Molecular Gas Depletion and Starbursts in Luminous Infrared Galaxy   Mergers
  Most luminous infrared galaxies (LIGs) are closely interacting/merging systems rich in molecular gas. We study here the relationship between the stage of the galaxy-galaxy interactions, the molecular gas mass, and the star formation rate as deduced from the infrared luminosity L_IR in LIGs. We find a correlation 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) in a sample of 50 LIG mergers, which shows that the molecular gas content decreases as merging advances. The starburst is due to enhanced star formation in pre-existing molecular clouds and not to the formation of more molecular clouds from HI.   The molecular content is being rapidly depleted due to the starbursts as merging progresses. This is further supported by an anti-correlation between L_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 first evidence connecting the depletion of molecular gas with starbursts in interacting galaxies. 
Yu Gao and Philip M. Solomon