Astronomy and Astrophysics, volume 301, 348-358 (1995/9-2)
ROSAT observations of compact groups of galaxies.
SARACCO P. and CILIEGI P.
Abstract (from CDS):
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 (HCG 16 and HCG 44), the X-ray emission originates from point-like sources spatially coincident with galaxies in the group. On the contrary, three groups (HCG 62, HCG 92 and HCG 97) show an extended emission clearly caused by hot intracluster gas. A Raymond-Smith hot plasma model provides an excellent fit to the X-ray spectra. The estimated temperatures are distributed in a quite narrow range (from 0.73 to 0.92keV) and are consistent, within the errors, with 0.9keV. The luminosity ranging from 0.75 to 5.11042erg/s. 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 (HCG 4, HCG 12 and HCG 15) are not sufficient to discriminate with certainty between diffuse 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 diffuse X-ray emission from compact group of galaxies, the IGM responsible for the emission is predominantly constituted by primordial gas. Upper limits to diffuse emission for the remaining 4 compact groups are given.