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

X-ray observations of 16 poor clusters containing central dominant galaxies have been performed with the IPC 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, the authors 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. Mean mass-to-light ratios are derived for the bright clusters and evidence for the presence of dark matter is presented. 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.