaxies: NGC 6166 in Abell 2199". The Astrophysical Journal. 807 (1): 56. arXiv:1411.2598. Bibcode:2015ApJ...807...56B. doi:10.1088/0004-637X/807/1/56. ISSN 1538-4357. This idea was entertained in the heyday of the cooling-flow problem, when we observed large amounts of X-ray-emitting, hot gas in clusters but could not measure temperature profiles. [...] This possibility is now regarded as a non-starter.
  73. ^ Fabian, A. C.; Nulsen, P. E. J. (1977). "Subsonic accretion of cooling gas in clusters of galaxies". Monthly Notices of the Royal Astronomical Society. 180 (3): 479. Bibcode:1977MNRAS.180..479F. doi:10.1093/mnras/180.3.479.
  74. ^ McDonald, M.; McNamara, B. R.; Voit, G. M.; Bayliss, M.; Benson, B. A.; Brodwin, M.; Canning, R. E. A.; Florian, M. K.; Garmire, G. P.; Gaspari, M.; Gladders, M. D.; Hlavacek-Larrondo, J.; Kara, E.; Reichardt, C. L.; Russell, H. R. (November 1, 2019). "Anatomy of a Cooling Flow: The Feedback Response to Pure Cooling in the Core of the Phoenix Cluster". The Astrophysical Journal. 885 (1): 63. arXiv:1904.08942. Bibcode:2019ApJ...885...63M. doi:10.3847/1538-4357/ab464c. ISSN 0004-637X.
  75. ^ "Galactic onion". European Space Agency. Archived from the original on August 6, 2020. Retrieved May 11, 2015.
  76. ^ Williams, M. J.; Bureau, M.; Cappellari, M. (2010). "Kinematic constraints on the stellar and dark matter content of spiral and S0 galaxies". Monthly Notices of the Royal Astronomical Society. 400 (4): 1665–1689. arXiv:0909.0680. Bibcode:2009MNRAS.400.1665W. doi:10.1111/j.1365-2966.2009.15582.x. S2CID 17940107.
  77. ^ Smith, Gene (March 6, 2000). "Galaxies — The Spiral Nebulae". University of California, San Diego Center for Astrophysics & Space Sciences. Archived from the original on March 27, 2023. Retrieved November 30, 2006.
  78. ^ Van den Bergh 1998, p. 17
  79. ^ Obreschkow, D.; Glazebrook, K. (February 28, 2014). "Fundamental Mass-Spin-Morphology Relation of Spiral Galaxies". The Astrophysical Journal. 784 (1): 26. arXiv:1312.4543. Bibcode:2014ApJ...784...26O. doi:10.1088/0004-637X/784/1/26. ISSN 0004-637X.
  80. ^ "Fat or flat: Getting galaxies into shape". Phys.org. February 27, 2014. Archived from the original on March 24, 2021.4
  81. ^ Bertin & Lin 1996, pp. 65–85
  82. ^ Belkora 2003, p. 355
  83. ^ Eskridge, P. B.; Frogel, J. A. (1999). "What is the True Fraction of Barred Spiral Galaxies?". Astrophysics and Space Science. 269/270: 427–430. Bibcode:1999Ap&SS.269..427E. doi:10.1023/A:1017025820201. S2CID 189840251.
  84. ^ Bournaud, F.; Combes, F. (2002). "Gas accretion on spiral galaxies: Bar formation and renewal". Astronomy & Astrophysics. 392 (1): 83–102. arXiv:astro-ph/0206273. Bibcode:2002A&A...392...83B. doi:10.1051/0004-6361:20020920. S2CID 17562844.
  85. ^ Knapen, Johan H.; Perez-Ramirez, Daniel; Laine, Seppo (2002). "Circumnuclear regions in barred spiral galaxies — II. Relations to host galaxies". Monthly Notices of the Royal Astronomical Society. 337 (3): 808–828. arXiv:astro-ph/0207258. Bibcode:2002MNRAS.337..808K. doi:10.1046/j.1365-8711.2002.05840.x. S2CID 10845683.
  86. ^ Alard, Christophe (2001). "Another bar in the Bulge". Astronomy and Astrophysics Letters. 379 (2): L44–L47. arXiv:astro-ph/0110491. Bibcode:2001A&A...379L..44A. doi:10.1051/0004-6361:20011487. S2CID 18018228.
  87. ^ Sanders, Robert (January 9, 2006). "Milky Way galaxy is warped and vibrating like a drum" (Press release). University of California, Berkeley. Archived from the original on January 18, 2014. Retrieved May 24, 2006.
  88. ^ Bell, G. R.; Levine, S. E. (1997). "Mass of the Milky Way and Dwarf Spheroidal Stream Membership". Bulletin of the American Astronomical Society. 29 (2): 1384. Bibcode:1997AAS...19110806B.
  89. ^ Santos, Miguel (March 21, 2016). "We Just Discovered a New Type of Colossal Galaxy". Futurism. Archived from the original on March 24, 2021. Retrieved March 21, 2016.
  90. ^ Ogle, Patrick M.; Lanz, Lauranne; Nader, Cyril; Helou, George (January 1, 2016). "Superluminous Spiral Galaxies". The Astrophysical Journal. 817 (2): 109. arXiv:1511.00659. Bibcode:2016ApJ...817..109O. doi:10.3847/0004-637X/817/2/109. ISSN 0004-637X. S2CID 35287348.
  91. ^ Gerber, R. A.; Lamb, S. A.; Balsara, D. S. (1994). "Ring Galaxy Evolution as a Function of "Intruder" Mass". Bulletin of the American Astronomical Society. 26: 911. Bibcode:1994AAS...184.3204G.
  92. ^ "ISO unveils the hidden rings of Andromeda" (Press release). European Space Agency. October 14, 1998. Archived from the original on May 5, 2006. Retrieved May 24, 2006.
