Groups and Clusters of Galaxies

Our Local Group is just one example of a galaxies in reasonably close proximity. It's pretty common to find galaxies in groups, small and large.

The galaxies M81 and M82 are the dominant members of a small group not far from our Local Group -- about 3.6 Mpc away. Perhaps you'd prefer the big picture .
A small group, Hickson Compact Group 87 as seen by HST

Color pictures can make some galaxy clusters stand out more clearly.

This black-and-white photograph from the Palomar Sky Survey hints at the presence of a cluster in the upper-left corner. But it's a lot more obvious in this color image from the Sloan Digital Sky Survey. And even more clearly in this big version of the SDSS picture.
A rich cluster of galaxies in the constellation Norma lies behind the Milky Way, about 80 Mpc away in the direction of the "Great Attractor". This image from the Wide-Field Imager on the ESO 2.2-m telescope in Chile shows an area half a degree on a side. It's a false-color composite, made by combining images taken through several near-infrared filters. The galaxies are clearly a different color than most of the stars in our Milky Way.

Cluster can look very different in the optical and X-ray regimes.

There's a small group of galaxies around NGC 2300. It looks pretty boring in X-rays -- there's not much emission.
But when we look at very large groups of tens or hundreds of galaxies, we see some curious features: X-rays in great abundance, and galaxies moving very fast.
The Virgo Cluster is the nearest big cluster. If you go outside in the spring, you can see many of its galaxies with a small telescope, in the constellation of Virgo.
The Coma Cluster is a much richer cluster, about six times farther away than the Virgo Cluster.

When we look at very distant clusters, we sometimes see evidence for substructure within a cluster. There are often more interacting galaxies than in nearby clusters, too. We are looking back in time, at a time when the clusters were still forming.

The Hercules Cluster has a recessional velocity of 11,000 km/s. It is noteworthy in that it contains a number of spiral galaxies, many of which are intereacting. This is a sure sign that the cluster still has substructure and is not fully virialized yet. (from the Electronic Universe Project ).
The galaxy cluster EIS 0046-2930 is at a redshit of about z=1; we are seeing it as it appeared several billion years ago.

Dark Matter in Galaxy Clusters

You may recall that the motions of satellites of our Milky Way indicate that our galaxy contains much more matter than we can see directly. The same is true in other groups of galaxies ... in fact, the discrepancy between the visible and invisible matter is even greater.

In the nineteen-thirties, astronomer Fritz Zwicky studied the Coma Cluster of galaxies, which is one of the nearest very rich clusters. The Coma Cluster is about d = 90 Mpc away from us, and contains hundreds of galaxies spread out over a roughly spherical volume of radius R = 3 Mpc. Zwicky measured two properties of the galaxies. First, he added up the visible light in the galaxies, due to stars. He found

   total optical luminosity   L(opt)  =  5 x 10^(12)   solar luminosities

Next, he used a spectrograph to measure the radial velocities of the galaxies: the speeds with which the galaxies were moving towards us, or away from us. He found a typical speed of V = 1000 km/s. If he assumed that the galaxies were moving in circular orbits around the center of the cluster -- which is not the case -- he derived a mass of

                                                        2 
   gravitational mass         M(grav) =  2.3 x 10^5  ( V  * R )
  (assuming circular orbits)
                                      =    7 x 10^(14)  solar masses

The actual orbits of galaxies are closer to radial plunges into the center of the cluster, then almost directly outwards. If one corrects for this non-circular motion, one finds an even greater mass

   gravitational mass         M(grav) =    3 x 10^(15)  solar masses
  (with realistic orbits)

Now, the ratio of these numbers is very big:


   gravitational mass       3 x 10^(15)             solar mass
  --------------------  =  -------------  =  600  ----------------
     optical light          5 x 10^(12)           solar luminosity

That's wierd! If the Coma Cluster galaxies were made entirely of stars like our sun, the ratio would be 1. If they were made of low-mass dwarf stars, the ratio would be around 10. But there are NO ordinary stars which can yield so much mass while producing so much light.

This discrepancy between the mass indicated by the amount of optical light and the mass inferred from gravitational motions was a mystery for decades. Was there some other explanation for the high speeds of galaxies in clusters, other than dark matter?

X-ray emission from clusters

When X-ray satellites were placed into space, astronomers discovered that most big clusters of galaxies, and even some small groups, radiated lots of X-rays. It appears that clusters are filled with very tenuous, but very hot, clouds of gas. The temperature of the gas may be 50 to 100 million degrees Kelvin, hot enough to produce X-rays.

Now, if one heats up gas, it expands. Here on Earth, if one could heat the entire atmosphere to such temperatures (which would be a Bad Thing for living creatures), the gas would expand upwards and outwards so rapidly that it would escape from Earth and fly off into interplanetary space. In fact, gas of that temperature would easily escape from the solar system: the Sun's gravity isn't strong enough to hold it.

In the Coma cluster (and in other clusters), the hot gas appears not to be expanding. In fact, it appears to be gathered together into a roughly spherical clump near the center of the cluster. If one calculates the force required to hold the gas together in a clump of the observed size, one finds -- again -- that the required gravitational mass is huge.

Could the hot gas itself be the matter which provides this gravitational force? Could the hot, X-ray emitting gas be the mysterious dark matter? If one uses the radiation from the gas to estimate its mass, one finds

   mass of hot gas            M(gas)  =    3 x 10^(14)  solar masses

which is

about 10 times the mass of the stars
but still about 1/10 of the gravitational mass required to explain the high speeds of galaxies in the cluster

So there is still a mystery: what is the dark matter in clusters (and in our Milky Way)? It must provide a strong gravitational pull, but emit very little radition in the optical, or the X-ray, or most other electromagnetic frequencies.