Defining the limits of galaxy clusters is imprecise as many clusters are still forming. In particular, clusters close to the Milky Way tend to be classified as galaxy clusters even when they are much smaller than more distant clusters.
In this collision between two clusters of galaxies, the stars pass between each other unhindered, while the hot, diffuse gas experiences friction and is left behind between the clusters. The gas dominates the visible mass budget of the clusters, being several times more massive than all the stars. Yet the regions with the stars show more gravitational lensing than the gas region, indicating that they are more massive than the gas. Some dark (since we don't see it), collision-less (or it would have been slowed, like the gas) matter is inferred to be present to account for the extra lensing around otherwise low-mass regions.[2]
This is actually a collision between two galaxy clusters. The galaxies and the dark matter seems to have separated out into separate dark and light cores.[3]
This is a recently coalesced merger of galaxy clusters, which has resulted in a ring of dark matter around the galaxies, yet to be redistributed.[4][5]
Named groups and clusters[]
This is a list of galaxy groups and clusters that are well known by something other than an entry in a catalog or list, or a set of coordinates, or a systematic designation.
Named for its size, El Gordo ("the fat one") is the biggest cluster found in the distant universe (at its distance and beyond), at the time of discovery in 2011, with a mass of 3 quadrillion suns. The second most massive galaxy cluster next to El Gordo is RCS2 J2327, a galaxy cluster with the mass of 2 quadrillion suns.
Also has a systematic designation of ACT-CL J0102-4915.[6][7][8]
Named in comparison with the Bullet Cluster, being of similar formation, except smaller.
Also has a systematic catalogue name . As of 2014, it was the lowest mass object that showed separation between the concentrations of dark matter and baryonic matter in the object.[11][12]
Coined by (author of "1000+ The Amateur Astronomers' Field Guide to Deep Sky Observing") to honor Deer Lick Gap in the mountains of North Carolina, from which he had especially fine views of the galaxy group.
Also referred to as the NGC 7331 Group, after the brightest member of the group.[13]
Named after its discoverer, Carl Seyfert. At the time it appeared to contain six external nebulae. It is also called the , after its brightest member.
There are actually only five galaxies in the sextet, and only four galaxies in the compact group. One of the galaxies is an ungravitationally bound background object. The other "galaxy" is instead an extension of the interacting system — a tidal stream caused by the merger. The group is, therefore, more properly called ; the name refers to the visual collection and not the group. HCG 79 lies 190 million light-years away in the Serpens Caput constellation.
There are actually only four galaxies in the compact group, the other galaxy is a foreground galaxy. The group is therefore more properly called , because the name refers to a visual collection and not a group. Thus, the real group is also called Stephan's Quartet.
The major nearby groups and clusters are generally named after the constellation they lie in. Many groups are named after the leading galaxy in the group. This represents an ad hoc systematic naming system.
Groups and clusters visible to the unaided eye[]
The Local Group contains the largest number of visible galaxies with the naked eye. However, its galaxies are not visually grouped together in the sky, except for the two Magellanic Clouds. The IC342/Maffei Group, the nearest galaxy group, would be visible by the naked eye if it were not obscured by the stars and dust clouds in the Milky Way's spiral arms.
The Centaurus A galaxy has been spotted with the naked eye by Stephen James O'Meara [16][17] and M83 has also reportedly been seen with the naked eye.[18]
Mly represents millions of light-years, a measure of distance.
Mpc represents millions of parsecs, a measure of distance (1 Mpc = 3.26 Mly).
z represents redshift, a measure of recessional velocity and inferred distance due to cosmological expansion. In this very nearby context, however, the observed redshift and recessional velocity are due to the Doppler shifting of the light.
Distances are measured from Earth, with Earth being at zero.
At the time, the BCG, was the most distant non-quasar galaxy.[47]
1960–1975
0.461
The measurement of 3C295's redshift in 1960 also defined its cluster's position. 3C 295 was also the most distant galaxy of the time.[48][49]
(fainter Hydra Cluster )
1951–1960
0.2
61 000
Attempts at measuring the redshift of the brightest cluster galaxy of this Hydra Cluster had been attempted for years before it had been successfully achieved. The BCG was also the most distant galaxy of the time.[48][50][51][52]
()
1936–1951
0.13
39 000
The BCG of this cluster was also the most distant galaxy of the time.[51][53]
()
1932 − 1936
0.075
23 000
The BCG of this cluster was the most distant galaxy at the time.[53][54]
1931–1932
19 700
The BCG of this cluster was the most distant galaxy known at the time.[51][54][55][56][57]
1930–1931
11 700
The BCG of this cluster was the highest redshift galaxy of the time.[57][58]
This cluster's distance was determined by one of the NGC objects lying in it, .[58][59]
(, )
1929
0.012
3 779
The BCG for this group was used to measure its redshift. Shortly after this was publicized, it was accepted that redshifts were an acceptable measure of inferred distance.[60]
This was the first noted cluster of "nebulae" that would become galaxies. The first redshifts to galaxies in the cluster were measured in the 1910s. Galaxies were not identified as such until the 1920s. The distance to the Virgo Cluster would have to wait until the 1930s.[21]
Mly represents millions of light-years, a measure of distance.
Mpc represents millions of parsecs, a measure of distance.
z represents redshift, a measure of recessional velocity and inferred distance due to cosmological expansion.
Distances are measured from Earth, with Earth being at zero.
In 2003 () at z=1.237, was found to be the most distant rich cluster, which lasted until 2005.[38][40][42]
In 2000, a cluster was announced in the field of quasar at z=1.31 (the quasar lies at z=2.69) [64]
In 1999, cluster (, ) was found at z=1.261 [43][46]
In 1995 and 2001, the cluster around was announced, at z=1.786 [65]
In 1992, observations of the field of cluster found what appears to be a background cluster near a quasar, also in the background. The quasar was measured at z=2.055 and it was assumed that the cluster would be as well.[66][67][68][69]
In 1975, and its galaxy cluster was incorrectly determined to lie at z=0.637 (actually z=0.218) [70][71]
In 1958, cluster and were estimated to have redshifts of z=0.29 and z=0.35 respectively. However, they were not spectroscopically determined.[48]
Farthest protoclusters[]
5 Farthest protoclusters
Galaxy protocluster
Distance
Notes
No entries yet
Mly represents millions of light-years, a measure of distance.
Mpc represents millions of parsecs, a measure of distance.
z represents redshift, a measure of recessional velocity and inferred distance due to cosmological expansion.
Distances are measured from Earth, with Earth being at zero.
z represents redshift, a measure of recessional velocity and inferred distance due to cosmological expansion.
Distances are measured from Earth, with Earth being at zero.
In 2002, a very large, very rich protocluster, or the most distant protosupercluster was found in the field of galaxy cluster , around the gravitationally lensed galaxy MS 1512-cB58, at z=2.724 [77][79]
False clusters[]
Sometimes clusters are put forward that are not genuine clusters or superclusters. Through the researching of member positions, distances, peculiar velocities, and binding mass, former clusters are sometimes found to be the product of a chance line-of-sight superposition.
The Cancer Cluster was found to be a random assortment of galaxy groups, and not a true cluster.[21]
The early identification of the Coma-Virgo Cloud of Nebulae was actually a mistaken identification due to the superposition of the Virgo Supercluster and Coma Supercluster, and not a
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