  93. ^ "Spitzer Reveals What Edwin Hubble Missed" (Press release). Harvard-Smithsonian Center for Astrophysics. May 31, 2004. Archived from the original on September 7, 2006. Retrieved December 6, 2006.
  94. ^ Barstow, M. A. (2005). "Irregular Galaxies". University of Leicester. Archived from the original on February 27, 2012. Retrieved December 5, 2006.
  95. ^ Fraknoi et al. 2023, pp. 879.
  96. ^ Pandya, Viraj; Romanowsky, Aaron J.; Laine, Seppo; Brodie, Jean P.; Johnson, Benjamin D.; Glaccum, William; Villaume, Alexa; Cuillandre, Jean-Charles; Gwyn, Stephen; Krick, Jessica; Lasker, Ronald; Martín-Navarro, Ignacio; Martinez-Delgado, David; van Dokkum, Pieter (May 1, 2018). "The Stellar Populations of Two Ultra-diffuse Galaxies from Optical and Near-infrared Photometry". The Astrophysical Journal. 858 (1): 29. arXiv:1711.05272. Bibcode:2018ApJ...858...29P. doi:10.3847/1538-4357/aab498. ISSN 0004-637X.
  97. ^ Overbye, Dennis (January 26, 2024). "What Do You Call a Galaxy Without Stars?". The New York Times. Archived from the original on February 2, 2024.
  98. ^ van Dokkum, Pieter G.; Abraham, Roberto; Merritt, Allison; Zhang, Jielai; Geha, Marla; Conroy, Charlie (January 2015). "Forty-Seven Milky Way-Sized, Extremely Diffuse Galaxies in the Coma Cluster". The Astrophysical Journal Letters. 798 (2): L45. arXiv:1410.8141. Bibcode:2015ApJ...798L..45V. doi:10.1088/2041-8205/798/2/L45. ISSN 2041-8205.
  99. ^ a b Mateo, Mario (1998). "Dwarf Galaxies of the Local Group". Annual Review of Astronomy and Astrophysics. 36 (1): 435–506. arXiv:astro-ph/9810070. Bibcode:1998ARA&A..36..435M. doi:10.1146/annurev.astro.36.1.435. ISSN 0066-4146. S2CID 119333888.
  100. ^ Phillipps, S.; Drinkwater, M. J.; Gregg, M. D.; Jones, J. B. (2001). "Ultracompact Dwarf Galaxies in the Fornax Cluster". The Astrophysical Journal. 560 (1): 201–206. arXiv:astro-ph/0106377. Bibcode:2001ApJ...560..201P. doi:10.1086/322517. S2CID 18297376.
  101. ^ Groshong, Kimm (April 24, 2006). "Strange satellite galaxies revealed around Milky Way". New Scientist. Archived from the original on July 2, 2015. Retrieved January 10, 2007.
  102. ^ Schirber, M. (August 27, 2008). "No Slimming Down for Dwarf Galaxies". ScienceNOW. Archived from the original on May 30, 2020. Retrieved August 27, 2008.
  103. ^ a b c "Galaxy Interactions". University of Maryland Department of Astronomy. Archived from the original on May 9, 2006. Retrieved December 19, 2006.
  104. ^ a b c "Interacting Galaxies". Swinburne University. Archived from the original on April 18, 2023. Retrieved December 19, 2006.
  105. ^ "Happy Sweet Sixteen, Hubble Telescope!". Hubble News Desk (Press release). NASA. April 24, 2006. Archived from the original on August 27, 2016. Retrieved August 10, 2006.
  106. ^ a b "Starburst Galaxies". Harvard-Smithsonian Center for Astrophysics. August 29, 2006. Archived from the original on March 16, 2019. Retrieved August 10, 2006.
  107. ^ Kennicutt, Robert C. Jr.; et al. (2005). Demographics and Host Galaxies of Starbursts. Starbursts: From 30 Doradus to Lyman Break Galaxies. Springer. p. 187. Bibcode:2005ASSL..329..187K. doi:10.1007/1-4020-3539-X_33. ISBN 978-1-4020-3538-8.
  108. ^ Smith, Gene (July 13, 2006). "Starbursts & Colliding Galaxies". University of California, San Diego Center for Astrophysics & Space Sciences. Archived from the original on December 17, 2023. Retrieved August 10, 2006.
  109. ^ Keel, William C. (September 2006). "Starburst Galaxies". University of Alabama. Archived from the original on August 31, 2023. Retrieved December 11, 2006.
  110. ^ Adams, David J.; et al. (2004). Jones, Mark H.; Lambourne, Robert J. A. (eds.). An Introduction to Galaxies and Cosmology. Cambridge University Press. pp. 142–144. ISBN 978-0-521-54623-2.
  111. ^ Kembhavi, Ajit K.; Narlikar, Jayant V. (2012) [1999]. "Radio Properties". Quasars and Active Galactic Nuclei: An Introduction. Cambridge University Press. pp. 214–272. ISBN 978-1-139-17440-4.
  112. ^ "Introductory Astronomy: Double Lobed Radio Galaxies". Department of Astronomy, University of Maryland. Archived from the original on September 1, 2022. Retrieved September 1, 2022.
  113. ^ Oei, Martijn S. S. L.; van Weeren, Reinout J.; Hardcastle, Martin J.; Botteon, Andrea; Shimwell, Tim W.; Dabhade, Pratik; Gast, Aivin R. D. J. G. I. B.; Röttgering, Huub J. A.; Brüggen, Marcus; Tasse, Cyril; Williams, Wendy L.; Shulevski, Aleksandar (April 1, 2022). "The discovery of a radio galaxy of at least 5 Mpc". Astronomy & Astrophysics. 660: A2. arXiv:2202.05427. Bibcode:2022A&A...660A...2O. doi:10.1051/0004-6361/202142778. ISSN 0004-6361.
  114. ^ Condon, J. J. (September 1992). "Radio Emission from Normal Galaxies". Annual Review of Astronomy and Astrophysics. 30 (1): 575–611. Bibcode:1992ARA&A..30..575C. doi:10.1146/annurev.aa.30.090192.003043. ISSN 0066-4146.
