Star-formation rates, molecular clouds, and the origin of the far-infrared luminosity of isolated and interacting galaxies

Abstract

The CO luminosities of 93 galaxies have been determined and compared with their IRAS far-infrared (FIR) luminosities. The galaxies have been classified into five types according to the degree of interaction, and the FIR to CO luminosity ratios have been determined as a function of interaction type. Strongly interacting/merging galaxies have L_FIR_/L_CO_ substantially higher than that of isolated galaxies, weakly interacting galaxies, or giant molecular clouds (GMCs); in particular galaxies with tidal tails/bridges are found to have L_FIR_/L_CO_ 9 times that of isolated galaxies. This effect is due to high FIR luminosities rather than low CO luminosities. Isolated and weakly interacting galaxies have L_FIR_/L_CO_ close to that of galactic GMCs. There is almost no difference between isolated galaxies and galaxies with companions unless there are morphological signs of a collision. The FIR luminosities of isolated and weakly interacting galaxies can be understood as a result of star formation similar to that in galactic GMCs. We conclude that if the high ratio L_FIR_/L_CO_ in strongly interacting/merging galaxies is due to star formation, then the rate of star formation per unit mass is higher than that of any galactic GMC, i.e., star formation is different and much more efficient than in any galactic cloud. The molecular gas depletion time in these systems is only 3 x 10^8^ yr. The CO luminosities and molecular masses of the FIR luminous interacting galaxies are among the highest observed for any spiral galaxies. They are gas-rich systems with about 10^10^ M_sun_ of molecular hydrogen and a high star-formation rate per unit mass.