Physics 321
Astrophysics II:  Lecture #22
Prof. Dale E. Gary

The Structure of the Universe

Clusters of Galaxies and the Structure of the Universe

Galaxies are not spread uniformly through space, but instead occur in clusters.  Our view has recently changed:
Local Group of Galaxies Virgo Cluster of Galaxies, Coma cluster
Galaxian Cannibalism
We already discussed, cD ellipticals at the centers of clusters appear to have eaten several other galaxies.  Galaxies are closely spaced relative to their diameters: This suggests that collisions and mergers are relatively common.  Here is a scenario for the distant future of the local group.
Velocity Dispersion and Gravitational Mass
To be a member of a group, a galaxy must have a velocity that is lower than the escape velocity.  By measuring the dispersion sr of radial velocities of members of a cluster, and making the reasonable assumption that they are gravitationally bound, we can estimate the total mass of the cluster:
M ~ 5 sr2R/G.
For the Coma cluster, sr = 977 km/s and R = 3 Mpc, which implies a mass of 3.3 x 1015Mo.  The mass estimate from the visual luminosity is only about 5 x 1012Lo, so the mass-to-light ratio is M / L = 660Mo/Lo.  This shows again the magnitude of the dark matter problem.

Some of the missing mass is found in a hot halo, called intracluster gas, filling the space between galaxies in the centers of clusters, and seen in X-rays.  However, the gas is very low density compared to similar gas in the galaxies themselves.  Still, adding up the mass in the Coma intracluster gas, it has a total mass of 3 x 1014Mo, some 20 times as much as currently in stars in the cluster's galaxies!  Spectral lines from the X-ray spectra show that it has highly ionized iron, silicon, and neon, indicating that it once was processed in stars.  How did so much mass escape from the cluster's galaxies?  The large amounts of intracluster gas was probably ejected during mergers early in the history of the cluster.

As we deepen our gaze further into space, we see that the clusters of galaxies are themselves grouped into larger groupings called superclusters.  The Virgo cluster is near the center of the Local Supercluster.

Redshift Surveys like the one below from Las Campanas Redshift Survey (LCRS) team, show the distribution of galaxies along slices in the sky.  See also the Sloan Digital Sky Survey.