Nature and distribution of dark matter: 2. Groups and clusters

Abstract

We study the mass-radius relationship for aggregates of galaxies, viz. binaries, small groups and clusters. The data are subjected to a simple best-fit analysis similar to the one carried out earlier for individual field galaxies. The analysis shows that: (i) The data on binary galaxies are consistent with the assumption that binaries are just two galaxies, each with an individual isothermal (M∝R) dark matter halo, moving under the mutual gravitational attraction, (ii) The data on the groups of galaxies are too scattered to obey a single power-law relation of the form M = kRᶯ with any degree of reliability, (iii) The data on groups and clusters fit better with a law of the form M = AR³ +BR. This form suggests the existence of two components in dark matter-one which is clustered around the galaxies (M∝R) and another which is distributed smoothly (M∝R3). The smooth distributions becomes significant only at scales ≥ 1 Mpc and hence does not affect binaries significantly. We briefly discuss the theoretical implications of this analysis.