Erratum: “Molecular Gas in Spiral Galaxies: A New Warm Phase at Large Galactocentric Distances?” (ApJ, 579, 270 [2002])

Abstract

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 (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.