  115. ^ Fraknoi et al. 2023, p. 906.
  116. ^ Zakamska, Nadia L.; Strauss, Michael A.; Krolik, Julian H.; Collinge, Matthew J.; Hall, Patrick B.; Hao, Lei; Heckman, Timothy M.; Ivezi, eljko; Richards, Gordon T.; Schlegel, David J.; Schneider, Donald P.; Strateva, Iskra; Vanden Berk, Daniel E.; Anderson, Scott F.; Brinkmann, Jon (November 2003). "Candidate Type II Quasars from the Sloan Digital Sky Survey. I. Selection and Optical Properties of a Sample at 0.3< Z". The Astronomical Journal. 126 (5): 2125–2144. arXiv:astro-ph/0309551. doi:10.1086/378610. ISSN 0004-6256.
  117. ^ Fraknoi et al. 2023, pp. 907–915.
  118. ^ a b Keel, William C. (2000). "Introducing Active Galactic Nuclei". University of Alabama. Archived from the original on April 27, 2012. Retrieved December 6, 2006.
  119. ^ a b Lochner, Jim. Gibb, Meredith (ed.). "A Monster in the Middle". Imagine the Universe!. NASA. Archived from the original on March 26, 2009. Retrieved December 20, 2006.
  120. ^ Peterson, Bradley M. (1997). An Introduction to Active Galactic Nuclei. Cambridge University Press. ISBN 978-0-521-47911-0.
  121. ^ Peterson, Bradley M. (August 1, 2008). "The central black hole and relationships with the host galaxy" (PDF). New Astronomy Reviews. Active Galactic Nuclei at the Highest Angular Resolution: Theory and Observations. 52 (6): 240–252. Bibcode:2008NewAR..52..240P. doi:10.1016/j.newar.2008.06.005. ISSN 1387-6473. S2CID 121460317.
  122. ^ Fraknoi et al. 2023, pp. 899–906.
  123. ^ Kovačević, Andjelka B; Yi, Tignfeng; Dai, Xinyu; Yang, Xing; Čvorović-Hajdinjak, Iva; Popović, Luka Č (May 21, 2020). "Confirmed short periodic variability of subparsec supermassive binary black hole candidate Mrk 231". Monthly Notices of the Royal Astronomical Society. 494 (3): 4069–4076. arXiv:2003.06359. doi:10.1093/mnras/staa737. ISSN 0035-8711.
  124. ^ Nemiroff, Robert; Bonnell, Jerry, eds. (November 10, 2023). "UHZ1: Distant Galaxy and Black Hole". Astronomy Picture of the Day. Retrieved February 18, 2024.
  125. ^ Bogdán, Ákos; Goulding, Andy D.; Natarajan, Priyamvada; Kovács, Orsolya E.; Tremblay, Grant R.; Chadayammuri, Urmila; Volonteri, Marta; Kraft, Ralph P.; Forman, William R.; Jones, Christine; Churazov, Eugene; Zhuravleva, Irina (January 2024). "Evidence for heavy-seed origin of early supermassive black holes from a z ≈ 10 X-ray quasar". Nature Astronomy. 8 (1): 126–133. arXiv:2305.15458. Bibcode:2024NatAs...8..126B. doi:10.1038/s41550-023-02111-9. ISSN 2397-3366. S2CID 258887541.
  126. ^ Blandford, R. D.; Narayan, R. (September 1992). "Cosmological Applications of Gravitational Lensing". Annual Review of Astronomy and Astrophysics. 30 (1): 311–358. doi:10.1146/annurev.aa.30.090192.001523. ISSN 0066-4146.
  127. ^ a b Heckman, Timothy M. (1980). "An Optical and Radio Survey of the Nuclei of Bright Galaxies" (PDF). Astronomy & Astrophysics. 87 (1–2): 152–164. Bibcode:1980A&A....87..152H.
  128. ^ Ho, Luis C.; Filippenko, Alexei V.; Sargent, Wallace L. W. (1997). "A Search for "Dwarf" Seyfert Nuclei. V. Demographics of Nuclear Activity in Nearby Galaxies". The Astrophysical Journal. 487 (2): 568–578. arXiv:astro-ph/9704108. Bibcode:1997ApJ...487..568H. doi:10.1086/304638. S2CID 16742031.
  129. ^ Pérez-Torres, Miguel; Mattila, Seppo; Alonso-Herrero, Almudena; Aalto, Susanne; Efstathiou, Andreas (December 2021). "Star formation and nuclear activity in luminous infrared galaxies: an infrared through radio review". The Astronomy and Astrophysics Review. 29 (1): 2. arXiv:2010.05072. Bibcode:2021A&ARv..29....2P. doi:10.1007/s00159-020-00128-x. ISSN 0935-4956. S2CID 253687419.
  130. ^ a b Bellocchi, E.; Pereira-Santaella, M.; Colina, L.; Labiano, A.; Sánchez-García, M.; Alonso-Herrero, A.; Arribas, S.; García-Burillo, S.; Villar-Martín, M.; Rigopoulou, D.; Valentino, F.; Puglisi, A.; Díaz-Santos, T.; Cazzoli, S.; Usero, A. (August 2022). "Compact molecular gas emission in local LIRGs among low- and high- z galaxies". Astronomy & Astrophysics. 664: A60. arXiv:2204.02055. Bibcode:2022A&A...664A..60B. doi:10.1051/0004-6361/202142802. ISSN 0004-6361.
  131. ^ Sanders, David B.; Mirabel, I. F. (September 1996). "Luminous Infrared Galaxies" (PDF). Annual Review of Astronomy and Astrophysics. 34 (1): 749–792 [772–773]. Bibcode:1996ARA&A..34..749S. doi:10.1146/annurev.astro.34.1.749. ISSN 0066-4146.
  132. ^ Sandage, Kron & Longair 1995, p. 43.
