EMISSION FROM HICKSONS COMPACT-GROUPS OF GALAXIES - RESULTS FROM THE ROSAT ALL-SKY SURVEY

We report on the detection of X-ray emission from 11 of Hickson's comp act groups of galaxies (HCGs) in the ROSAT All-Sky Survey (RASS). One of these is clearly part of a much richer system with an X-ray luminos ity comparable to that of Abell clusters. With two other detections, t he X-ray emission is dominated by pointlike and spectrally soft source s which can be identified with active galactic nuclei within the group . The remaining eight detections constitute the first statistically re levant sample of compact groups of galaxies truly seen in X-rays. Some of the sources are clearly extended; for the other ones it is, at the sensitivity of the RASS, not clear whether the X-ray emission is due to the presence of a hot intragroup gas, to individual galaxies, or to galaxy-galaxy interactions in these objects, although the former is s uggested by the hardness of the emission typical of a thermal bremsstr ahlung spectrum with T-gas approximate to 1 keV. We find the HCG's X-r ay luminosity to be almost independent of the optical richness of the group; however, there seems to be a strong correlation with the spiral fraction and the morphological type of the dominant galaxy. A compari son between the X-ray luminosity functions (XLFs) of HCGs and clusters of galaxies suggests that, at X-ray luminosities of some 10(42) h(50) (-2) ergs s(-1), compact groups of galaxies contribute at the similar to 10% level to the XLF of galactic systems. The majority of the latte r thus have to be loose groups and poor clusters of galaxies.