Clumpy diffuse X - ray emission from the spiral - rich compact galaxy group HCG 16

We carefully reanalyze the ROSAT PSPC X-ray spectro-photometric observations of HCG 16 (Arp 318), and compare them to optical and radio data. Its X-ray morphology resembles its morphology at 20 cm, seen by the NVSS. In particular, we detect diffuse emission in eight regions filling half of the 200h−1 50 kpc (8. ′7) radius circle around the optical center of the group: one region encompassing galaxies a & b, two regions surrounding the group galaxies c & d, a clumpy region roughly 140h−1 50 kpc from the group galaxies, which may be gas ejected from one of the galaxies, plus regions respectively associated with a background radio-source, a probable background radio-source, a foreground star and a background group or cluster. The bolometric X-ray luminosity of the diffuse emission, excluding the regions associated with radio galaxies, is Lbol X = 2.3 × 1041 h−2 50 erg s−1, i.e., half of the 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 keVwith 90% confidence with a best fit T = 0.27 keV). At these low temperatures, the correction for photoelectric absorption in the estimate of bolometric luminosity is a factor 3.5 and varies rapidly with temperature, hence an uncertain bolometric luminosity. The clumpy distribution of hot diffuse gas in HCG 16 is illustrated by the low mean X-ray surface brightness and hot gas density of the regions of undetected emission within 8′ (at most 1/4 and 1/6 of those of the detected gas, assuming both have same temperature, metallicity and clumpiness). 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. In any event, the clumpy gas distribution, and high luminosity given the low temperature suggest that most of the diffuse gas originates from galaxies, either through tidal stripping or through galactic winds driven by supernova remnants. Therefore, no spiral-only HCGs are known with regular diffuse emission tracing a gravitational potential. Our results highlight the need for a careful 2D spatial analysis and multi-wavelength study of the diffuse X-ray emission from groups, suggesting that other compact groups could be significantly contaminated by superimposed X-ray sources. Send offprint requests to: S. Dos Santos