Molecular gas in spiral galaxies: a new warm phase at large galactocentric distances?

There is now strong evidence suggesting that the CO 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 in large galactocentric distances where we show that any unaccounted H2 gas phase is likely to be diffuse (n ∼ 5− 20 cm−3) and warmer (Tkin ∼ 50− 100 K) than the cool (Tkin ∼ 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. Subject headings: dark matter: galaxies—interstellar matter: molecular gas— galaxies: individual: NGC 891, Galaxy—molecules: CO, H2