Editing Galaxy formation and evolution (section)

==Formation of disk galaxies==
The earliest stage in the evolution of galaxies is their formation. When a galaxy forms, it has a disk shape and is called a spiral galaxy due to spiral-like "arm" structures located on the disk. There are different theories on how these disk-like distributions of stars develop from a cloud of matter: however, at present, none of them exactly predicts the results of observation.

=== Top-down theories ===
[[Olin J. Eggen]], [[Donald Lynden-Bell]], and [[Allan Sandage]]<ref>{{cite journal | last1 = Eggen | first1 = O. J. | last2 = Lynden-Bell | first2 = D. | last3 = Sandage | first3 = A. R. | title = Evidence from the motions of old stars that the Galaxy collapsed | bibcode = 1962ApJ...136..748E | date = 1962 | journal = [[The Astrophysical Journal]] | volume = 136 | pages = 748 | doi = 10.1086/147433}}</ref> in 1962, proposed a theory that disk galaxies form through a monolithic collapse of a large gas cloud. The distribution of matter in the early universe was in clumps that consisted mostly of dark matter. These clumps interacted gravitationally, putting tidal torques on each other that acted to give them some angular momentum. As the [[Baryon|baryonic matter]] cooled, it dissipated some energy and contracted toward the center. With angular momentum conserved, the matter near the center speeds up its rotation. Then, like a spinning ball of pizza dough, the matter forms into a tight disk. Once the disk cools, the gas is not gravitationally stable, so it cannot remain a singular homogeneous cloud. It breaks, and these smaller clouds of gas form stars. Since the dark matter does not dissipate as it only interacts gravitationally, it remains distributed outside the disk in what is known as the [[Dark matter halo|dark halo]]. Observations show that there are stars located outside the disk, which does not quite fit the "pizza dough" model. It was first proposed by [[Leonard Searle]] and Robert Zinn <ref>{{cite journal | last = Searle | first = L. |author2=Zinn, R. | date = 1978 | journal = [[The Astrophysical Journal]] | title = Compositions of halo clusters and the formation of the galactic halo | bibcode = 1978ApJ...225..357S | volume = 225 | pages = 357–379 | doi = 10.1086/156499| doi-access = free }}</ref> that galaxies form by the coalescence of smaller progenitors. Known as a top-down formation scenario, this theory is quite simple yet no longer widely accepted.

=== Bottom-up theory ===
More recent theories include the clustering of dark matter halos in the bottom-up process. Instead of large gas clouds collapsing to form a galaxy in which the gas breaks up into smaller clouds, it is proposed that matter started out in these “smaller” clumps (mass on the order of [[globular cluster]]s), and then many of these clumps merged to form galaxies,<ref>{{Cite journal|last1=White|first1=Simon|last2=Rees|first2=Martin|date=1978|title=Core condensation in heavy halos: a two-stage theory for galaxy formation and clustering.|journal=MNRAS|doi=10.1093/mnras/183.3.341|bibcode = 1978MNRAS.183..341W|volume=183|issue=3|pages=341–358|doi-access=free}}</ref> which then were drawn by gravitation to form [[galaxy cluster]]s. This still results in disk-like distributions of baryonic matter with dark matter forming the halo for all the same reasons as in the top-down theory. Models using this sort of process predict more small galaxies than large ones, which matches observations.

Astronomers do not currently know what process stops the contraction. In fact, theories of disk galaxy formation are not successful at producing the rotation speed and size of disk galaxies. It has been suggested that the radiation from bright newly formed stars, or from an [[active galactic nucleus]] can slow the contraction of a forming disk. It has also been suggested that the [[dark matter]] halo can pull the galaxy, thus stopping disk contraction.<ref>{{Cite book|title=Cosmic Collisions: The Hubble Atlas of Merging Galaxies.|last1=Christensen|first1=L.L.|last2=de Martin|first2=D.|last3=Shida|first3=R.Y.|publisher=Springer|year=2009|isbn=9780387938530}}</ref>

The [[Lambda-CDM model]] is a cosmological model that explains the formation of the universe after the [[Big Bang]]. It is a relatively simple model that predicts many properties observed in the universe, including the relative frequency of different galaxy types; however, it underestimates the number of thin disk galaxies in the universe.<ref>{{Cite journal|last1=Steinmetz|first1=Matthias|last2=Navarro|first2=Julio F.|date=2002-06-01|title=The hierarchical origin of galaxy morphologies|journal=New Astronomy|volume=7|issue=4|pages=155–160|doi=10.1016/S1384-1076(02)00102-1|arxiv = astro-ph/0202466 |bibcode = 2002NewA....7..155S |citeseerx=10.1.1.20.7981|s2cid=14153669 }}</ref> The reason is that these galaxy formation models predict a large number of mergers. If disk galaxies merge with another galaxy of comparable mass (at least 15 percent of its mass) the merger will likely destroy, or at a minimum greatly disrupt the disk, and the resulting galaxy is not expected to be a disk galaxy (see next section). While this remains an unsolved problem for astronomers, it does not necessarily mean that the Lambda-CDM model is completely wrong, but rather that it requires further refinement to accurately reproduce the population of galaxies in the universe.