Cold gas and star formation in a merging galaxy sequence

We explore the evolution of the cold gas (molecular and neutral hydrogen) a nd star formation activity during galaxy interactions, using a merging gala xy sequence comprising both pre- and post-merger candidates. Data for this study come from the literature, but are supplemented by some new radio obse rvations presented here. First, we confirm that the ratio of far-infrared l uminosity to molecular hydrogen mass (L-FIR/M(H-2); star formation efficien cy) increases close to nuclear coalescence. After the merging of the two nu clei there is evidence that the star formation efficiency declines again to values typical of ellipticals. This trend can be attributed to M(H-2) depl etion arising from interaction induced star formation. However, there is si gnificant scatter, likely to arise from differences in the interaction deta ils (e.g., disc-to-bulge ratio, geometry) of individual systems. Secondly, we find that the central molecular hydrogen surface density, Sigma(H2), inc reases close to the final stages of the merging of the two nuclei. Such a t rend, indicating gas inflows caused by gravitational instabilities during t he interaction, is also predicted by numerical simulations. Furthermore, th ere is evidence for a decreasing fraction of cold gas mass from early inter acting systems to merger remnants, attributed to neutral hydrogen conversio n 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 fo rmation activity, is also investigated. Although this ratio is on average h igher than that for isolated spirals, we find a marginal increase along the merging sequence, attributed to the relative insensitivity of disc star fo rmation to interactions. However, a similar result is also obtained for the nuclear radio emission, although galaxy interactions are believed to signi ficantly affect the activity (star formation, AGN) in the central galaxy re gions. Nevertheless, the nuclear-radio to blue-band luminosity ratio is sig nificantly elevated compared with that for isolated spirals. Finally, we fi nd that the FIR-radio flux ratio distribution of interacting galaxies is co nsistent with star formation being the main energizing source.