  133. ^ Sandage, Kron & Longair 1995, p. 73.
  134. ^ a b Chamba, Nushkia (2020). "A historical perspective on the concept of galaxy size". Research Notes of the American Astronomical Society. 4 (7): 117. arXiv:2010.07946. Bibcode:2020RNAAS...4..117C. doi:10.3847/2515-5172/aba951. S2CID 223953458.
  135. ^ Mushotzky, Richard. "Measuring Galaxy Luminosities" (PDF). ASTR620: Galaxies. University of Maryland Astronomy Department. Retrieved February 14, 2024.
  136. ^ Sparke & Gallagher 2000, 1.3.1
  137. ^ Fouque, P.; Paturel, G. (September 1985). "Standard photometric diameters of galaxies. II - Reduction of the ESO, UGC, MCG catalogue". Astronomy & Astrophysics. 150 (2): 192–204. Bibcode:1985A&A...150..192F.
  138. ^ "Results for object Large Magellanic Cloud". NASA/IPAC Extragalactic Database. California Institute of Technology. Retrieved March 8, 2024.
  139. ^ "Results for object MESSIER 087 (NGC 4486)". NASA/IPAC Extragalactic Database. California Institute of Technology. Retrieved March 8, 2024.
  140. ^ "Results for object MESSIER 031 (Andromeda Galaxy)". NASA/IPAC Extragalactic Database. California Institute of Technology. Retrieved March 8, 2024.
  141. ^ De Vaucouleurs, Gérard (1948). "Recherches sur les Nebuleuses Extragalactiques". Annales d'Astrophysique. 11: 247. Bibcode:1948AnAp...11..247D.
  142. ^ Fish, Robert A. (1963). "Significance of a Luminosity Concentration Law in Elliptical Galaxies". The Astronomical Journal. 68: 72. Bibcode:1963AJ.....68R..72F. doi:10.1086/109075.
  143. ^ Sérsic, José Luis (1968). "On the formation of galaxies by fragmentation". Bulletin of the Astronomical Institutes of Czechoslovakia. 19: 105. Bibcode:1968BAICz..19..105S.
  144. ^ Conselice, Christopher J.; Bershady, Matthew A.; Jangren, Anna (2000). "The Asymmetry of Galaxies: Physical Morphology for Nearby and High-Redshift Galaxies". The Astrophysical Journal. 529 (2): 886–910. arXiv:astro-ph/9907399. Bibcode:2000ApJ...529..886C. doi:10.1086/308300. S2CID 118962524.
  145. ^ Burgarella, D.; Buat, V.; Donas, J.; Milliard, B.; Chapelon, S. (2001). "The ultraviolet visibility and quantitative morphology of galactic disks at low and high redshift". Astronomy & Astrophysics. 369 (2): 421–431. arXiv:astro-ph/0101344. Bibcode:2001A&A...369..421B. doi:10.1051/0004-6361:20010107. S2CID 858029.
  146. ^ Lauberts, Andris; Valentijn, Edwin A. (1989). The surface photometry catalogue of the ESO-Uppsala galaxies. Bibcode:1989spce.book.....L.
  147. ^ Petrosian, Vahe (1976). "Surface Brightness and Evolution of Galaxies". The Astrophysical Journal. 210: L53. Bibcode:1976ApJ...209L...1P. doi:10.1086/182301.
  148. ^ "The Petrosian magnitude". Sloan Digital Sky Survey. Archived from the original on August 2, 2023. Retrieved April 20, 2024.
  149. ^ a b Graham, Alister W.; Driver, Simon P.; Petrosian, Vahe; Conselice, Christopher J.; Bershady, Matthew A.; Crawford, Steven M.; Goto, Tomotsugu (2005). "Total Galaxy Magnitudes and Effective Radii from Petrosian Magnitudes and Radii". The Astronomical Journal. 130 (4): 1535–1544. arXiv:astro-ph/0504287. Bibcode:2005AJ....130.1535G. doi:10.1086/444475. S2CID 11517686.
  150. ^ Jarret, Tom; Rosenberg, Jessica (February 6, 1997). "Circular and Fixed Elliptical Apertures: The Petrosian and Isophotal Photometry". Comparison of GALWORKS and IRAF's STSDAS Photometry in a Polar Field, WISE Science Data Center. California Institute of Technology. Retrieved April 20, 2024.
  151. ^ "Measures of SDSS flux, magnitudes, and diameters". NASA/IPAC Extragalactic Database. July 19, 2017. Retrieved April 20, 2024.
  152. ^ a b Jarrett, T. H.; Chester, T.; Cutri, R.; Schneider, S. E.; Huchra, J. P. (2003). "The 2MASS Large Galaxy Atlas". The Astronomical Journal. 125 (2): 525–554. Bibcode:2003AJ....125..525J. doi:10.1086/345794. S2CID 117784410.
  153. ^ a b Beck, Rainer (2007). "Galactic magnetic fields". Scholarpedia. 2 (8): 2411. Bibcode:2007SchpJ...2.2411B. doi:10.4249/scholarpedia.2411.
  154. ^ "Construction Secrets of a Galactic Metropolis" (Press release). European Southern Observatory. Archived from the original on March 24, 2021. Retrieved October 15, 2014.
  155. ^ a b "Protogalaxies". Harvard-Smithsonian Center for Astrophysics. November 18, 1999. Archived from the original on March 25, 2008. Retrieved January 10, 2007.
  156. ^ Firmani, C.; Avila-Reese, Vladimir (2003). "Physical processes behind the morphological Hubble sequence". Revista Mexicana de Astronomía y Astrofísica. 17: 107–120. arXiv:astro-ph/0303543. Bibcode:2003RMxAC..17..107F.
  157. ^ Skibba, Ramin (January 10, 2023). "Astronomers May Have Just Spotted the Universe's First Galaxies". Wired. Retrieved January 10, 2023.
