Editing Galaxy formation and evolution (section)

== Galaxy quenching ==
[[File:Eso1516a.jpg|thumb|Star formation in what are now "dead" galaxies sputtered out billions of years ago.<ref>{{cite web|title=Giant Galaxies Die from the Inside Out|url=http://www.eso.org/public/news/eso1516/|website=www.eso.org|publisher=European Southern Observatory|access-date=21 April 2015}}</ref>]]{{Main|Quenching (astronomy)}}
One observation that must be explained by a successful theory of galaxy evolution is the existence of two different populations of galaxies on the galaxy color-magnitude diagram. Most galaxies tend to fall into two separate locations on this diagram: a "red sequence" and a "blue cloud". Red sequence galaxies are generally non-star-forming elliptical galaxies with little gas and dust, while blue cloud galaxies tend to be dusty star-forming spiral galaxies.<ref>{{Cite book|title=An Introduction to Modern Astrophysics|last1=Carroll|first1=Bradley W.|last2=Ostlie|first2=Dale A.|publisher=Pearson|year=2007|isbn=978-0805304022|location=New York}}</ref><ref>{{Cite journal|last1=Blanton|first1=Michael R.|last2=Hogg|first2=David W.|last3=Bahcall|first3=Neta A.|last4=Baldry|first4=Ivan K.|last5=Brinkmann|first5=J.|last6=Csabai|first6=István|last7=Daniel Eisenstein|last8=Fukugita|first8=Masataka|last9=Gunn|first9=James E.|date=2003-01-01|title=The Broadband Optical Properties of Galaxies with Redshifts 0.02 < z < 0.22|journal=The Astrophysical Journal|language=en|volume=594|issue=1|pages=186|doi=10.1086/375528|issn=0004-637X|arxiv = astro-ph/0209479 |bibcode = 2003ApJ...594..186B |s2cid=67803622 }}</ref>

As described in previous sections, galaxies tend to evolve from spiral to elliptical structure via mergers. However, the current rate of galaxy mergers does not explain how all galaxies move from the "blue cloud" to the "red sequence". It also does not explain how star formation ceases in galaxies. Theories of galaxy evolution must therefore be able to explain how star formation turns off in galaxies. This phenomenon is called galaxy "quenching".<ref>{{Cite journal|last1=Faber|first1=S. M.|last2=Willmer|first2=C. N. A.|last3=Wolf|first3=C.|last4=Koo|first4=D. C.|last5=Weiner|first5=B. J.|last6=Newman|first6=J. A.|last7=Im|first7=M.|last8=Coil|first8=A. L.|author8-link=Alison Coil|last9=C. Conroy|date=2007-01-01|title=Galaxy Luminosity Functions to z 1 from DEEP2 and COMBO-17: Implications for Red Galaxy Formation|journal=The Astrophysical Journal|language=en|volume=665|issue=1|pages=265–294|doi=10.1086/519294|issn=0004-637X|arxiv = astro-ph/0506044 |bibcode = 2007ApJ...665..265F |s2cid=15750425 }}</ref>

[[Star formation|Stars form]] out of cold gas (see also the [[Kennicutt–Schmidt law]]), so a galaxy is quenched when it has no more cold gas. However, it is thought that quenching occurs relatively quickly (within 1 billion years), which is much shorter than the time it would take for a galaxy to simply use up its reservoir of cold gas.<ref>{{Cite journal|last=Blanton|first=Michael R.|date=2006-01-01|title=Galaxies in SDSS and DEEP2: A Quiet Life on the Blue Sequence?|journal=The Astrophysical Journal|language=en|volume=648|issue=1|pages=268–280|doi=10.1086/505628|issn=0004-637X|arxiv = astro-ph/0512127 |bibcode = 2006ApJ...648..268B |s2cid=119426210 }}</ref><ref name=":0">{{Cite journal|last1=Gabor|first1=J. M.|last2=Davé|first2=R.|last3=Finlator|first3=K.|last4=Oppenheimer|first4=B. D.|date=2010-09-11|title=How is star formation quenched in massive galaxies?|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=407|issue=2|pages=749–771|doi=10.1111/j.1365-2966.2010.16961.x|doi-access=free |issn=0035-8711|arxiv = 1001.1734 |bibcode = 2010MNRAS.407..749G |s2cid=85462129 }}</ref> Galaxy evolution models explain this by hypothesizing other physical mechanisms that remove or shut off the supply of cold gas in a galaxy. These mechanisms can be broadly classified into two categories: (1) preventive feedback mechanisms that stop cold gas from entering a galaxy or stop it from producing stars, and (2) ejective feedback mechanisms that remove gas so that it cannot form stars.<ref>{{Cite journal|last1=Kereš|first1=Dušan|last2=Katz|first2=Neal|last3=Davé|first3=Romeel|last4=Fardal|first4=Mark|last5=Weinberg|first5=David H.|date=2009-07-11|title=Galaxies in a simulated ΛCDM universe – II. Observable properties and constraints on feedback|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=396|issue=4|pages=2332–2344|doi=10.1111/j.1365-2966.2009.14924.x|doi-access=free |issn=0035-8711|arxiv = 0901.1880 |bibcode = 2009MNRAS.396.2332K |s2cid=4500254 }}</ref>

