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:

• Old -- clusters more or less spherical groups, rather uniformly arranged throughout space
• New -- clusters arrange into chain-like superclusters, on edges of vast voids (bubbles) of empty space

Local Group of Galaxies

• Consists of over 30 members (new ones being discovered all the time).
• Dominated by Andromeda galaxy and Milky Way, also contains M33, a large, faint spiral.  Most are low mass, low luminosity dwarfs.
• Total size, ~ 1 Mpc.

Virgo Cluster of Galaxies, Coma cluster

• Giant and supergiant (cD class) ellipticals lie near center
• Virgo: 19% by number are elliptical, 68% are spirals, but probably many faint dwarfs that cannot be seen.
• Coma: 85% are elliptical or S0, only 15% are spirals or irregulars
• Virgo cluster is very nearby, only ~15 Mpc, covers 7^o of sky!
• Coma cluster may contain as many as 10,000 galaxies

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:

• planets spaced ~ 10⁵ times their diameters.
• stars spaced ~ 10⁶ times their diameters.
• galaxies spaced only ~ 100 times (Andromeda is only ~ 20 Milky Way diameters away).

This suggests that collisions and mergers are relatively common.  Here is a scenario for the distant future of the local group.

To be a member of a group, a galaxy must have a velocity that is lower than the escape velocity.  By measuring the dispersion s_r 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 s_r²R/G.

For the Coma cluster, s_r = 977 km/s and R = 3 Mpc, which implies a mass of 3.3 x 10¹⁵M_o.  The mass estimate from the visual luminosity is only about 5 x 10¹²L_o, so the mass-to-light ratio is M / L = 660M_o/L_o.  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 10¹⁴M_o, 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.