View source for Gaia Sausage

{{Short description|Remains of a galaxy merger in the Milky Way}}
{{Infobox galaxy
| name               = Gaia Sausage
| image              = [[File:Simulated Gaia-Enceladus debris.jpg|300px|Artist’s impression of debris of the Gaia-Enceladus galaxy.]]
| image_size         = 
| upright            =
| caption            = 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.
| epoch              = 
| pronounce          = <!-- {{IPA-en||}} -->
| constellation name =multiple 
| ra                 = <!-- {{RA|00|00|00}} or {{Deg2HMS|000.0000|sup=yes}} -->
| dec                = <!-- {{DEC|±00|00|00}} -->
| z                  = 
| h_radial_v         = 
| gal_v              = 
| dist_pc            = <!-- {{cvt|000|+/-|00|kpc|kly|order=flip|lk=on}}{{hub|0.73}} --> <!-- comoving distance -->
| dist_ly            = <!-- {{cvt|000|+/-|00|kpc|kly|order=flip|lk=on}}{{hub|0.73}} -->
| group_cluster      =[[Local Group]] 
| type               =[[Dwarf galaxy]] 
| mass               = {{Val|5|e=10}}
| mass_light_ratio   = <!-- In solar units  -->
| size               = <!-- In light years  -->
| stars              = 
| appmag_v           = 
| appmag_b           = 
| absmag_v           = 
| size_v             = 
| half_light_radius_pc        = <!-- physical dimension, pc preferred -->
| half_light_radius_arcminsec = <!-- apparent dimension -->
| h1_scale_length_pc          = <!-- physical dimension, pc preferred -->
| h1_scale_length_arcminsec   = <!-- apparent dimension -->
| xray_radius_pc              = <!-- physical dimension, pc preferred -->
| xray_radius_arcminsec       = <!-- apparent dimension -->
| notes              = 
| names              =Gaia Enceladus <br/> Enceladus Galaxy 
| references         = 
}}
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&nbsp;billion years ago. At least eight [[globular cluster]]s were added to the Milky Way along with 50&nbsp;billion [[solar mass]]es of stars, gas and [[dark matter]].<ref name=Myeong_etal>{{cite journal |last1=Myeong |first1=G.C.
 |last2=Evans |first2=N.W.
 |last3=Belokurov |first3=V.
 |last4=Sanders |first4=J.L.
 |last5=Koposov |first5=S.
 |title=The Sausage globular clusters
 |journal=The Astrophysical Journal |volume=863 |issue=2 |pages=L28
 |date=2018|bibcode=2018ApJ...863L..28M |arxiv=1805.00453 |doi=10.3847/2041-8213/aad7f7|s2cid=67791285
 |doi-access=free
 }}</ref> It represents the last major merger of the Milky Way.<ref name=ESA-GAIA-60892> {{cite web |url= https://sci.esa.int/web/gaia/-/60892-galactic-ghosts-gaia-uncovers-major-event-in-the-formation-of-the-milky-way |title= Galactic Ghosts: Gaia Uncovers Major Event in the Formation of the Milky Way Galaxy |date= 31 October 2018 |work= Gaia |publisher= ESA }} </ref><ref name="Skibba">{{cite journal |last1=Skibba |first1=Ramin |title=A galactic archaeologist digs into the Milky Way's history |journal=Knowable Magazine |date=10 June 2021 |doi=10.1146/knowable-060921-1 |s2cid=236290725 |url=https://knowablemagazine.org/article/physical-world/2021/a-galactic-archaeologist-digs-milky-ways-history |access-date=4 August 2022|doi-access=free }}</ref>