  158. ^ McMahon, Richard (2006). "Astronomy: Dawn after the dark age". Nature. 443 (7108): 151–2. Bibcode:2006Natur.443..151M. doi:10.1038/443151a. PMID 16971933. S2CID 28977650.
  159. ^ Bouwens, R. J.; et al. (2013). "Photometric Constraints on the Redshift of z ~ 10 Candidate UDFj-39546284 from Deeper WFC3/IR+ACS+IRAC Observations over the HUDF". The Astrophysical Journal Letters. 765 (1): L16. arXiv:1211.3105. Bibcode:2013ApJ...765L..16B. doi:10.1088/2041-8205/765/1/L16.
  160. ^ Wall, Mike (December 12, 2012). "Ancient Galaxy May Be Most Distant Ever Seen". Space.com. Archived from the original on May 4, 2019. Retrieved December 12, 2012.
  161. ^ a b Robertson, B. E.; et al. (2023). "Identification and properties of intense star-forming galaxies at redshifts z > 10". Nature Astronomy. 7 (5): 611–621. arXiv:2212.04480. Bibcode:2023NatAs...7..611R. doi:10.1038/s41550-023-01921-1. S2CID 257968812.
  162. ^ Curtis-Lake, Emma; Carniani, Stefano; Cameron, Alex; et al. (2023). "Spectroscopy of four metal-poor galaxies beyond redshift ten" (PDF). Nature Astronomy. 7: 622–632. arXiv:2212.04568. doi:10.1038/s41550-023-01918-w. hdl:1887/3716631.
  163. ^ Cesari, Thaddeus (December 9, 2022). "NASA's Webb Reaches New Milestone in Quest for Distant Galaxies" (Press release). NASA. Retrieved April 23, 2024.
  164. ^ Ilie, Cosmin; Paulin, Jillian; Freese, Katherine (2023). "Supermassive Dark Star candidates seen by JWST?". Proceedings of the National Academy of Sciences. 120 (30): e2305762120. arXiv:2304.01173. Bibcode:2023PNAS..12005762I. doi:10.1073/pnas.2305762120. PMC 10372643. PMID 37433001.
  165. ^ Wang, Bingjie; et al. (November 2023). "UNCOVER: Illuminating the Early Universe—JWST/NIRSpec Confirmation of z > 12 Galaxies". The Astrophysical Journal Letters. 957 (2): L34. arXiv:2308.03745. Bibcode:2023ApJ...957L..34W. doi:10.3847/2041-8213/acfe07. ISSN 2041-8205.
  166. ^ Cooper, Keith (November 14, 2023). "James Webb Space Telescope finds 2 of the most distant galaxies ever seen". Space.com. Retrieved April 23, 2024.
  167. ^ Atkinson, Nancy (August 17, 2022). "Here's the Largest Image JWST Has Taken So Far". Universe Today. Retrieved August 18, 2022.
  168. ^ Whitt, Kelly Kizer (August 18, 2022). "Webb's largest image of galaxies yet". Earth & Sky. Retrieved August 19, 2022.
  169. ^ Overbye, Dennis (August 23, 2022). "How the Webb Telescope Expanded My Universe". The New York Times. Retrieved August 24, 2022.
  170. ^ a b c d Roberts-Borsani, Guido; et al. (May 17, 2023). "The nature of an ultra-faint galaxy in the cosmic dark ages seen with JWST". Nature. 618 (7965): 480–483. arXiv:2210.15639. Bibcode:2023Natur.618..480R. doi:10.1038/s41586-023-05994-w. PMID 37198479. S2CID 258741077.
  171. ^ a b c Ober, Holly (June 1, 2023). "Astrophysicists confirm the faintest galaxy ever seen in the early universe". Phys.org. Retrieved June 12, 2023.
  172. ^ Overbye, Dennis (January 5, 2024). "The Early Universe Was Bananas". The New York Times. Archived from the original on January 6, 2024. Retrieved January 6, 2024.
  173. ^ Pandya, Viraj; et al. (2024). "Galaxies Going Bananas: Inferring the 3D Geometry of High-Redshift Galaxies with JWST-CEERS". The Astrophysical Journal. 963 (1): 54. arXiv:2310.15232. Bibcode:2024ApJ...963...54P. doi:10.3847/1538-4357/ad1a13.
  174. ^ "Signatures of the Earliest Galaxies". European Space Agency. Archived from the original on August 6, 2020. Retrieved September 15, 2015.
  175. ^ Eggen, Olin J.; Lynden-Bell, Donald; Sandage, Allan R. (1962). "Evidence from the motions of old stars that the Galaxy collapsed". The Astrophysical Journal. 136: 748. Bibcode:1962ApJ...136..748E. doi:10.1086/147433.
  176. ^ Searle, L.; Zinn, R. (1978). "Compositions of halo clusters and the formation of the galactic halo". The Astrophysical Journal. 225 (1): 357–379. Bibcode:1978ApJ...225..357S. doi:10.1086/156499.
  177. ^ Heger, Alexander; Woosley, Stanford E. (2002). "The Nucleosynthetic Signature of Population III". The Astrophysical Journal. 567 (1): 532–543. arXiv:astro-ph/0107037. Bibcode:2002ApJ...567..532H. doi:10.1086/338487. S2CID 16050642.
  178. ^ Barkana, Rennan; Loeb, Abraham (2001). "In the beginning: the first sources of light and the reionization of the Universe" (PDF). Physics Reports. 349 (2): 125–238. arXiv:astro-ph/0010468. Bibcode:2001PhR...349..125B. doi:10.1016/S0370-1573(01)00019-9. S2CID 119094218. Archived from the original (PDF) on March 14, 2021.
  179. ^ Sobral, David; et al. (June 4, 2015). "Evidence for POPIII-like Stellar Populations in the Most Luminous LYMAN-α Emitters at the Epoch of Re-ionisation: Spectroscopic Confirmation". The Astrophysical Journal. 808 (2): 139. arXiv:1504.01734. Bibcode:2015ApJ...808..139S. doi:10.1088/0004-637x/808/2/139. S2CID 18471887.
