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

The role of star formation in luminous and ultraluminous infrared galaxies isa hotly debated issue: while it is clear that starbursts play a large role inpowering the IR luminosity in these galaxies, the relative importance ofpossible enshrouded AGNs is unknown. It is therefore important to betterunderstand the role of star forming gas in contributing to the infraredluminosity in IR-bright galaxies. The J=3 level of 12CO lies 33K above groundand has a critical density of ~1.5 X 10^4 cm^-3. The 12CO(J=3-2) line serves asan effective tracer for warm-dense molecular gas heated by active starformation. Here we report on 12CO (J=3-2) observations of 17 starburst spirals,LIRGs and ULIRGs which we obtained with the Heinrich Hertz SubmillimeterTelescope on Mt. Graham, Arizona. Our main results are the following: 1. We find a nearly linear relationbetween the infrared luminosity and warm-dense molecular gas such that theinfrared luminosity increases as the warm-dense molecular gas to the power0.92; We interpret this to be roughly consistent with the recent results of Gao& Solomon (2004a,b). 2. We find L_IR/M_H2 ratios ranging from ~10 to ~128L_sun/M_sun using a standard CO-H2 conversion factor of 3 X 10^20 cm^-2 (K kms^-1)^-1. If this conversion factor is ~an order of magnitude less, assuggested in a recent statistical survey (Yao et al. 2003), then 2-3 of ourobjects may have significant contributions to the L_IR by dust-enshrouded AGNs.Comment: 15 Pages, 2 figures, Accepted for Publication in Ap