Re - analysis of ROSAT - PSPC X -

We carefully reanalyze the ROSAT PSPC X-ray spectro-photometric observations of HCG 16 (Arp 318) and compare them to optical and radio data. We detect diffuse emission in six regions filling half of the 8 radius circle around the optical center of the group: one region encompassing galaxies a & b, two regions surrounding the group galaxies c & d, one associated with a background radio-source, one associated with a probable background radio-source, and finally a clumpy region roughly 140 h 50 kpc from the group galaxies, which may be gas ejected from one of the galaxies. The bolometric X-ray luminosity of the diffuse emission, excluding the regions associated with radio galaxies, is L X = 7 × 10 40 h 50 erg s , i.e., nearly 7 times lower than the corresponding luminosity found by Ponman et al. (1996). The region that is offset from the galaxies contributes half of the diffuse X-ray luminosity of the group. The diffuse emission is cool (T < 0.55 keV with 90% confidence). At most 18% of the group is in the form of baryons, at least half of which is in cold gas. The irregular X-ray morphology of the diffuse emission rules out a (nearly) virialized nature for HCG 16, unless intergalactic gas had sufficiently high specific entropy to be unable to collapse with the group. Our results demonstrate the need for a careful 2D spatial analysis and multi-wavelength study of the diffuse X-ray emission from compact groups. We therefore expect other compact groups to have been misclassified as X-ray luminous for similar reasons as HCG 16, and suggest that the frequency of high density compact groups is lower than previously inferred from X-ray studies.