The X-ray emitting gas in poor clusters with central dominant galaxies

Abstract

X-ray observations of 16 poor clusters containing central dominant galaxies have been performed with the Imaging Proportional Counter of the Einstein Observatory. Twelve clusters were detected, and in each case the X-ray emission is centered on the dominant galaxy. For the six brightest clusters, we find extended X-ray emission that is smooth, centrally peaked, reasonably symmetric, and reaches in one case to nearly 1 Mpc and possibly farther. X-ray temperatures are in the range 1-5 keV. The X-ray surface brightness implies density profiles that are roughly inversely propor- tional to radius. The low temperatures and high central densities observed for the brightest clusters give central cooling times less than a Hubble time, implying that radiative accretion flows may be present. Assuming isothermahty and hydrostatic equilibrium, we find total binding masses of - 1013 Mq within 0.5 Mpc. The X-ray gas has ~ 10-20% of this mass. The mean mass-to-light ratio for the bright clusters is ~ 70-100 MQ/LQ. Comparison of the deduced distribution of binding mass with the light distribution of the central galaxies of four clusters shows that M/L rises to over 200 Mq/Lq in the galaxy halos. Thus these halos, like the clusters themselves, must possess dark matter. Although the dominant galaxies in poor clusters appear to lack the distended, luminous envelopes characteristic of rich cluster cD’s, they possess many optical and X-ray morphological similarities. In particular, the X-ray data show clearly that the dominant galaxies sit at the bottoms of the poor cluster gravitational potential wells. This suggests a similar origin for dominant galaxies in poor and rich clusters, possibly through mergers and cannibalism of cluster galaxies. It is the luminosity of the distended cD envelope that reflects the relative wealth of the cluster environment.