Warm, dense molecular gas in the ISM of starbursts, LIRGs, and ULIRGs

The role of star formation in luminous and ultraluminous infrared galaxies (LIRGs, L-IR >= 10(11) L-circle dot; ULIRGs, L-IR >= 10(12) L-circle dot) 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 (CO)-C-12 lies 33 K above ground and has a critical density of similar to 1.5; 10(4) cm(-3). The (CO)-C-12 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 L-IR/M-H2 (warm, dense) ratios ranging from similar to 38 to similar to 482 L-circle dot/M-circle dot using a modified CO-H-2 conversion factor of 8.3; 10(19) cm(-2) (K km s(-1))(-1) derived in this paper.