Warm and Cold Molecular Gas in Galaxies

New and archival interferometric ¹²CO (1-->0) data sets from six nearby galaxies are combined with H₂ 2.122 μm and Hα maps to explore in detail the interstellar medium in different star-forming galaxies. We investigate the relation between warm (H₂ at T~2000 K) and cold (CO at T~50 K) molecular gas from 100 pc to 2 kpc scales. On these scales, the ratio of warm-to-cold molecular hydrogen correlates with the f_ν(60μm)/f_ν(100μm) ratio, which tracks the star formation activity level. This result also holds for the global properties of galaxies from a much larger sample drawn from the literature. The trend persists for over 3 orders of magnitude in the mass ratio, regardless of source nuclear activity.