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

We carefully reanalyze the ROSAT PSPC X-ray spectro-photometric observation s of HCG 16 (Arp 318), and compare them to optical and radio data. Its X-ra y morphology resembles its morphology at 20 cm, seen by the NVSS. In partic ular, we detect diffuse emission in eight regions filling half of the 200 h (50)(-1) kpc (8.'7) radius circle around the optical center of the group: o ne region encompassing galaxies a & b, two regions surrounding the group ga laxies c & d, a clumpy region roughly 140 h(50)(-1) kpc from the group gala xies, which may be gas ejected from one of the galaxies, plus regions respe ctively associated with a background radio-source, a probable background ra dio-source, a foreground star and a background group or cluster. The bolome tric X-ray luminosity of the diffuse emission, excluding the regions associ ated with radio galaxies, is L-X(bol) = 2.3 x 10(41) h(50)(-2) 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 luminosit y of the group. The diffuse emission is cool (T < 0.55 keV with 90% confide nce with a best fit T = 0.27 keV). At these low temperatures, the correctio n for photoelectric absorption in the estimate of bolometric luminosity is a factor 3.5 and varies rapidly with temperature, hence an uncertain bolome tric luminosity. The clumpy distribution of hut diffuse gas in HCG 16 is illustrated by the low mean X-ray surface brightness and hot gas density of the regions of und etected emission within 8' (at most 1/4 and 1/6 of those of the detected ga s, assuming both have same temperature, metallicity and clumpiness). The ir regular X-ray morphology of the diffuse emission rules out a (nearly) viria lized nature for HCG 16, unless intergalactic gas had sufficiently high spe cific entropy to be unable to collapse with the group. In any event, the cl umpy gas distribution, and high luminosity given the low temperature sugges t that most of the diffuse gas originates from galaxies, either through tid al stripping or through galactic winds driven by supernova remnants. Theref ore, 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.