On the molecular gas content and SFR in Hickson Compact Groups: enhanced or deficient? ⋆

Aims. We study the effect of the extreme environment in Hickson Compact groups (HCGs) on the molecular gas mass, MH2 , and the star formation rate (SFR) of galaxies as a function of atomic hydrogen (HI) content and evolutionary phase of the group. Methods. We have selected a redshift limited (D<100 Mpc) sample of 88 galaxies in 20 HCGs with available atomic hydrogen (HI) VLA maps, covering a wide range of HI deficiencies and evoluti onary phases of the groups, and containing at least one spiral galaxy. We derived the far-infrared (FIR) luminosity (LFIR) from IRAS data and used it as a tracer of the star formation rate (SFR). We calculated the HI mass (MHI), LFIR and MH2 deficiencies. Results. The mean deficiencies of LFIR and MH2 of spiral galaxies in HCGs are close to 0, indicating that the ir average SFR and molecular gas content are similar to those of isolated galaxies. However, there are indications of an excess in MH2 (∼ 50%) in spiral galaxies in HCGs which can be interpreted, assuming that there is no systematic difference in the CO-to-H2 conversion factor, as either an enhanced molecular gas content or as a higher concentration of the molecular component towards the center in comparison to galaxies in lower density environments. In contrast, the mean MHI of spiral galaxies in HCGs is only 12% of the expected value. The specific star formation rate (sSFR = SFR/stellar mass) tends to be lower for galaxies with a higher MH2 or MHI deficiency. This trend is not seen for the star formation effi ciency (SFE=SFR/MH2), which is very similar to isolated galaxies. We found tentative indications for an enhancement of MH2 in spiral galaxies in HCGs in an early evolutionary phase and a decrease in later phases. We suggest that this might be due to an enhancement of the conversion from atomic to molecular gas due to on-going tidal interactions in an early evolutionary phase, followed by HI stripping and a decrease of the molecular gas content because of lack of replenishment. Conclusions. The properties of MH2 and LFIR in galaxies in HCGs are surprisingly similar to those of isol ated galaxies, in spite of the much higher def(MHI) of the former. The trends of the sSFR and def(MH2 ) with def(MHI) and the evolutionary state indicate, however, that the ongoing interaction might have some effect on the molecular gas and SF.