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Astron. Astrophys. 352, 1-18 (1999)

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

S. Dos Santos 1 and G.A. Mamon 1,2

1 Institut d'Astrophysique (CNRS UPR 341), 98 bis Bd Arago, 75014 Paris, France (santos,gam@iap.fr)
2 DAEC (CNRS UMR 8631), Observatoire de Paris, 92195 Meudon, France

Received 18 November 1998 / Accepted 21 July 1999


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 [FORMULA] 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 [FORMULA] 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 [FORMULA], 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 ([FORMULA] with 90% confidence with a best fit [FORMULA]). 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 [FORMULA] (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.

Key words: galaxies: clusters: general – galaxies: clusters: individual: HCG 16 – X-rays: galaxies – X-rays: general

Send offprint requests to: S. Dos Santos

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© European Southern Observatory (ESO) 1999

Online publication: November 23, 1999