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

There is now strong evidence suggesting that the (12)CO J = 1-0 transition, widely used to trace H(2) 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 similar to 5-20 cm(-3)) and warmer (T(kin) similar to 50-100 K) than the cool (T(kin) similar to 15-20 K) CO-luminous one. Moreover, we find that a high value of the H(2) 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 H(2) component, which may be contributing significantly to the mass and pressure of spiral disks beyond their optical radius.