Editing Andromeda Galaxy (section)

=== Formation and history ===

[[File:Andromeda Galaxy 560mm FL.jpg|thumb|Processed image of the Andromeda Galaxy, with enhancement of [[H-alpha]] to highlight its star-forming regions]]
A major [[Galaxy merger|merger]] occurred 2 to 3&nbsp;billion years ago at the Andromeda location, involving two galaxies with a mass ratio of approximately 4.<ref name="Hammer et al. 2018">{{cite journal |last1=Hammer |first1=F |last2=Yang |first2=Y B |last3=Wang |first3=J L |last4=Ibata |first4=R |last5=Flores |first5=H |last6=Puech |first6=M |title=A 2–3 billion year old major merger paradigm for the Andromeda galaxy and its outskirts |journal=Monthly Notices of the Royal Astronomical Society |date=1 April 2018 |volume=475 |issue=2 |pages=2754–2767 |doi=10.1093/mnras/stx3343|doi-access=free |arxiv=1801.04279 }}</ref><ref name="D'Souza and Bell">{{cite journal |last1=D’Souza |first1=Richard |last2=Bell |first2=Eric F. |title=The Andromeda galaxy's most important merger about 2 billion years ago as M32's likely progenitor |journal=Nature Astronomy |date=September 2018 |volume=2 |issue=9 |pages=737–743 |doi=10.1038/s41550-018-0533-x |arxiv=1807.08819 |bibcode=2018NatAs...2..737D |s2cid=256713163 |url=https://www.nature.com/articles/s41550-018-0533-x |language=en |issn=2397-3366}}</ref>

The discovery of a recent merger in the Andromeda galaxy was first based on interpreting its anomalous age-velocity dispersion relation,<ref name="Dorman et al. 2014">{{Cite journal |last1=Dorman |first1=Claire E. |last2=Guhathakurta |first2=Puragra |last3=Seth |first3=Anil C. |last4=Weisz |first4=Daniel R. |last5=Bell |first5=Eric F. |last6=Dalcanton |first6=Julianne J. |last7=Gilbert |first7=Karoline M. |last8=Hamren |first8=Katherine M. |last9=Lewis |first9=Alexia R. |last10=Skillman |first10=Evan D. |last11=Toloba |first11=Elisa |last12=Williams |first12=Benjamin F. |date=9 April 2015 |title=A clear age-velocity dispersion correlation in Andromeda's stellar disk |journal=The Astrophysical Journal |volume=803 |issue=1 |pages=24 |arxiv=1502.03820 |bibcode=2015ApJ...803...24D |doi=10.1088/0004-637X/803/1/24 |s2cid=119223754}}</ref> as well as the fact that 2&nbsp;billion years ago, star formation throughout Andromeda's disk was much more active than today.<ref name="Williams et al. 2015">{{Cite journal |last1=Williams |first1=Benjamin F. |last2=Dalcanton |first2=Julianne J. |last3=Dolphin |first3=Andrew E. |last4=Weisz |first4=Daniel R. |last5=Lewis |first5=Alexia R. |last6=Lang |first6=Dustin |last7=Bell |first7=Eric F. |last8=Boyer |first8=Martha |last9=Fouesneau |first9=Morgan |last10=Gilbert |first10=Karoline M. |last11=Monachesi |first11=Antonela |last12=Skillman |first12=Evan |date=5 June 2015 |title=A Global Star-forming Episode in M31 2-4 Gyr Ago |journal=The Astrophysical Journal |volume=806 |issue=1 |pages=48 |arxiv=1504.02120 |bibcode=2015ApJ...806...48W |doi=10.1088/0004-637X/806/1/48 |s2cid=118435748}}</ref>

Modeling<ref name="Hammer et al. 2018"/> of this violent collision shows that it has formed most of the galaxy's (metal-rich) [[galactic halo]], including the Giant Stream,<ref name="Ibata et al. 2001">{{cite journal |last1=Ibata |first1=Rodrigo |last2=Irwin |first2=Michael |last3=Lewis |first3=Geraint |last4=Ferguson |first4=Annette M. N. |last5=Tanvir |first5=Nial |title=A giant stream of metal-rich stars in the halo of the galaxy M31 |journal=Nature |date=July 2001 |volume=412 |issue=6842 |pages=49–52 |doi=10.1038/35083506|pmid=11452300 |arxiv=astro-ph/0107090 |bibcode=2001Natur.412...49I |s2cid=4413139 }}</ref> and also the extended thick disk, the young age thin disk, and the static 10 kpc ring. During this epoch, its rate of [[star formation]] would have been [[Starburst galaxy|very high]], to the point of becoming a [[luminous infrared galaxy]] for roughly 100 million years. Modeling also recovers the bulge profile, the large bar, and the overall halo density profile.

Andromeda and the [[Triangulum Galaxy]] (M33) might have had a very close passage 2–4&nbsp;billion years ago, but it seems unlikely from the last measurements from the Hubble Space Telescope.<ref name="Patel et al. 2017">{{cite journal |last1=Patel |first1=Ekta |last2=Besla |first2=Gurtina |last3=Sohn |first3=Sangmo Tony |title=Orbits of massive satellite galaxies – I. A close look at the Large Magellanic Cloud and a new orbital history for M33 |journal=Monthly Notices of the Royal Astronomical Society |date=1 February 2017 |volume=464 |issue=4 |pages=3825–3849 |doi=10.1093/mnras/stw2616|doi-access=free |hdl=10150/623269 |hdl-access=free }}</ref>