Editing Protogalaxy

{{Short description|Gas cloud forming into a galaxy}}
{{for|the 2010 action-adventure video game|ProtoGalaxy}}

In [[physical cosmology]], a '''protogalaxy''', which could also be called a "'''primeval galaxy'''", is a cloud of [[gas]] which is forming into a [[galaxy]]. It is believed that the rate of [[star formation]] during this period of [[galaxy formation and evolution|galactic evolution]] will determine whether a galaxy is a [[spiral galaxy|spiral]] or [[elliptical galaxy]]; a slower star formation tends to produce a spiral galaxy. The smaller clumps of gas in a protogalaxy form into [[star]]s.

The term "protogalaxy" itself is generally accepted to mean "Progenitors of the present day (normal) galaxies, in the early stages of formation." However, the "early stages of formation" is not a clearly defined phrase. It could be defined as: "The first major burst of [[star formation]] in a progenitor of a present day elliptical galaxy"; "The peak merging [[Epoch (astronomy)|epoch]] of [[dark halo]]s of the fragments which assemble to produce an average galaxy today"; "A still gaseous body before any star formation has taken place."; or " an over-dense region of [[dark matter]] in the very [[early universe]], destined to become [[gravitationally bound]] and to collapse."<ref name=EAAV3>{{cite book|last1=Djorgovski|first1=S.G.|title=Encyclopedia of Astronomy and Astrophysics|volume=3|date=2001|publisher=Institute of Physics Publishing, Nature Publishing Group|location=Dirac House, Temple Back, Bristol|isbn=978-0-333-75088-9|pages=2159–2165|edition=1st}}</ref>

==Formation==

It is thought that the early universe began with a nearly uniform distribution (each particle an equal distance from the next) of matter and dark matter. The dark matter then began to clump together under [[gravitational attraction]] due to the [[Structure formation#Cosmological perturbations|initial density perturbation spectrum]] caused by [[quantum fluctuation]]s.<ref name="EAAV3" /> This derives from [[uncertainty principle|Heisenberg's uncertainty principle]] which shows that there can be tiny temporary changes in the amount of energy in empty space.{{citation needed|date=May 2019}} Particle/[[antiparticle]] pairs can form from this energy through [[mass–energy equivalence]], and gravitational pull causes other nearby particles to move towards it, disturbing the even distribution and creating a centre of gravity, pulling nearby particles closer. When this happens at the universe's present size it is negligible, but the state of these tiny fluctuations as the universe began expanding from a single point left an impression which scaled up as the universe expanded, resulting in large areas of increased density. The gravity of these denser clumps of dark matter then caused nearby matter to start falling into the denser region.<ref name=HotU>{{cite book|last1=Seagrave|first1=Wyken|title=History of the Universe|date=2012|publisher=Penny Press|url=http://historyoftheuniverse.com/index.php?p=firstStars.htm|access-date=18 July 2014|archive-url=https://web.archive.org/web/20140728163133/http://historyoftheuniverse.com/index.php?p=firstStars.htm|archive-date=28 July 2014|url-status=dead}}</ref> This sort of process was reportedly observed and analysed by Nilsson et al. in 2006.<ref name=CAoDMH>{{cite journal|last1=Nilsson|first1=K.K.|display-authors=etal|title=A Lyman-α blob in the GOODS South field: evidence for cold accretion onto a dark matter halo|journal=Astronomy and Astrophysics|date=June 2006|volume=452|issue=3|pages=L23–L26|doi=10.1051/0004-6361:200600025|arxiv = astro-ph/0512396 |bibcode = 2006A&A...452L..23N |s2cid=14837456}}</ref><ref name=RBU>{{cite web|title=Rare Blob Unveiled: Evidence For Hydrogen Gas Falling Onto A Dark Matter Clump?|url=https://www.sciencedaily.com/releases/2006/07/060703163148.htm|website=ScienceDaily.com|access-date=22 July 2014}}</ref> This resulted in the formation of clouds of gas, predominantly [[hydrogen]], and the first stars began to form within these clouds. These clouds of gas and early stars, many times smaller than our galaxy, were the first protogalaxies.<ref name=PGTCDM>{{cite web|last1=Johnston|first1=Hamish|title=Proto-galaxies tip cold dark matter|url=http://physicsworld.com/cws/article/news/2007/nov/28/proto-galaxies-tip-cold-dark-matter|website=Physicsworld.com|access-date=18 July 2014|date=2007-11-28}}</ref>

[[File:Stellar Fireworks Finale.jpg|thumbnail|An illustration showing protogalaxies colliding]]
The established theory is that the groups of small protogalaxies were attracted together by gravity and collided, which resulted in the formation of the much larger "adult" galaxies we have today.<ref name=PGTCDM/> This follows the process of hierarchical assembly, which is an ongoing process where larger bodies are continually formed from the merging of smaller ones.<ref name=EAAV3/><ref>{{cite book|last1=Freeman|first1=K|last2=Larson|first2=R.C|last3=Tinsley|first3=B|title=Galaxies: Sixth Advanced Course of the Swiss Society of Astronomy and Astrophysics|date=1976|publisher=[[Geneva Observatory]]|location=Sauverny, Switzerland|pages=75–82}}</ref>

==Properties==

===Composition===
Since there had been no previous [[star formation]] to create other elements, protogalaxies would have been made up almost entirely of hydrogen and helium. The hydrogen would bond to form H<sub>2</sub> molecules, with some exceptions.<ref name=TSNtDaPG>{{cite journal|last1=Whalen|first1=Daniel|title=The Supernova That Destroyed a Protogalaxy: Prompt Chemical Enrichment and Supermassive Black Hole Growth|display-authors=etal|journal=The Astrophysical Journal|date=16 August 2013|volume=774|issue=1|doi=10.1088/0004-637X/774/1/64|arxiv = 1305.6966 |bibcode = 2013ApJ...774...64W|pages=64|s2cid=59289675}}</ref> This would change as star formation began and produced more elements through the process of [[nuclear fusion]].

