Editing Galaxy formation and evolution (section)

==Galaxy mergers and the formation of elliptical galaxies==
[[File:14-23000-Sparky-MassiveGalaxyFormation-20140827.jpg|thumb|200px|left|Artist's image of a firestorm of star birth deep inside the core of a young, growing elliptical galaxy]]
[[File:NGC4676.jpg|thumb|200px|right|NGC 4676 ([[Mice Galaxies]]) is an example of a present merger.]]
[[File:Antennae galaxies xl.jpg|thumb|200px|right|The [[Antennae Galaxies]] are a pair of colliding galaxies – the bright, blue knots are young stars that have recently ignited as a result of the merger.]]
[[File:Abell S740, cropped to ESO 325-G004.jpg|thumb|200px|left|[[ESO 325-G004]], a typical elliptical galaxy]]
{{Main|Galaxy merger}}
Elliptical galaxies (most notably [[Type-cD galaxy|supergiant ellipticals]], such as [[ESO 306-17]]) are among some of the [[List of largest galaxies|largest known thus far]]. Their stars are on orbits that are randomly oriented within the galaxy (i.e. they are not rotating like disk galaxies). A distinguishing feature of elliptical galaxies is that the velocity of the stars does not necessarily contribute to flattening of the galaxy, such as in spiral galaxies.<ref>{{Cite book|title=Hot Interstellar Matter in Elliptical Galaxies|last=Kim|first=Dong-Woo|publisher=Springer|year=2012|isbn=978-1-4614-0579-5|location=New York}}</ref> Elliptical galaxies have central [[supermassive black hole]]s, and the masses of these black holes correlate with the galaxy's mass.

Elliptical galaxies have two main stages of evolution. The first is due to the supermassive black hole growing by accreting cooling gas. The second stage is marked by the black hole stabilizing by suppressing gas cooling, thus leaving the elliptical galaxy in a stable state.<ref>{{Cite journal|last1=Churazov|first1=E.|last2=Sazonov|first2=S.|last3=Sunyaev|first3=R.|last4=Forman|first4=W.|last5=Jones|first5=C.|last6=Böhringer|first6=H.|date=2005-10-01|title=Supermassive black holes in elliptical galaxies: switching from very bright to very dim|journal=Monthly Notices of the Royal Astronomical Society: Letters|language=en|volume=363|issue=1|pages=L91–L95|doi=10.1111/j.1745-3933.2005.00093.x|doi-access=free |issn=1745-3925|arxiv = astro-ph/0507073 |bibcode = 2005MNRAS.363L..91C |s2cid=119379229 }}</ref> The mass of the black hole is also correlated to a property called [[Velocity dispersion|sigma]] which is the dispersion of the velocities of stars in their orbits.  This relationship, known as the [[M–sigma relation|M-sigma relation]], was discovered in 2000.<ref>{{Cite journal|last1=Gebhardt|first1=Karl|last2=Bender|first2=Ralf|last3=Bower|first3=Gary|last4=Dressler|first4=Alan|last5=Faber|first5=S. M.|last6=Filippenko|first6=Alexei V.|last7=Richard Green|last8=Grillmair|first8=Carl|last9=Ho|first9=Luis C.|date=2000-01-01|title=A Relationship between Nuclear Black Hole Mass and Galaxy Velocity Dispersion|journal=The Astrophysical Journal Letters|language=en|volume=539|issue=1|pages=L13|doi=10.1086/312840|issn=1538-4357|arxiv = astro-ph/0006289 |bibcode = 2000ApJ...539L..13G |s2cid=11737403 }}</ref> Elliptical galaxies mostly lack disks, although some [[Galaxy bulge|bulges]] of disk galaxies resemble elliptical galaxies. Elliptical galaxies are more likely found in crowded regions of the universe (such as [[galaxy cluster]]s).

Astronomers now see elliptical galaxies as some of the most evolved systems in the universe. It is widely accepted that the main driving force for the evolution of elliptical galaxies is [[Galaxy merger|mergers]] of smaller galaxies. Many galaxies in the universe are gravitationally bound to other galaxies, which means that they will never escape their mutual pull. If those colliding galaxies are of similar size, the resultant galaxy will appear similar to neither of the progenitors,<ref>{{Cite journal|last=Barnes|first=Joshua E.|date=1989-03-09|title=Evolution of compact groups and the formation of elliptical galaxies|journal=Nature|language=en|volume=338|issue=6211|pages=123–126|doi=10.1038/338123a0|bibcode = 1989Natur.338..123B |s2cid=4249960 }}</ref> but will instead be elliptical.  There are many types of galaxy mergers, which do not necessarily result in elliptical galaxies, but result in a structural change.  For example, a minor merger event is thought to be occurring between the Milky Way and the Magellanic Clouds.

Mergers between such large galaxies are regarded as violent, and the frictional interaction of the gas between the two galaxies can cause gravitational [[shock wave]]s, which are capable of forming new stars in the new elliptical galaxy.<ref>{{Cite web|url=https://www.noao.edu/outreach/current/collide_hilite.html|title=Current Science Highlights: When Galaxies Collide|website=www.noao.edu|access-date=2016-04-25}}</ref> By sequencing several images of different galactic collisions, one can observe the timeline of two spiral galaxies merging into a single elliptical galaxy.<ref>{{Cite web|url=http://curious.astro.cornell.edu/about-us/96-the-%20universe/galaxies/formation-and-evolution/530-what-happens-when-galaxies-collide-beginner|title=What happens when galaxies collide? (Beginner) - Curious About Astronomy? Ask an Astronomer|last=Saintonge|first=Amelie|website=curious.astro.cornell.edu|access-date=2016-04-25}}</ref>

In the [[Local Group]], the Milky Way and the [[Andromeda Galaxy]] are gravitationally bound, and currently approaching each other at high speed. Simulations show that the Milky Way and Andromeda are on a collision course, and are expected to collide in less than five billion years. During this collision, it is expected that the Sun and the rest of the Solar System will be ejected from its current path around the Milky Way.  The remnant could be a giant elliptical galaxy.<ref>{{Cite journal|last1=Cox|first1=T. J.|last2=Loeb|first2=Abraham|date=2008-05-01|title=The collision between the Milky Way and Andromeda|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=386|issue=1|pages=461–474|doi=10.1111/j.1365-2966.2008.13048.x|doi-access=free |issn=0035-8711|arxiv = 0705.1170 |bibcode = 2008MNRAS.386..461C |s2cid=14964036 }}</ref>