Molecular Gas in Spiral Galaxies: A New Warm Phase at Large Galactocentric Distances?

There is now strong evidence suggesting that the ¹²CO J=1-0 transition, widely used to trace H₂ gas, significantly underestimates its mass in metal-poor regions. In spiral disks such regions are found at large galactocentric distances, where we show that any unaccounted H₂ gas phase is likely to be diffuse (n~5-20 cm^-3) and warmer (T_kin~50-100 K) than the cool (T_kin~15-20 K) CO-luminous one. Moreover, we find that a high value of the H₂ formation rate on grains, suggested by recent observational work, can compensate for the reduction of the available grain surface in the metal-poor part of typical galactic disks and thus enhance this CO-poor H₂ component, which may be contributing significantly to the mass and pressure of spiral disks beyond their optical radius.