Molecular gas depletion and starbursts in luminous infrared galaxy mergers

Most luminous infrared galaxies (LIGs) are closely interacting/merging systems that are rich in molecular gas. Here we study 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 (the 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 preexisting molecular clouds and not to the formation of more molecular clouds from atomic gas. Because of the starbursts, the molecular content is being rapidly depleted as merging progresses. This is further supported by an anticorrelation between L(IR)/M(H(2)), the global measure of the star formation rate per unit gas mass, and the projected separation that implies an enhanced star formation "efficiency" in late-stage mergers compared with that of early mergers. This is the first evidence connecting the depletion of molecular gas with starbursts in interacting galaxies.