Effects of Interaction‐induced Activities in Hickson Compact Groups: CO and Far‐Infrared Study

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Abstract
A study of 2.6 mm CO J = 1 → 0 and far-infrared (FIR) emission in a distance-limited (z < 0.03) complete sample of Hickson compact group (HCG) galaxies was conducted in order to examine the effects of their unique environment on the interstellar medium of component galaxies and to search for a possible enhancement of star formation and nuclear activity. Ubiquitous tidal interactions in these dense groups would predict enhanced activities among the HCG galaxies compared to isolated galaxies. Instead, their CO and FIR properties (thus, "star formation efficiency") are surprisingly similar to isolated spirals. The CO data for 80 HCG galaxies presented here (including 10 obtained from the literature) indicate that the spirals globally show the same H2 content as the isolated comparison sample, although 20% are deficient in CO emission. Because of their large optical luminosity, low metallicity is not likely the main cause for the low CO luminosity. The CO deficiency appears linked with the group evolution, and gas exhaustion through past star formation and removal of the external gas reserve by tidal stripping of the outer H I disk offer a possible explanation. The IRAS data for the entire redshift-limited complete sample of 161 HCG galaxies were reanalyzed using ADDSCAN/SCANPI, improving the sensitivity by a factor of 3-5 over the existing Point Source Catalog (PSC) and resolving better the contribution from individual galaxies. The new analysis of the IRAS data confirms the previous suggestion that FIR emission in HCG galaxies is similar to isolated, Virgo Cluster, and weakly interacting galaxies. Their H2 and FIR characteristics yield a star formation efficiency that is similar to that of these comparison samples. A factor 2 enhancement in the 25-100 μm flux ratio among the HCG spirals is found, which suggests intense localized nuclear starburst activity similar to that of H II galaxies. A number of early-type galaxies in HCGs are detected in CO and FIR, lending further support to the idea that tidal interactions and tidally induced evolution of the groups and member galaxies are important in our sample.
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