  180. ^ Overbye, Dennis (June 17, 2015). "Traces of Earliest Stars That Enriched Cosmos Are Spied". The New York Times. Archived from the original on June 29, 2019. Retrieved June 17, 2015.
  181. ^ O'Callaghan, Jonathan (December 6, 2022). "Astronomers Grapple with JWST's Discovery of Early Galaxies". Scientific American. Retrieved December 6, 2022.
  182. ^ "Simulations Show How Growing Black Holes Regulate Galaxy Formation" (Press release). Carnegie Mellon University. February 9, 2005. Archived from the original on March 31, 2012. Retrieved January 7, 2007.
  183. ^ Massey, Robert; Heward, Anita (April 21, 2007). "Caught in the act; forming galaxies captured in the young Universe". Royal Astronomical Society. Archived from the original on November 15, 2013. Retrieved April 20, 2007.
  184. ^ Noguchi, Masafumi (1999). "Early Evolution of Disk Galaxies: Formation of Bulges in Clumpy Young Galactic Disks". The Astrophysical Journal. 514 (1): 77–95. arXiv:astro-ph/9806355. Bibcode:1999ApJ...514...77N. doi:10.1086/306932. S2CID 17963236.
  185. ^ Baugh, Carlton; Frenk, Carlos (May 1999). "How are galaxies made?". Institute of Physics. Archived from the original on April 26, 2007. Retrieved January 16, 2007.
  186. ^ Gonzalez, Guillermo (1998). Rebolo, Rafael; Martin, Eduardo L.; Osorio, Maria Rosa Zapatero (eds.). The Stellar Metallicity — Planet Connection (PDF). Brown Dwarfs and Extrasolar Planets. ASP Conference Series. Vol. 134. pp. 431–437. Bibcode:1998ASPC..134..431G.
  187. ^ Moskowitz, Clara (September 25, 2012). "Hubble Telescope Reveals Farthest View Into Universe Ever". Space.com. Archived from the original on May 5, 2020. Retrieved September 26, 2012.
  188. ^ Rojas, Randall R.; Vogeley, Michael S.; Hoyle, Fiona; Brinkmann, Jon (May 10, 2005). "Spectroscopic Properties of Void Galaxies in the Sloan Digital Sky Survey". The Astrophysical Journal. 624 (2): 571–585. arXiv:astro-ph/0409074. Bibcode:2005ApJ...624..571R. doi:10.1086/428476. ISSN 0004-637X.
  189. ^ Conselice, Christopher J. (February 2007). "The Universe's Invisible Hand". Scientific American. Vol. 296, no. 2. pp. 35–41. Bibcode:2007SciAm.296b..34C. doi:10.1038/scientificamerican0207-34.
  190. ^ Ford, H.; et al. (April 30, 2002). "The Mice (NGC 4676): Colliding Galaxies With Tails of Stars and Gas". Hubble News Desk (Press release). NASA. Archived from the original on September 7, 2016. Retrieved May 8, 2007.
  191. ^ Struck, Curtis (1999). "Galaxy Collisions". Physics Reports. 321 (1–3): 1–137. arXiv:astro-ph/9908269. Bibcode:1999PhR...321....1S. doi:10.1016/S0370-1573(99)00030-7. S2CID 119369136.
  192. ^ Buser, Roland (2000). "The Formation and Early Evolution of the Milky Way Galaxy". Science. 287 (5450): 69–74. Bibcode:2000Sci...287...69B. doi:10.1126/science.287.5450.69. PMID 10615051.
  193. ^ Kruijssen, J M Diederik; Pfeffer, Joel L; Chevance, Mélanie; Bonaca, Ana; Trujillo-Gomez, Sebastian; Bastian, Nate; Reina-Campos, Marta; Crain, Robert A; Hughes, Meghan E (October 2020). "Kraken reveals itself – the merger history of the Milky Way reconstructed with the E-MOSAICS simulations". Monthly Notices of the Royal Astronomical Society. 498 (2): 2472–2491. arXiv:2003.01119. doi:10.1093/mnras/staa2452.
  194. ^ Young, Monica (November 13, 2020). "Star Clusters reveal the "Kraken" in the Milky Way's Past". Sky & Telescope. Archived from the original on November 15, 2020. Retrieved November 15, 2020.
  195. ^ Panter, B.; Jimenez, R.; Heavens, A. F.; Charlot, S. (2007). "The star formation histories of galaxies in the Sloan Digital Sky Survey". Monthly Notices of the Royal Astronomical Society. 378 (4): 1550–1564. arXiv:astro-ph/0608531. Bibcode:2007MNRAS.378.1550P. doi:10.1111/j.1365-2966.2007.11909.x. S2CID 15174718.
  196. ^ Kennicutt, Robert C. Jr.; Tamblyn, Peter; Congdon, Charles E. (1994). "Past and future star formation in disk galaxies". The Astrophysical Journal. 435 (1): 22–36. Bibcode:1994ApJ...435...22K. doi:10.1086/174790.
  197. ^ Knapp, Gillian R. (1999). Star Formation in Early Type Galaxies. Vol. 163. Astronomical Society of the Pacific. p. 119. arXiv:astro-ph/9808266. Bibcode:1999ASPC..163..119K. ISBN 978-1-886733-84-8. OCLC 41302839.
  198. ^ a b Adams, Fred; Laughlin, Greg (July 13, 2006). "The Great Cosmic Battle". Astronomical Society of the Pacific. Archived from the original on May 13, 2012. Retrieved January 16, 2007.
  199. ^ Choi, Charles Q. (May 13, 2015). "Cosmic 'Murder Mystery' Solved: Galaxies Are 'Strangled to Death'". Space.com. Archived from the original on March 24, 2021. Retrieved May 14, 2015.
  200. ^ Pobojewski, Sally (January 21, 1997). "Physics offers glimpse into the dark side of the Universe". University of Michigan. Archived from the original on January 21, 2005. Retrieved January 13, 2007.
