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 in large galactocentric distances where we show that any unaccounted H _ { 2 } gas phase is likely to be diffuse ( n \sim 5 - 20 cm ^ { -3 } ) and warmer ( T _ { kin } \sim 50 - 100 K ) than the cool ( T _ { kin } \sim~ { } 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 .