The detection of diffuse emission in HCG 16 with XMM-Newton

We report results obtained from analysis of the XMM-Newton observation of the compact group of galaxies HCG 16. It is a peculiar system composed of 7 spirals, 6 of which are active, and its nature as a bound system has been much debated. The EPIC camera observations give new insights into the X-ray parameters describing the physical status of the group. We detect diffuse X-ray emission with a rather elliptical morphology which extends to at least a radius of 135 $h^{-1}_{50}$ kpc from the group centre. The spectrum within this region is well modelled by a thermal plasma with a temperature of 0.49 ± 0.17 keV, and a non-zero metallicity. We measure a bolometric X-ray luminosity of $9.6\times 10^{40}$ $h^{-2}_{50}$ erg s-1which may be only a small fraction of the total luminosity because of the limit in spatial detection arising from the high background level. Despite its low temperature and luminosity, HCG 16 obeys the $L_{\rm X}-T$ relation obtained for brighter galaxy groups even if it lies in a very extreme position. The properties of the diffuse emission confirm the bound nature of HCG 16 even if the gas trapped in the potential well may not yet be virialized. This reopens the debate about the real nature of spiral-dominated galaxy groups, and on their role in a more general cosmological context