The Mass of the Virgo Cluster

The lists of radial velocities now include results for thirty-two members of the Virgo Cluster, thus giving for the first time sufficient data to determine some of the physical characteristics of a cluster of nebulae. A comparison of the velocities of fainter members of the cluster with those of brighter members shows that the line-of-sight velocity of a nebula has no dependence on its magnitude; hence, equipartition apparently does not hold in the cluster. The distribu- tion of the velocities in right ascension and declination shows that the cluster is not in rotation and that there is no central concentration of high velocities. This result is tak- en to mean that the cluster is neither condensing nor breaking up, but is a fairly stable assemblage, more or less held together by its gravitational field. From the observed distribution function for radial velocity is derived the distribu- tion function for space velocity. For an assumed distance of 2 X 106 parsecs this func- don leads to 2 X Io~~ g or i~'~ 0 as a value of the mass of the cluster. On the basis of ~oo nebulae in the cluster, the mass per nebula is 2 X io" 0. Although far larger than Hubble's value of io9 0 for the mass of an average nebula, other evidence lends support to the high value obtained from the Virgo Cluster. It is possible that both figures are correct and that the difference represents a great mass of internebular material within the cluster