  201. ^ Argudo-Fernández, M.; Verley, S.; Bergond, G.; Duarte Puertas, S.; Ramos Carmona, E.; Sabater, J.; Fernández Lorenzo, M.; Espada, D.; Sulentic, J.; Ruiz, J. E.; Leon, S. (June 2015). "Catalogues of isolated galaxies, isolated pairs, and isolated triplets in the local Universe". Astronomy & Astrophysics. 578: A110. arXiv:1504.00117. Bibcode:2015A&A...578A.110A. doi:10.1051/0004-6361/201526016. ISSN 0004-6361.
  202. ^ Karachentsev, I. D.; Makarov, D. I.; Karachentseva, V. E.; Melnyk, O. V. (January 2011). "Catalog of nearby isolated galaxies in the volume z < 0.01". Astrophysical Bulletin. 66 (1): 1–27. arXiv:1103.3990. Bibcode:2011AstBu..66....1K. doi:10.1134/S1990341311010019. ISSN 1990-3413.
  203. ^ Melnyk, O.; Karachentseva, V.; Karachentsev, I. (August 1, 2015). "Star formation rates in isolated galaxies selected from the Two-Micron All-Sky Survey". Monthly Notices of the Royal Astronomical Society. 451 (2): 1482–1495. arXiv:1504.07990. doi:10.1093/mnras/stv950. ISSN 1365-2966.
  204. ^ Hirschmann, Michaela; De Lucia, Gabriella; Iovino, Angela; Cucciati, Olga (August 1, 2013). "Isolated galaxies in hierarchical galaxy formation models – present-day properties and environmental histories". Monthly Notices of the Royal Astronomical Society. 433 (2): 1479–1491. arXiv:1302.3616. Bibcode:2013MNRAS.433.1479H. doi:10.1093/mnras/stt827. ISSN 1365-2966.
  205. ^ Wang, Wenting; White, Simon D. M. (August 21, 2012). "Satellite abundances around bright isolated galaxies: Satellite abundances". Monthly Notices of the Royal Astronomical Society. 424 (4): 2574–2598. arXiv:1203.0009. Bibcode:2012MNRAS.424.2574W. doi:10.1111/j.1365-2966.2012.21256.x.
  206. ^ "Groups & Clusters of Galaxies". Chandra X-ray Observatory. Harvard–Smithsonian Center for Astrophysics. Archived from the original on February 22, 2024. Retrieved January 15, 2007.
  207. ^ Ricker, Paul. "When Galaxy Clusters Collide". San Diego Supercomputer Center. University of California, San Diego. Archived from the original on August 2, 2023. Retrieved April 21, 2024.
  208. ^ Pompei, Emanuela; Dahlem, Michael; Iovino, Angela (November 24, 2006). Dahlem, Michael (ed.). "Optical and radio survey of Southern Compact Groups of galaxies". University of Birmingham Astrophysics and Space Research Group. Archived from the original on June 13, 2007. Retrieved January 15, 2007.
  209. ^ Ponman, Trevor (February 25, 2005). "Galaxy Systems: Groups". University of Birmingham Astrophysics and Space Research Group. Archived from the original on February 15, 2009. Retrieved January 15, 2007.
  210. ^ Girardi, Marisa; Giuricin, G. (2000). "The Observational Mass Function of Loose Galaxy Groups". The Astrophysical Journal. 540 (1): 45–56. arXiv:astro-ph/0004149. Bibcode:2000ApJ...540...45G. doi:10.1086/309314. S2CID 14059401.
  211. ^ "Hubble Pinpoints Furthest Protocluster of Galaxies Ever Seen" (Press release). NASA, European Space Agency. Archived from the original on June 12, 2018. Retrieved January 22, 2015.
  212. ^ Dubinski, John (1998). "The Origin of the Brightest Cluster Galaxies". The Astrophysical Journal. 502 (2): 141–149. arXiv:astro-ph/9709102. Bibcode:1998ApJ...502..141D. doi:10.1086/305901. S2CID 3137328. Archived from the original on May 14, 2011.
  213. ^ "ATLASGAL Survey of Milky Way Completed". European Southern Observatory. Archived from the original on March 24, 2021. Retrieved March 7, 2016.
  214. ^ Bahcall, Neta A. (1988). "Large-scale structure in the Universe indicated by galaxy clusters". Annual Review of Astronomy and Astrophysics. 26 (1): 631–686. Bibcode:1988ARA&A..26..631B. doi:10.1146/annurev.aa.26.090188.003215.
  215. ^ Mandolesi, Nazzareno; et al. (1986). "Large-scale homogeneity of the Universe measured by the microwave background". Letters to Nature. 319 (6056): 751–753. Bibcode:1986Natur.319..751M. doi:10.1038/319751a0. S2CID 4349689.
  216. ^ Horváth, István; Bagoly, Zsolt; Hakkila, Jon; Tóth, L. Viktor (2015). "New data support the existence of the Hercules–Corona Borealis Great Wall". Astronomy & Astrophysics. 584: A48. arXiv:1510.01933. Bibcode:2015A&A...584A..48H. doi:10.1051/0004-6361/201424829. S2CID 56073380.
  217. ^ Horváth, István; Bagoly, Zsolt; Hakkila, Jon; Tóth, L. Viktor (2014). "Anomalies in the GRB spatial distribution". Proceedings of Science: 78. arXiv:1507.05528. Bibcode:2014styd.confE..78H. doi:10.22323/1.233.0078.
  218. ^ van den Bergh, Sidney (2000). "Updated Information on the Local Group". Publications of the Astronomical Society of the Pacific. 112 (770): 529–536. arXiv:astro-ph/0001040. Bibcode:2000PASP..112..529V. doi:10.1086/316548. S2CID 1805423.
  219. ^ Tully, Richard Brent (1982). "The Local Supercluster". The Astrophysical Journal. 257: 389–422. Bibcode:1982ApJ...257..389T. doi:10.1086/159999.