===Mechanics===
Once a protogalaxy begins to form, all particles bound by its gravity begin to [[free fall]] towards it. The time taken for this free-fall to conclude can be approximated using the [[Free-fall time#infall to a point source of gravity|free-fall equations]]. Most galaxies have completed this free-fall stage to become stable elliptical or disk galaxies, the disks taking longer to fully form. The formation of galaxy clusters takes much longer and is still in progress now.<ref name="EAAV3" /> This stage is also where galaxies acquire most of their [[angular momentum]]. A protogalaxy acquires this due to gravitational influence from neighbouring dense clumps in the early universe, and the further the gas is away from the centre, the more spin it gets.<ref name=EPiG>{{cite book|last1=Gilmore|first1=Gerard|last2=Wyse|first2=Rosemary F.G.|last3=Kuijken|first3=Konrad|title=Evolutionary Phenomena in Galaxies|date=1989|publisher=[[Cambridge University Press]]|location=Cambridge, UK|isbn=0-521-37193-7|page=194|edition=1st}}</ref>

===Luminosity===
The luminosity of protogalaxies comes from two sources. First and foremost is the [[radiation]] from nuclear fusion of Hydrogen into [[helium]] in early stars. This early burst of star formation is thought to have made a protogalaxy's luminosity comparable to a present-day [[starburst galaxy]] or a [[quasar]]. The other is the release of excess [[gravitational binding energy]].<ref name="EAAV3" />
The primary wavelength expected from a protogalaxy is a variety of [[Ultraviolet|UV]] called [[Lyman-alpha]], which is the wavelength emitted by Hydrogen gas when it is [[ionised]] by radiation from a star.<ref name="EAAV3" /><ref name=PGTCDM/>

==Detection==
Protogalaxies can theoretically still be seen today, as the light from the farthest reaches of the universe takes a very long time to reach Earth, in some places long enough that we see them at the stage where they are populated by protogalaxies.
There have been many attempts to find protogalaxies with telescopes over the last 30 years because of the value of such a discovery in confirming how galaxies form, but the sheer distance any light would have to travel for it to be old enough to come from a protogalaxy is very large. This, coupled with the fact that the Lyman-alpha wavelength is quite readily absorbed by dust, made some astronomers think protogalaxies may be too faint to detect.<ref>{{cite web|last1=Bothun|first1=Gregory D|title=Protogalaxies|url=http://ned.ipac.caltech.edu/level5/ESSAYS/Bothun/bothun.html|website=Caltech.edu|access-date=18 July 2014}}</ref>

In 1996, a protogalaxy candidate was discovered by Yee et al. using the Canadian Network for Observational Cosmology (CNOC). The object was a disk-like galaxy at high [[redshift]] with a very high luminosity.<ref>{{cite journal|last1=Yee|first1=H.K.C.|display-authors=etal|title=A Proto-Galaxy Candidate at z=2.7 Discovered by its Young Stellar Population|journal=Astronomical Journal|date=May 1996|volume=111|page=1783|doi=10.1086/117916|arxiv = astro-ph/9602121 |bibcode = 1996AJ....111.1783Y |s2cid=1421568}}</ref> It was later debated that the incredible luminosity was caused by the [[gravitational lens]]ing of a foreground [[galactic cluster]].<ref>{{cite journal|last1=Williams|first1=L.L.R.|last2=Lewis|first2=G.F.|title=The giant protogalaxy cB 58: an artefact of gravitational lensing?|journal=Monthly Notices of the Royal Astronomical Society|date=August 1996|volume=281|issue=3|pages=L35–L39|arxiv = astro-ph/9605062 |bibcode = 1996MNRAS.281L..35W|doi=10.1093/mnras/281.3.l35|doi-access=free |s2cid=14392384}}</ref>

In 2006, K. Nilsson et al. reported finding a "blob" emitting Lyman alpha UV radiation. Analysis concluded that this was a giant cloud of hydrogen gas falling onto a clump of dark matter in the early universe, creating a protogalaxy.<ref name="CAoDMH" /><ref name="RBU" />

In 2007, Michael Rauch et al.<ref>{{cite journal|last1=Rauch|first1=Michael|title=A Population of Faint Extended Line Emitters and the Host Galaxies of Optically Thick QSO Absorption Systems|journal=The Astrophysical Journal|date=July 2008|volume=681|issue=2|pages=856–880|arxiv = 0711.1354 |bibcode = 2008ApJ...681..856R |doi = 10.1086/525846 |s2cid=16974679}}</ref> were using the [[Very Large Telescope|VLT]] to search for a signal from intergalactic gas, when they spotted dozens of discrete objects emitting large amounts of the Lyman-alpha type UV radiation. They concluded that these 27 objects were examples of protogalaxies from 11 billion years ago.<ref name=PGTCDM/>

==See also==
* [[Big Bang]]
* [[Galaxy formation and evolution]]
* [[GOODS-N-774]]

==References==
{{Reflist}}

==External links==
{{Wiktionary|protogalaxy}}
*[https://web.archive.org/web/20160304123604/http://universe-review.ca/F05-galaxy07.htm Universe-Review:Formation and Evolution of Galaxy]
{{Galaxy}}

[[Category:Protogalaxies]]
[[Category:Physical cosmology]]