Xmm-Newton and Gemini observations of eight RASSCALS galaxy groups

We study the distribution of gas pressure and entropy in eight groups of galaxies belonging to the ROSAT All-Sky Survey/Center for Astrophysics Loose Systems (RASSCALS). We use archival and proprietary XMM-Newton observations, supplementing the X-ray data with redshifts derived from the literature; we also list 127 new redshifts measured with the Gemini North telescope. The groups are morphologically heterogeneous in both the optical and the X-ray, and several suffer from superpositions with background galaxies or clusters of galaxies. Nevertheless, they show remarkable self-similarity in their azimuthally averaged entropy and temperature profiles. The entropy increases with radius; the behavior of the entropy profiles is consistent with an increasing broken power law with inner and outer slope 0.92(-0.05)(+0.04) and 0.42(-0.04)(+0.05) (68% confidence), respectively. There is no evidence of a central, isentropic core, and the entropy distribution in most of the groups is flatter at large radii than in the inner region, challenging earlier reports, as well as theoretical models predicting large isentropic cores or asymptotic slopes of 1.1 as r --> infinity. The pressure profiles are consistent with a self-similar decreasing broken power law in radius; the inner and outer slopes are -0.78(-0.03)(+0.04) and 1.7(-0.3)(+0.1), respectively. The results suggest that the larger scatter in the entropy distribution reflects the varied gasdynamical histories of the groups; the regularity and self-similarity of the pressure profiles is a sign of a similarity in the underlying dark matter distributions.