Hercules, or Her, is a dwarf spheroidal galaxy situated in the Hercules constellation and discovered in 2006 in data obtained by the Sloan Digital Sky Survey.[4] The galaxy is located at a distance of about 140 kpc from the Sun and moves away from the Sun with a velocity of about 45 km/s.[4][5] It is classified as a dwarf spheroidal galaxy (dSph). It has a noticeably elongated (ratio of axes ~ 3:1) shape with a half-light radius of about 350 pc.[4][6][note 1] This elongation may be caused by tidal forces acting from the Milky Way galaxy, meaning that Her is being tidally disrupted now.[3] Her also shows some gradient of velocities across the galaxy's body[7] and is embedded into a faint stellar stream,[3] which also points towards its ongoing tidal disruption.
Hercules Dwarf Galaxy[1] | |
---|---|
Observation data (J2000 epoch) | |
Constellation | Hercules |
Right ascension | 16h 31m 02s[1] |
Declination | +12° 47′ 30″[1] |
Distance | 479+26 −23 kly (147+8 −7 kpc),[2] 430+20 −20 kly (133+6 −6 kpc[3]) |
Apparent magnitude (V) | 14.7 ± 0.5[4] |
Characteristics | |
Type | dSph[4] |
Apparent size (V) | 16.8′[4] |
Other designations | |
Her,[1] PGC 4713560 |
Her is one of the smallest and faintest satellites of the Milky Way—its integrated luminosity is about 30,000 times that of the Sun (absolute visible magnitude of about −6.6),[note 2] which is comparable to the luminosity of a typical globular cluster.[4][6] However, its total mass is about 7 million solar masses, which means the galaxy's mass to light ratio is around 330. A high mass to light ratio implies that Her is dominated by dark matter.[5][note 3]
The stellar population of Her consists mainly of old stars formed more than 12 billion years ago.[3] The metallicity of these old stars is also very low at [Fe/H] ≈ −2.58 ± 0.51,[note 4] which means that they contain 400 times less heavy elements than the Sun.[8] The stars of Her were probably among the first stars to form in the Universe. Currently there is no star formation in Her.[3] Measurements have so far failed to detect neutral hydrogen in it—the upper limit is 466 solar masses.[9]