One theorized preventive mechanism called “strangulation” keeps cold gas from entering the galaxy. Strangulation is likely the main mechanism for quenching star formation in nearby low-mass galaxies.<ref>{{Cite journal|last1=Peng|first1=Y.|last2=Maiolino|first2=R.|last3=Cochrane|first3=R.|title=Strangulation as the primary mechanism for shutting down star formation in galaxies|journal=Nature|volume=521|issue=7551|pages=192–195|doi=10.1038/nature14439|arxiv = 1505.03143 |bibcode = 2015Natur.521..192P|pmid=25971510|year=2015|s2cid=205243674 }}</ref> The exact physical explanation for strangulation is still unknown, but it may have to do with a galaxy's interactions with other galaxies. As a galaxy falls into a galaxy cluster, gravitational interactions with other galaxies can strangle it by preventing it from accreting more gas.<ref>{{Cite journal|last1=Bianconi|first1=Matteo|last2=Marleau|first2=Francine R.|last3=Fadda|first3=Dario|title=Star formation and black hole accretion activity in rich local clusters of galaxies|journal=Astronomy & Astrophysics|volume=588|doi=10.1051/0004-6361/201527116|arxiv = 1601.06080 |bibcode = 2016A&A...588A.105B|page=A105|year=2016|s2cid=56310943 }}</ref> For galaxies with massive [[dark matter halo]]s, another preventive mechanism called “virial [[Shock wave|shock]] heating” may also prevent gas from becoming cool enough to form stars.<ref name=":0" />

Ejective processes, which expel cold gas from galaxies, may explain how more massive galaxies are quenched.<ref>{{Cite journal|last1=Kereš|first1=Dušan|last2=Katz|first2=Neal|last3=Fardal|first3=Mark|last4=Davé|first4=Romeel|last5=Weinberg|first5=David H.|date=2009-05-01|title=Galaxies in a simulated ΛCDM Universe – I. Cold mode and hot cores|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=395|issue=1|pages=160–179|doi=10.1111/j.1365-2966.2009.14541.x|doi-access=free |issn=0035-8711|arxiv = 0809.1430 |bibcode = 2009MNRAS.395..160K |s2cid=15020915 }}</ref> One ejective mechanism is caused by supermassive black holes found in the centers of galaxies. Simulations have shown that gas accreting onto supermassive black holes in galactic centers produces high-energy [[astrophysical jet|jet]]s; the released energy can expel enough cold gas to quench star formation.<ref>{{Cite journal|last1=Di Matteo|first1=Tiziana|author1-link=Tiziana Di Matteo (astrophysicist)|last2=Springel|first2=Volker|last3=Hernquist|first3=Lars|title=Energy input from quasars regulates the growth and activity of black holes and their host galaxies|journal=Nature|volume=433|issue=7026|pages=604–607|doi=10.1038/nature03335|arxiv = astro-ph/0502199 |bibcode = 2005Natur.433..604D|pmid=15703739|year=2005|s2cid=3007350 |url=https://cds.cern.ch/record/821559|type=Submitted manuscript}}</ref>

Our own Milky Way and the nearby Andromeda Galaxy currently appear to be undergoing the quenching transition from star-forming blue galaxies to passive red galaxies.<ref>{{Cite journal|last1=Mutch|first1=Simon J.|last2=Croton|first2=Darren J.|last3=Poole|first3=Gregory B.|date=2011-01-01|title=The Mid-life Crisis of the Milky Way and M31|journal=The Astrophysical Journal|language=en|volume=736|issue=2|pages=84|doi=10.1088/0004-637X/736/2/84|issn=0004-637X|arxiv = 1105.2564 |bibcode = 2011ApJ...736...84M |s2cid=119280671 }}</ref>