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.