Editing Galaxy (section)

=== Ellipticals ===
{{Main|Elliptical galaxy}}

The Hubble classification system rates elliptical galaxies on the basis of their ellipticity, ranging from E0, being nearly spherical, up to E7, which is highly elongated. These galaxies have an [[ellipsoid]]al profile, giving them an elliptical appearance regardless of the viewing angle. Their appearance shows little structure and they typically have relatively little [[interstellar medium|interstellar matter]]. Consequently, these galaxies also have a low portion of [[open cluster]]s and a reduced rate of new star formation. Instead, they are dominated by generally older, more [[stellar evolution|evolved stars]] that are orbiting the common center of gravity in random directions. The stars contain low abundances of heavy elements because star formation ceases after the initial burst. In this sense they have some similarity to the much smaller [[globular cluster]]s.<ref name="elliptical">{{cite web
 |last1=Barstow |first1=M. A.
 |date=2005
 |title=Elliptical Galaxies
 |url=http://www.star.le.ac.uk/edu/Elliptical.shtml
 |archive-url=https://web.archive.org/web/20120729081504/http://www.star.le.ac.uk/edu/Elliptical.shtml
 |archive-date=2012-07-29
 |publisher=[[Leicester University]] Physics Department
 |access-date=June 8, 2006
}}</ref>

====Type-cD galaxies====

[[Image:Galaxy Cluster Abell 1413.jpg|thumb|right|300px|The galaxy cluster [[Abell 1413]] is dominated by this cD elliptical galaxy designated Abell 1413 BCG. It has an isophotal diameter of over 800,000 light-years across. Note the [[gravitational lens]]ing.]]

The [[List of largest galaxies|largest galaxies]] are the [[Type-cD galaxy|type-cD galaxies]].
First described in 1964 by a paper by Thomas A. Matthews and others,<ref name=Matthews>{{cite journal | url=https://ui.adsabs.harvard.edu/abs/1964ApJ...140...35M/abstract | bibcode=1964ApJ...140...35M | title=A Discussion of Galaxies Indentified &#91;sic&#93; with Radio Sources | last1=Matthews | first1=Thomas A. | last2=Morgan | first2=William W. | last3=Schmidt | first3=Maarten | journal=[[The Astrophysical Journal]] | year=1964 | volume=140 | page=35 | doi=10.1086/147890 }}</ref> they are a subtype of the more general class of D galaxies, which are giant elliptical galaxies, except that they are much larger. They are popularly known as the '''supergiant elliptical galaxies''' and constitute the largest and most luminous galaxies known. These galaxies feature a central elliptical nucleus with an extensive, faint halo of stars extending to megaparsec scales.<ref name="NASAVlog">{{cite web | url=https://www.nasa.gov/content/discoveries-highlights-tracing-the-growth-of-galaxies |publisher=[[NASA]]|title=Tracing the Growth of Galaxies |access-date=April 20, 2024| date=6 February 2017 }}</ref> The profile of their surface brightnesses as a function of their radius (or distance from their cores) falls off more slowly than their smaller counterparts.<ref name=Tonry>{{cite book | chapter-url=https://ui.adsabs.harvard.edu/abs/1987IAUS..127...89T/abstract | bibcode=1987IAUS..127...89T | last1=Tonry | first1=John L. | title=Structure and Dynamics of Elliptical Galaxies | chapter=Properties of cD Galaxies | year=1987 | volume=127 | page=89 | doi=10.1007/978-94-009-3971-4_7 | isbn=978-90-277-2586-8 | s2cid=117980521 }}</ref>

