Introduction

Recall that the MW, M31, and some 40 small irregular galaxies within 2Mpc or so from the MW, are part of a gravitationally bound system, called the Local Group. Several other such bound groups occur at larger distances from the MW, and searches over larger volumes have revealed the existence of 1000s of such groups of galaxies.

Within about 20Mpc from the MW are several clusters of galaxies, containing 10s of large galaxies, and hundreds of smaller ones each. The Virgo cluster of galaxies contains the big elliptical M87. We already discussed the big cD galaxy NGC1399 at the centre of the Fornax galaxy cluster. The typical total mass of these clusters is estimated at around $10^{14}\hbox{$M_\odot$}$. At 90Mpc we find the Coma cluster of galaxies, with a mass of may be as large as $10^{15}\hbox{$M_\odot$}$. Clusters such as Coma contain the most massive known galaxies.

In the 1950s, George Abell and collaborators eye balled photographic plates to look for rich clusters of galaxies, by finding regions with a large over density of galaxies. He found some 4500, and ranked them from poor to rich, according to the number of galaxies they had. Paul Hickson later performed a similar exercise looking for `Hickson' compact galaxy groups.

Clusters of galaxies are the most massive virialised structures in the Universe. Their crossing time^9.1 is sufficiently small that each galaxy has had the opportunity to cross the system several times, and hence the system has had time to dynamically relax to a near equilibrium situation, i.e. the system is close to being in virial equilibrium. Although more massive bound systems exist, these have crossing times which are so large that, given the age of the Universe, they have not had time to become dynamically relaxed.