Molecular Gas Depletion and Starbursts in Luminous Infrared Galaxy Mergers

Most luminous infrared galaxies (LIGs) are closely interacting/mergingsystems rich in molecular gas. We study here the relationship between the stageof the galaxy-galaxy interactions, the molecular gas mass, and the starformation rate as deduced from the infrared luminosity L_IR in LIGs. We find acorrelation between the CO(1-0) luminosity (a measure of molecular mass M(H_2))and the projected separation of merger nuclei (indicator of merging stages) ina sample of 50 LIG mergers, which shows that the molecular gas contentdecreases as merging advances. The starburst is due to enhanced star formationin pre-existing molecular clouds and not to the formation of more molecularclouds from HI. The molecular content is being rapidly depleted due to the starbursts asmerging progresses. This is further supported by an anti-correlation betweenL_IR/M(H_2), the global measure of the star formation rate per unit gas mass,and the projected separation implying an enhanced star formation ``efficiency''in late stage mergers compared to that of early mergers. This is the firstevidence connecting the depletion of molecular gas with starbursts ininteracting galaxies.Comment: 14 pages, 2 postscript figures, to appear in ApJ Letter