Rosat Observations of Compact Groups of Galaxies

The detection of X-ray emission from Hickson's compact groups of galaxies (HCGs) and its origin are the subject of this work. A search for X-ray emission from compact groups revealed detection from 8 out of the 12 HCG images extracted from the ROSAT public archive. For two of them (HCG16 and HCG44), the Xray emission originates from point-like sources spatially coincident with galaxies in the group. On the contrary, three groups (HCG62, HCG92 and HCG97) show an extended emission clearly caused by hot intracluster gas. A Raymond-Smith hot plasma model provides an excellent t to the X-ray spectra. The estimated temperatures are distributed in a quite narrow range (from 0.73 to 0.92 keV) and are consistent, within the errors, with 0.9 keV. The luminosity ranging from 0.75 to 5:1 1042erg s 1. The most relevant result is the low metal abundance surely detected in two of them and likely in a third that characterizes the hot gas cloud responsible for the X-ray emission. The data concerning the remaining 3 detected compact groups (HCG4, HCG12 and HCG15) are not su cient to discriminate with certainty between di use and/or point-like X-ray emission. However the results of the spectral analysis point to the presence of a hot gas again with low metal abundance. Our results support the statement that in case of di use X-ray emission from compact group of galaxies, the IGM responsible for the emission is predominantly constituted by primordial gas. Upper limits to di use emission for the remaining 4 compact groups are given.