Coma Cluster / Supercluster

Sir William Friedrich Wilhelm Herschel (1738-1822, portrait) was the first astronomer to notice the concentration of " fine nebulae" in the constellation of Coma Berenices, near the north galactic pole (William Herschel, "On the Construction of the Heavens," in Philosophical Trans., Royal Society, London, 75, 213, 1785). Now sometimes called the Coma Cluster of galaxies (Abell/ACO/A 1656) or Supercluster with the addition of Leo Cluster or A 1367, this large-scaled structure of the local universe is located around 320 to 370 million light-years (ly) away from Sol. Because Coma lies well off the plane of the Milky Way (unlike the Perseus Cluster, or the A3627 cluster), it is largely unobscured by any gas and dust or any foreground stars. Located more than five times farther away from Sol than the prominent Virgo Cluster of galaxies, astronomers have estimated that more than 3,000 galaxies within a diameter of 20 million ly are part of the Coma Supercluster, making it very rich and dense. Nearly spherical, Coma is composed of mostly elliptical and lenticular galaxies within a one-megaparsec diameter, with some spiral galaxies towards the outer reaches. Most of these galaxies have been estimated to be as old as the Cosmos, as much as 13+ billion years old.

? © Renée C. Kraan-Korteweg and
Ofer Lahav, Scientific American
(October 1998): pp. 50-57

Larger map.

The Coma Cluster (or Supercluster)
is located near the Milky Way's
north pole, and so is relatively
easy to observe, unlike the Great
Attractor which is obscured by
the dust of the Milky Way's disk.

The Coma Cluster is an ideal subject for observation. Less than five percent of Abell clusters contain as many galaxies as Coma does, and none are as close to Sol. Because Coma is not obscured by the dust clouds of the Milky Way, it can be seen without interference. Since the advent of X-ray imaging, several phenomena have been observed in galaxy clusters and continue to be studied in Coma and elsewhere. Astronomers have used the estimated distance of Sol to the Coma Cluster as an important "cosmic yardstick" for scaling the overall size of the universe (more discussion at HubbleSite.org).

Steve Snowden, ROSAT, MPE




Larger x-ray image.




Coma's larger, brighter central cluster
appears to be merging with a fainter
group of galaxies to the lower right
(more from CXC and ROSAT).

Coma is now thought to be a galactic supercluster in the making, with several subunits in the throes of merging. Astronomers have observed a plume of stars about 500,000 ly (130,000 parsecs or pc) long in the heart of the central cluster -- around four times the diameter of the Milky Way. Its total luminosity is less than that of the Milky Way, the equivalent of about three Large Magellanic Clouds. The plume and other objects (including a patch of stars following in the wake of spiral galaxy NGC 4911 as it plunges into the core of the cluster at more than 1,000 km/s and a fourth smaller arc may be tidal debris left galaxy-galaxy or galaxy-cluster interactions within the past several hundred million years. Over time, the plume should fade into the background sea of stars that permeates the Coma Cluster (more discussion and images).

Deiss et al, 1997;
Effelsberg Telescope




Larger radio image.





Coma's core cluster of galaxies
and a neighboring cluster subunit
emit diffuse radio haloes
(Deiss et al, 1997).

One or two immense cD galaxies lies near Coma's center. These elliptical galaxies have a very extended stellar envelope of hydrogen, and are very massive compared to normal ellipticals and spirals. Frequently found with multiple nuclei, the diameter of cD galaxies typically ranges around 20 times larger than those of spirals or giant ellipticals (up to 65 million ly or 2 million parsecs across), and their envelopes may be the accumulated debris of tidal interactions. Commonly located in the core of rich galaxy clusters, such galaxies appear to have grown by the tidal capture of smaller neighbors.

XMM-Newton, ESA



Larger x-ray image.



X-ray emission clearly visible from the two
very bright giant elliptical galaxies, NGC 4889
and NGC 4874, lying either side of the heart of
the cluster, and other individual sources are
seen in Coma's outlying regions (more from
XMM-Newton).

Among dwarf galaxies, dwarf ellipticals follow the same distribution of giant ellipticals in Coma. However, dwarf spheroidals are lacking in the core, and this absence has been interpreted by some astronomers as evidence for tidal disruption of such spheroidal galaxies in Coma core. Some astronomers also find that faint galaxies describe a more regular cluster than do bright galaxies (for which the effect of subclustering is stronger), and that finding has been interpreted as evidence of the ongoing accretion of groups of galaxies onto the cluster. On May 28, 2007, however, astronomers announced the discovery of thousands of previously undetected dwarf galaxies in two small regions within the core of the Coma Cluster using the infrared Spitzer Space Telescope (see Spitzer press release; Jenkins et al, 2007; and APOD).

What distinguishes Coma from most other clusters of galaxies is the scarcity of spirals. Some peculiar spirals do belong to Coma, and that at least 16 of the spectroscopically confirmed members of Coma were spirals or irregulars. Three spirals in Coma have a surprisingly low HI abundance for their luminosity, when compared to similar spirals in the field (i.e. they were "HI-deficient"). Hence, these spirals may have passed through Coma and have been stripped of part of their gas. Subsequent studies showed that the HI-deficiency mostly concerns spirals in the core of Coma, and not spirals in the Coma supercluster. This definitely proved the existence of a population of cluster spirals (note however that Coma spirals are not H2-deficient).

© Anthony P. Fairall,
An Atlas of Nearby Large-scale
Structures, from Large-scale
Structures in the Universe
(Used with permission)




Larger map.




The Coma Cluster or
Supercluster lies near
the "Coma Wall" of
galaxies (more maps
and discussion on
Large-scale Structures
in the Universe).

As a rich and compact galactic supercluster, Coma shows a strong central condensation and spherical symmetry, unlike the closer Virgo (which includes the Local Group and the Milky Way) and Perseus superclusters. Coma's visible mass, however, has long been known to be insufficent to maintain its strong symmetry. Hence, the cluster's existence provides strong evidence of the existence of unseen, dark matter that is presumed to be providing the gravitational pull for holding the cluster together. (More discussion is available from Andrea Bivano's historical review of the Coma Cluster.

Vikhlinin et al, 2001; CXC, SAO, NASA




Larger x-ray image.




Around 1.5 million light-years across,
Coma's central region around galaxy
NGC 4889 (left) and NGC 4874 contains
a diffuse cloud of hot gas (at 10 to
20 million degrees) that is held by
the cluster's enormous gravity, (more
from CXC).

Like other galaxy clusters, Coma's center consists of a diffuse cloud of hot gas held by the cluster's gravity which was found to emit X-rays in 1971. In a central region around 1.5 million ly across, thousands of galaxies are enveloped by a vast 100 million-degree Celsius gas cloud. Concentrations of cooler (10 to 20 million degrees) gas have been found around the cluster's large galaxies of NGC 4889 and NGC 4874, which are 10,000 light years in diameter and may be produced by matter ejected from stars in the galaxies over a period of about a billion years. The relatively cool gas clouds may in a delicate balance whereby the energy lost by X-ray radiation ("radiative cooling") is closely balanced by energy gained by heat conduction from the hot gas of the cluster. Because of the large difference in temperature between the hot gas and the much cooler clouds, this balance would require that the heat flow from the hot gas be greatly reduced, perhaps by the magnetic fields in the galaxies that separate the hot and cold components. (More discussion is available from the Chandra X-Ray Observatory and Vikhlinin et al, 2001.)