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Most galaxies are found in groups, yet due to the difficulty of discerning them from the field, galaxy groups have not been investigated as thoroughly as clusters. One well-defined sample of groups, Hickson Compact Groups (HCG's), are an exception to the rule. Although HCG's have been surveyed in many wavelengths, only with the advent of ROSAT have sufficiently sensitive X-ray observations of a large number of them been possible. Similarly, large-format CCD's have made it feasible to make deep optical observations of the groups as a whole, rather than just the component galaxies.
We have systematically analyzed ROSAT PSPC observations of 12 HCG's plus the NCG~2300 group of galaxies, and have obtained three-color photometry for eight of these groups. Approximately two-thirds of the groups have extended X-ray emission, and, in at least half of these, the emission is due to diffuse gas in the group potential at a temperature of $k$T $\sim 1$ keV. Although HCG's as a whole are spiral rich, all but one of the groups with extended emission have a spiral fraction of less than 50\%. For the groups with diffuse emission, we find baryon fractions of 5--19\%, similar to values found in clusters of galaxies; however, the median value of the gas-to-stellar-mass ratio is 5\%, two orders of magnitude less than what is found in clusters but greater than that of individual elliptical galaxies.
Even though they lack diffuse X-ray emission, many of the spiral-rich HCG's in our sample show optical signs of interaction such as tidal tails. Similarly, there is no clear correlation between the presence of fine structure in ellipticals in a group and the presence of diffuse X-ray emission. However, the group with the highest X-ray luminosity, HCG 94, has an asymmetric red envelope surrounding the two brightest galaxies, suggesting that we are witnessing the tidal destruction of these galaxies and the possible formation of a cD.
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