On the molecular gas content and SFR in Hickson compact groups: enhanced or deficient?

Abstract

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 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 evolutionary phases of the groups and containing at least one spiral galaxy. We observed the CO(1–0) and CO(2–1) lines with the IRAM 30 m telescope for 47 galaxies. Together with literature data, our sample contains CO(1–0) spectra for 86 galaxies. We derived the far-infrared (FIR) luminosity (LFIR) from IRAS data and used it as a tracer of the SFR. We calculated the HI mass (MHI), LFIR, and MH2 deficiencies, based on the values expected from LB and LK in isolated galaxies from the AMIGA sample. We limited our statistical analysis to spiral galaxies, since the large number of upper limits did not allow drawing strong conclusions about MH2and LFIR in early-type galaxies.

Results. The mean deficiencies of LFIR and MH2 of spiral galaxies in HCGs are close to 0, indicating that their average SFR and molecular gas content are similar to those of isolated galaxies. However, there are indications of an excess of 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 SFR (sSFR = SFR/stellar mass) tends to be lower for galaxies with higher MH2 or MHI deficiency. This trend is not seen for the star formation efficiency (SFE = SFR/MH2), which is very similar to isolated galaxies. We found tentative indications of 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 ongoing tidal interactions in an early evolutionary phase, followed by HI stripping and a decrease in 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 isolated 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.