Sixteen of the 34 galaxies listed in the Messier catalogue are members of the Virgo cluster, which lies near the north galactic pole and is a good example of an irregular cluster Its mean redshift translates to about 1140 km s-1 implying a distance of only 16Mpc, and the spread in velocities is about 670 kms-1. It should be noted that the Virgo cluster of galaxies is part of the chain of systems used to determine distances independent of redshift.

There is little in the way of a central concentration of galaxies, although the ellipticals are somewhat more spherically symmetric than the rest. The whole cluster seems to be highly structured. No marked separation of galaxy types is observed. The galaxy M87 (NGC4486) is a prominent member of the Virgo cluster. Short photographic exposures reveal a jet, and deep surveys show an extensive halo of globular clusters.

The X-ray luminosity of the Virgo cluster is about 2 per cent of the Perseus cluster. It is centred upon M87, which lies about 1° from the cluster centre. The emission extends for about 50 arc min and again it is probably due to hot gas, although emission from compact sources in the galaxy and globular cluster halo may con¬tribute also. The X-ray luminosity of clusters of galaxies seems to be correlated with velocity dispersion: the clusters with the higher velocities are the more luminous. These clusters are likely to be more massive and contain more gas, thus more X-ray emission is expected.

Many groups and chains of galaxies lie between the Virgo cluster and the Local Group. The apparent flattening and concentration of these systems has encouraged G. de Vaucouleurs to name them the Local, or Virgo, Supercluster. The physical and dynamical reality of this supercluster is in doubt, and it is not clear how chance groupings of objects interfere with the interpretation.

Comments are closed.