  220. ^ Tempel, Elmo (September 1, 2014). "Cosmology: Meet the Laniakea supercluster". Nature. 513 (7516): 41–42. Bibcode:2014Natur.513...41T. doi:10.1038/513041a. PMID 25186896.
  221. ^ Rieke, George Henry (2012). Measuring the Universe: A Multiwavelength Perspective. Cambridge University Press. p. xi. ISBN 978-0-521-76229-8.
  222. ^ "Near, Mid & Far Infrared". Infrared Processing and Analysis Center. California Institute of Technology. Archived from the original on December 30, 2006. Retrieved January 2, 2007.
  223. ^ Fraknoi et al. 2023, pp. 144, 202.
  224. ^ "The Effects of Earth's Upper Atmosphere on Radio Signals". NASA. Archived from the original on May 7, 2012. Retrieved August 10, 2006.
  225. ^ Max Planck Society (December 14, 2006). "Giant Radio Telescope Imaging Could Make Dark Matter Visible". ScienceDaily. Archived from the original on July 3, 2017. Retrieved January 2, 2007.
  226. ^ "NASA Telescope Sees Black Hole Munch on a Star". Galaxy Evolution Explorer. NASA. December 5, 2006. Archived from the original on April 25, 2010. Retrieved January 2, 2007.
  227. ^ Dunn, Robert (August 9, 2005). "An Introduction to X-ray Astronomy". Institute of Astronomy X-Ray Group. Archived from the original on February 24, 2024. Retrieved January 2, 2007.
  228. ^ "Webb reveals structure in 19 spiral galaxies". ESA. January 29, 2024. Retrieved January 30, 2024.

Bibliography edit

  • Belkora, Leila (2003). Minding the Heavens: the Story of our Discovery of the Milky Way. CRC Press. ISBN 978-0-7503-0730-7. Archived from the original on March 24, 2021. Retrieved July 25, 2018.
  • Bertin, Giuseppe; Lin, Chia-Chiao (1996). Spiral Structure in Galaxies: a Density Wave Theory. MIT Press. ISBN 978-0-262-02396-2. Archived from the original on March 24, 2021. Retrieved July 25, 2018.
  • Binney, James; Merrifield, Michael (1998). Galactic Astronomy. Princeton University Press. ISBN 978-0-691-00402-0. OCLC 39108765. Archived from the original on March 24, 2021. Retrieved July 25, 2018.
  • Dickinson, Terence (2004). The Universe and Beyond (4th ed.). Firefly Books. ISBN 978-1-55297-901-3. OCLC 55596414.
  • Fraknoi, Andrew; et al. (2023). Astronomy 2e. OpenStax. ISBN 978-1-951693-50-3.
  • Heidarzadeh, Tofigh (2008). A History of Physical Theories of Comets, from Aristotle to Whipple. Springer. ISBN 978-1-4020-8322-8. Archived from the original on March 24, 2021. Retrieved July 25, 2018.
  • Mo, Houjun; van den Bosch, Frank; White, Simon (2010). Galaxy Formation and Evolution. Cambridge University Press. ISBN 978-0-521-85793-2. Archived from the original on March 24, 2021. Retrieved July 25, 2018.
  • Kepple, George R.; Sanner, Glen W. (1998). The Night Sky Observer's Guide, Volume 1. Willmann-Bell. ISBN 978-0-943396-58-3. Archived from the original on March 24, 2021. Retrieved July 25, 2018.
  • Merritt, David R. (2013). Dynamics and Evolution of Galactic Nuclei. Princeton University Press. ISBN 978-1-4008-4612-2. Archived from the original on March 24, 2021. Retrieved July 25, 2018.
  • Mohamed, Mohaini (2000). Great Muslim Mathematicians. Penerbit UTM. ISBN 978-983-52-0157-8. OCLC 48759017. Archived from the original on March 24, 2021. Retrieved July 25, 2018.
  • Paul, Erich R. (1993). The Milky Way Galaxy and Statistical Cosmology, 1890–1924. Cambridge University Press. ISBN 978-0-521-35363-2. Archived from the original on March 24, 2021. Retrieved July 25, 2018.
  • Sandage, Allan R.; Kron, Richard G.; Longair, Malcolm S. (1995). The Deep Universe. Saas-Fee Advanced Course, Swiss Society for Astrophysics and Astronomy. Vol. 23. Springer-Verlag. ISBN 978-3-54-058913-6.
  • Sparke, Linda S.; Gallagher, John S. (2000). Galaxies in the Universe: An Introduction. Cambridge University Press. ISBN 978-0-521-59740-1. Archived from the original on March 24, 2021. Retrieved July 25, 2018.
  • Van den Bergh, Sidney (1998). Galaxy Morphology and Classification. Cambridge University Press. ISBN 978-0-521-62335-3. Archived from the original on March 24, 2021. Retrieved July 25, 2018.
  • Waller, William H.; Hodge, Paul W. (2003). Galaxies and the Cosmic Frontier. Harvard University Press. ISBN 978-0-674-01079-6.
  • Wright, Thomas (1750). An Original Theory or New Hypothesis of the Universe. London: Chapelle. Archived from the original on April 30, 2021. Retrieved April 21, 2024. ... the stars are not infinitely dispersed and distributed in a promiscuous manner throughout all the mundane space, without order or design, ... this phænomenon [is] no other than a certain effect arising from the observer's situation, ... To a spectator placed in an indefinite space, ... it [i.e., the Milky Way (Via Lactea)] [is] a vast ring of stars ...

External links edit

  • NASA/IPAC Extragalactic Database (NED)
  • NED Redshift-Independent Distances
  • Galaxies on In Our Time at the BBC
  • An Atlas of The Universe
  • Galaxies – Information and amateur observations
  • Galaxy Zoo – citizen science galaxy classification project
  • "A Flight Through the Universe, by the Sloan Digital Sky Survey" – animated video from Berkeley Lab