==Etymology==
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 (<math>\boldsymbol{v}_r</math>) versus azimuthal velocity (<math>\boldsymbol{v}_\theta</math>) of stars (see [[Spherical coordinate system]]), using data from the [[Gaia Mission]].<ref name=Myeong_etal/> The stars that have merged with the Milky Way have orbits that are highly elongated. The outermost points of their orbits are around 20&nbsp;[[kiloparsec]]s from the [[Galactic Center]] at what is called the "halo break".<ref name=Deason>{{cite journal |last1=Deason |first1=Alis |last2=Belokurov |first2=Vasily |last3=Koposov |first3=Sergey |last4=Lancaster |first4=Lachlan |title=Apocenter Pile-Up: Origin of the stellar halo density break |journal=The Astrophysical Journal |volume=862 |issue=1 |pages=L1 |date=2018|bibcode=2018ApJ...862L...1D |arxiv=1805.10288 |doi=10.3847/2041-8213/aad0ee|s2cid=118936735 |doi-access=free }}</ref> These stars had previously been seen in Hipparcos data <ref>{{cite journal |last1=Chiba |first1=Masashi |last2=Beers |first2=Timothy C. |title=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 |journal=The Astronomical Journal |date=June 2000 |volume=119 |issue=6 |pages=2843–2865 |doi=10.1086/301409|bibcode=2000AJ....119.2843C |arxiv=astro-ph/0003087 |s2cid=16620828 }}</ref> and identified as originаting from an accreted galaxy.<ref>{{cite journal |last1=Brook |first1=Chris B. |last2=Kawata |first2=Daisuke |last3=Gibson |first3=Brad K. |last4=Flynn |first4=Chris |title=Galactic Halo Stars in Phase Space: A Hint of Satellite Accretion? |journal=The Astrophysical Journal |date=10 March 2003 |volume=585 |issue=2 |pages=L125–L129 |doi=10.1086/374306|bibcode=2003ApJ...585L.125B |arxiv=astro-ph/0301596 |s2cid=16936195 }}</ref>

The name "Enceladus" refers to the mythological giant [[Enceladus (Giant)|Enceladus]], who was buried under [[Mount Etna]] and caused [[earthquake]]s. Thus this former galaxy was buried in the Milky Way, and caused the puffing up of the thick disc.<ref name=ESA-GAIA-60892/>

==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]].<ref name=Myeong_etal/>

===The nature of NGC 2808===
[[File:NGC 2808 HST.jpg|thumb|250px|[[NGC 2808]], possible old core of Gaia Sausage]]
[[NGC 2808]] is another globular-like cluster of the Sausage. It is composed of three generations of stars, all born within 200 million years of the formation of the cluster.<ref>{{cite journal
 | author=Piotto, G. |display-authors=etal  | title=A Triple Main Sequence in the Globular Cluster NGC 2808
 | journal=The Astrophysical Journal | volume=661| issue=1
 | pages=L53–L56 |date=May 2007
 | doi=10.1086/518503 | bibcode=2007ApJ...661L..53P |arxiv = astro-ph/0703767 |s2cid=119376556 }}</ref>

One theory to account for three generations of stars is that NGC 2808 is the former core of the Sausage.<ref name=Myeong_etal/> This could also account for its stellar population of over a million stars, which is unusually large for a globular cluster.

=== Stars ===
The stars from this dwarf orbit the Milky Way core with extreme [[orbital eccentricity|eccentricities]] on the order of about 0.9. Their [[metallicity]] is also typically higher than other halo stars, with most having [Fe/H] > &minus;1.7&nbsp;dex, i.e., at least 2% of the solar value<ref name=Deason/><ref>{{cite journal|last1=Iorio|first1=Giuliano|date=2021|title=Chemo-kinematcs of the Gaia RR Lyrae: the halo and the disc|journal=Monthly Notices of the Royal Astronomical Society|volume=502|issue=4|pages=5686–5710|doi=10.1093/mnras/stab005|bibcode=2021MNRAS.502.5686I|last2=Belokurov|first2=Vasily|doi-access=free |arxiv=2008.02280}}</ref>

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]].<ref name=Myeong_etal/>

==See also==
* [[Field of Streams]]
* [[Monoceros Ring]]
* [[Omega Centauri]]
* [[M32p]], large galaxy merged into Andromeda Galaxy, responsible for its thick disc and most halo stars
* [[J1124+4535]]
* [[FSR 1758]], globular cluster likely from a dwarf galaxy which merged into the Milky Way
* [[Kraken galaxy]], another proposed large galaxy merged into Milky Way but earlier, contributed at least some of the surviving 150 globular clusters to the Milky Way.

