Warm, Dense Molecular Gas in the ISM of Starbursts, LIRGs, and ULIRGs

The role of star formation in luminous and ultraluminous infrared galaxies (LIRGs, LIR ≥ 1011 L☉; ULIRGs, LIR ≥ 1012 L☉) is a hotly debated issue: while it is clear that starbursts play a large role in powering the IR luminosity in these galaxies, the relative importance of possible enshrouded AGNs is unknown. It is therefore important to better understand the role of star-forming gas in contributing to the infrared luminosity in IR-bright galaxies. The J = 3 level of 12CO lies 33 K above ground and has a critical density of ~1.5 × 104 cm-3. The 12CO J = 3-2 line serves as an effective tracer for warm, dense molecular gas heated by active star formation. Here we report on 12CO J = 3-2 observations of 17 starburst spiral galaxies, LIRGs, and ULIRGs, which we obtained with the Heinrich Hertz Submillimeter Telescope on Mount Graham, Arizona. Our main results are as follows. (1) We find a nearly linear relation between the infrared luminosity and warm, dense molecular gas such that the infrared luminosity increases as the warm, dense molecular gas to the power 0.92; we interpret this to be roughly consistent with the recent results of Gao & Solomon. (2) We find LIR/M ratios ranging from ~38 to ~482 L☉/M☉ using a modified CO-H2 conversion factor of 8.3 × 1019 cm-2 (K km s-1)-1 derived in this paper.