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 200 h _ { 50 } ^ { -1 } { kpc } ( 8 \hbox { $ . { } ^ { \prime } $ } 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 140 h _ { 50 } ^ { -1 } 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 L _ { X } ^ { bol } = 2.3 \times 10 ^ { 41 } h _ { 50 } ^ { -2 } erg s ^ { -1 } , i.e. , half of the luminosity found by .
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 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 ^ { \prime } ( 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 .