MOLECULAR GAS IN SPIRAL GALAXIES: A NEW WARM PHASE AT LARGE GALACTOCENTRIC DISTANCES?

摘要

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 H_2 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 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.