The formation of these cD galaxies remains an active area of research, but the leading model is that they are the result of the mergers of smaller galaxies in the environments of dense clusters, or even those outside of clusters with random overdensities.<ref>{{Cite journal |last1=Tovmassian |first1=Hrant M. |last2=Andernach |first2=Heinz |date=2012-12-11 |title=On the formation of cD galaxies and their parent clusters: Formation of cD galaxies and parent clusters |journal=[[Monthly Notices of the Royal Astronomical Society]] |language=en |volume=427 |issue=3 |pages=2047–2056 |doi=10.1111/j.1365-2966.2012.22044.x|doi-access=free |arxiv=1212.0238 }}</ref> These processes are the mechanisms that drive the formation of fossil groups or fossil clusters, where a large, relatively isolated, supergiant elliptical resides in the middle of the cluster and are surrounded by an extensive cloud of X-rays as the residue of these galactic collisions. Another older model posits the phenomenon of [[cooling flow]], where the heated gases in clusters collapses towards their centers as they cool, forming stars in the process,<ref>{{Cite journal |last1=Bender |first1=Ralf |last2=Kormendy |first2=John |last3=Cornell |first3=Mark E. |last4=Fisher |first4=David B. |date=2015-06-30 |title=Structure and Formation of cD Galaxies: NGC 6166 in Abell 2199 |url=https://iopscience.iop.org/article/10.1088/0004-637X/807/1/56 |journal=[[The Astrophysical Journal]] |volume=807 |issue=1 |pages=56 |doi=10.1088/0004-637X/807/1/56 |arxiv=1411.2598 |bibcode=2015ApJ...807...56B |hdl=1959.3/404318 |issn=1538-4357 |quote=This idea was entertained in the heyday of the cooling-flow problem, when we observed large amounts of X-ray-emitting, hot gas in clusters but could not measure temperature profiles. [...] This possibility is now regarded as a non-starter.}}</ref> a phenomenon observed in clusters such as [[Perseus Cluster|Perseus]],<ref name=Fabian>{{cite journal | bibcode=1977MNRAS.180..479F | title=Subsonic accretion of cooling gas in clusters of galaxies | last1=Fabian | first1=A. C. | last2=Nulsen | first2=P. E. J. | journal=[[Monthly Notices of the Royal Astronomical Society]] | year=1977 | volume=180 | issue=3 | page=479 | doi=10.1093/mnras/180.3.479 | doi-access=free }}</ref> and more recently in the [[Phoenix Cluster]].<ref>{{Cite journal |last1=McDonald |first1=M. |last2=McNamara |first2=B. R. |last3=Voit |first3=G. M. |last4=Bayliss |first4=M. |last5=Benson |first5=B. A. |last6=Brodwin |first6=M. |last7=Canning |first7=R. E. A. |last8=Florian |first8=M. K. |last9=Garmire |first9=G. P. |last10=Gaspari |first10=M. |last11=Gladders |first11=M. D. |last12=Hlavacek-Larrondo |first12=J. |last13=Kara |first13=E. |last14=Reichardt |first14=C. L. |last15=Russell |first15=H. R. |date=2019-11-01 |title=Anatomy of a Cooling Flow: The Feedback Response to Pure Cooling in the Core of the Phoenix Cluster |journal=[[The Astrophysical Journal]] |volume=885 |issue=1 |pages=63 |doi=10.3847/1538-4357/ab464c |doi-access=free |arxiv=1904.08942 |bibcode=2019ApJ...885...63M |issn=0004-637X}}</ref>

==== Shell galaxy ====
[[File:NGC 3923 Elliptical Shell Galaxy.jpg|thumb|[[NGC 3923]] Elliptical Shell Galaxy (Hubble photograph)]]

A shell galaxy is a type of elliptical galaxy where the stars in its halo are arranged in concentric shells. About one-tenth of elliptical galaxies have a shell-like structure, which has never been observed in spiral galaxies. These structures are thought to develop when a larger galaxy absorbs a smaller companion galaxy—that as the two galaxy centers approach, they start to oscillate around a center point, and the oscillation creates gravitational ripples forming the shells of stars, similar to ripples spreading on water. For example, galaxy [[NGC 3923]] has over 20 shells.<ref>{{cite web|title = Galactic onion|url = http://www.spacetelescope.org/images/potw1519a/|publisher = [[European Space Agency]]|access-date = 2015-05-11|archive-date = August 6, 2020|archive-url = https://web.archive.org/web/20200806221639/https://www.spacetelescope.org/images/potw1519a/|url-status = live}}</ref>