Virial-mass estimates for systems of galaxies

Astronomy and Astrophysics – Astrophysics

Scientific paper

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Astrophysics, Galactic Clusters, Galaxies, Mass Distribution, Virial Theorem, Dynamic Stability, Kinetic Energy, Potential Energy, Radial Velocity, Spatial Distribution

Scientific paper

Two questions associated with the application of the virial theorem to systems of galaxies are considered: allowance for galaxy clumpiness in potential-energy determinations and the effect of errors in radial-velocity determinations on system kinetic-energy estimates. A clumpiness factor (C) is introduced which represents the ratio of true cluster potential energy to the potential energy calculated from a smoothed density distribution. C is found to be approximately 2-3 for the Neyman-Scott (1959) galaxy-clumpiness model; this is taken to indicate that virial masses based on galaxy counts in zones should be decreased by a corresponding amount. An exact unbiased estimate of the kinetic energy of a system of galaxies is obtained which takes into account errors in radial-velocity determinations. Application of the results to data for the NGC 1023 group yields a virial-mass ratio estimate of about 0.6, which confirms the stability of this group.

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