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Gaia Sausage

## Summary

The Gaia Sausage or Gaia Enceladus is the remains of a dwarf galaxy (the Sausage Galaxy, or Gaia-Enceladus-Sausage, or Gaia-Sausage-Enceladus) that merged with the Milky Way about 8–11 billion years ago. At least eight globular clusters were added to the Milky Way along with 50 billion solar masses of stars, gas and dark matter.[1]

Artist’s impression of debris from the Gaia-Sausage-Enceladus galaxy. Yellow arrows represent the positions and velocities of stars originating from the dwarf galaxy, the data taken from a simulated merger with the Milky Way with similar properties to the one believed to have occurred.
Observation data

The "Gaia Sausage" is so-called because of the characteristic sausage shape of the population in a chart of velocity space, in particular a plot of radial (${\displaystyle {\boldsymbol {v}}_{r}}$) versus azimuthal velocity (${\displaystyle {\boldsymbol {v}}_{\theta }}$) of stars (See spherical coordinate system), using data from the Gaia Mission.[1] The stars that have merged with the Milky Way have orbits that are highly elongated. The outermost points of their orbits are around 20 kiloparsecs from the galactic centre at what is called the "halo break".[2] These stars had previously been seen in Hipparcos data [3] and identified as originаting from an accreted galaxy.[4]

## Components

### Globular clusters

The globular clusters firmly identified as former Sausage members are Messier 2, Messier 56, Messier 75, Messier 79, NGC 1851, NGC 2298, and NGC 5286.[1]

### NGC 2808: Globular cluster or old core?

NGC 2808, possible old core of Gaia Sausage

NGC 2808 is another globular-like cluster of Sausage. This cluster is composed of three generations of stars, all born within 200 million years of the formation of the cluster.[5]

One theory to account for three generations of stars is that NGC 2808 is the former core of Sausage.[1] This is also an explanation for the number of stars, more than a million, which is unusually large for a globular cluster.

### Stars

The stars from this dwarf orbit the Milky Way core with extreme eccentricities on the order of about 0.9. Their metallicity is also typically higher than other halo stars, with most having [Fe/H] > −1.7 dex, i.e., at least 2% of the solar value[2][6]

The "Gaia Sausage" reconstructed the Milky Way by puffing up the thin disk to make it a thick disk, whilst the gas it brought into the Milky Way triggered a fresh round of star formation and replenished the thin disk. The debris from the dwarf galaxy provides most of the metal-rich part of the galactic halo.[1]

## References

1. Myeong, G.C.; Evans, N.W.; Belokurov, V.; Sanders, J.L.; Koposov, S. (2018). "The Sausage globular clusters". The Astrophysical Journal. 863 (2): L28. arXiv:1805.00453. Bibcode:2018ApJ...863L..28M. doi:10.3847/2041-8213/aad7f7. S2CID 67791285.
2. ^ a b Deason, Alis; Belokurov, Vasily; Koposov, Sergey; Lancaster, Lachlan (2018). "Apocenter Pile-Up: Origin of the stellar halo density break". The Astrophysical Journal. 862 (1): L1. arXiv:1805.10288. Bibcode:2018ApJ...862L...1D. doi:10.3847/2041-8213/aad0ee. S2CID 118936735.
3. ^ Chiba, Masashi; Beers, Timothy C. (June 2000). "Kinematics of Metal-poor Stars in the Galaxy. III. Formation of the Stellar Halo and Thick Disk as Revealed from a Large Sample of Nonkinematically Selected Stars". The Astronomical Journal. 119 (6): 2843–2865. arXiv:astro-ph/0003087. Bibcode:2000AJ....119.2843C. doi:10.1086/301409. S2CID 16620828.
4. ^ Brook, Chris B.; Kawata, Daisuke; Gibson, Brad K.; Flynn, Chris (10 March 2003). "Galactic Halo Stars in Phase Space: A Hint of Satellite Accretion?". The Astrophysical Journal. 585 (2): L125–L129. arXiv:astro-ph/0301596. Bibcode:2003ApJ...585L.125B. doi:10.1086/374306. S2CID 16936195.
5. ^ Piotto, G.; et al. (May 2007). "A Triple Main Sequence in the Globular Cluster NGC 2808". The Astrophysical Journal. 661 (1): L53–L56. arXiv:astro-ph/0703767. Bibcode:2007ApJ...661L..53P. doi:10.1086/518503. S2CID 119376556.
6. ^ Iorio, Giuliano; Belokurov, Vasily (2021). "Chemo-kinematcs of the Gaia RR Lyrae: the halo and the disc". Monthly Notices of the Royal Astronomical Society. 502 (4): 5686–5710. Bibcode:2021MNRAS.502.5686I. doi:10.1093/mnras/stab005.