==References==
{{Reflist}}

==Further reading==
* {{cite journal |title=Co-formation of the disc and the stellar halo |journal=Monthly Notices of the Royal Astronomical Society |volume=478 |number=1 |pages= 611–619 |date=July 2018 |arxiv=1802.03414 |doi=10.1093/mnras/sty982 |bibcode= 2018MNRAS.478..611B |last1=Belokurov |first1=V. |last2=Erkal |first2=D. |last3=Evans |first3=N.W. |last4=Koposov |first4=S.E. |last5=Deason |first5=A.J.|doi-access=free }}
* {{cite journal |title=The Milky Way halo in action space |date=April 2018 |arxiv=1802.03351 |doi=10.3847/2041-8213/aab613 |bibcode=2018ApJ...856L..26M |journal=The Astrophysical Journal Letters |volume=856 |issue=2 |page=L26 |last1=Myeong |first1=G.C. |last2=Evans |first2=N.W. |last3=Belokurov |first3=V. |last4=Sanders |first4=J.L. |last5=Koposov |first5=S.E.|s2cid=73518200 |doi-access=free }}
* {{cite arXiv  |title= The Shards of ω&nbsp;Centauri |eprint=1804.07050 |class=astro-ph.GA |date=April 2018 |last1=Myeong |first1=G.C. |last2=Evans |first2=N.W. |last3=Belokurov |first3=V. |last4= Sanders |first4=J.L. |last5=Koposov |first5=S.E.}}
* {{Cite journal|last1=Chaplin|first1=William J.|last2=Serenelli|first2=Aldo M.|last3=Miglio|first3=Andrea|last4=Morel|first4=Thierry|last5=Mackereth|first5=J. Ted|last6=Vincenzo|first6=Fiorenzo|last7=Kjeldsen|first7=Hans|last8=Basu|first8=Sarbani|last9=Ball|first9=Warrick H.|last10=Stokholm|first10=Amalie|last11=Verma|first11=Kuldeep|date=Jan 13, 2020|title=Age dating of an early Milky Way merger via asteroseismology of the naked-eye star ν Indi|url=https://www.nature.com/articles/s41550-019-0975-9.epdf?referrer_access_token=Ca7nRnyr3C49uBgmsm-4sdRgN0jAjWel9jnR3ZoTv0Odljv0OfEChFx0q4V7_rzUC-EEaxJWxx7dsC-D_gi7hRCWTw5MofCSJ5jynvhYK7dc8srNhZu5QWmnxu9pK7Wd9goIIrhLQCkOzCIuFTJica29pOpMKI2x4uL5ht7W5hv1Bafba_TKip_1JvofCVnF-hZgcMZgxvazegSCaC9HJEJneYq0RAtNGpbryR6_upe0exooqN00Lm-0ZdMcLO_5t3DUlbzw7Sgq-SvBmEw7eUkW9MM08ogPyuQfhF3h7hpS3EeI6QixsOpEIpw3gIfZ&tracking_referrer=www.newscientist.com|journal=Nature Astronomy|volume=4|issue=4|language=en|pages=382–389|doi=10.1038/s41550-019-0975-9|issn=2397-3366|arxiv=2001.04653|bibcode=2020NatAs...4..382C|hdl=1721.1/128912|s2cid=210166431}}

==External links==
* {{YouTube |8T2EdRZ_iE4 |Gaia Sausage Simulation}}
* {{cite news |url= https://www.cmu.edu/news/stories/archives/2018/july/gaia-sausage.html |title= The Gaia Sausage: The major collision that changed the Milky Way galaxy |date=4 July 2018 |first= Jocelyn |last= Duffy |publisher= Carnegie Mellon University}}
* {{cite news |url= https://www.cam.ac.uk/research/news/the-gaia-sausage-the-major-collision-that-changed-the-milky-way |title= The Gaia Sausage: The major collision that changed the Milky Way |publisher= University of Cambridge |first= Sarah |last= Collins |date= 4 July 2018 }}

{{Milky Way}}

[[Category: Gaia-Enceladus| ]]
[[Category:Milky Way]]
[[Category:Dwarf galaxies]]
[[Category:Hypothetical galaxies]]