The Dynamical Stability of Stephan's Quintet.

Abstract

The dynamical stability of the compact group of galaxies known as Stephan's Quintet has been reexamined. Estimates for the masses of the members of this group have been derived from their luminosi-ties and from what appear to be the best available estimates for their mass-to-light ratios. Strong indications are found that the five members are all highly luminous. It follows that their masses, too, are probably significantly larger than those of average galaxies. Estimates for the internal kinetic and potential energies of the quintet have been derived on the tentative assumption that no significant fraction of the total mass of the cluster is in the form of intergalactic matter. These results have been used in applying the virial theorm to the group. When account is taken of the uncertainties in the projection factors and in the observed radial velocities, it is shown that one is by no means driven to the conclusion that the virial theorem appropriate to a cluster in dynamical equilibrium is violated and that Stephan's Quintet is rapidly disrupting. Further, the observations strongly suggest that two of the members of the group form a double galaxy within the larger structure of the quintet. If this is the case, then there is strong and independent support for the view that the virial theorem as applied to this group is satisfied to at least a reasonable approximation and that Stephan's Quintet is therefore in, or near, dynamical equilibrium and is not disrupting significantly in a time scale of a billion years.