Cold gas and star formation in a merging galaxy sequence

We explore the evolution of the cold gas (molecular and neutral hydrogen) and star formation activity during galaxy interactions, using a merging galaxy sequence comprising both pre- and post-merger candidates. Data for this study come from the literature, but are supplemented by some new radio observations presented here. First, we confirm that the ratio of far-infrared luminosity to molecular hydrogen mass (LFIRM(H2); star formation efficiency) increases close to nuclear coalescence. After the merging of the two nuclei there is evidence that the star formation efficiency declines again to values typical of ellipticals. This trend can be attributed to M(H2) depletion arising from interaction induced star formation. However, there is significant scatter, likely to arise from differences in the interaction details (e.g., disc-to-bulge ratio, geometry) of individual systems. Secondly, we find that the central molecular hydrogen surface density, ΣH2, increases close to the final stages of the merging of the two nuclei. Such a trend, indicating gas inflows caused by gravitational instabilities during the interaction, is also predicted by numerical simulations. Furthermore, there is evidence for a decreasing fraction of cold gas mass from early interacting systems to merger remnants, attributed to neutral hydrogen conversion into other forms (e.g., stars, hot gas) and molecular hydrogen depletion resulting from ongoing star formation. The evolution of the total-radio to blue-band luminosity ratio, reflecting the total (disc and nucleus) star formation activity, is also investigated. Although this ratio is on average higher than that for isolated spirals, we find a marginal increase along the merging sequence, attributed to the relative insensitivity of disc star formation to interactions. However, a similar result is also obtained for the nuclear radio emission, although galaxy interactions are believed to significantly affect the activity (star formation, AGN) in the central galaxy regions. Nevertheless, the nuclear-radio to blue-band luminosity ratio is significantly elevated compared with that for isolated spirals. Finally, we find that the FIR–radio flux ratio distribution of interacting galaxies is consistent with star formation being the main energizing source.