CO EXCITATION AND H2 MASSES OF INFRARED-LUMINOUS GALAXIES

We mapped the CO(2-->1) and CO(1-->0) emission from four infrared-luminous galaxies, Arp 193, Arp 220, Mrk 231, and VII Zw 31, with the IRAM 30 m telescope. These maps show the molecular gas is concentrated in the central regions. The CO(2-->1)/CO(1-->0) brightness temperature ratio for these galaxies is low, 0.6-0.75, indicating the CO is subthermally excited in regions of moderate H2 density, almost-equal-to 400 cm-3. We infer the intrinsic CO(1-->0) brightness temperatures are between 6 and 13 K, even if the gas kinetic temperatures are much higher. For these galaxies, the H2 mass-to-CO luminosity ratio is similar to that measured for giant molecular clouds in the Milky Way molecular ring.