Astronomy and Astrophysics, volume 328, 471-482 (1997/12-2)
H2 and its relation to CO in the LMC and other Magellanic irregular galaxies.
Abstract (from CDS):
H2 column densities towards CO clouds in the LMC and SMC are estimated from their far-infrared surface brightness and HI column density. The newly derived H2 column densities imply N(H2)/I(CO) conversion factors (in units of 1021mol/cm2/(K.km/s)) XLMC=1.3±0.2 and XSMC=12±2. LMC and SMC contain total (warm) H2 masses of 1.0±0.3x108M☉ and 0.75±0.25x108M☉ respectively. Local H2/HI mass ratios similar to those in LMC and SMC are found in the magellanic irregulars NGC 55, 1569, 4214, 4449 and 6822 and in the extragalactic HII region complexes NGC 604, 595 and 5461 in M 33 and M 101 respectively. In these HII regions and in NGC 4449, we find X=1-2; in NGC 55, 4214 and 6822 X=3-6 again in units of 1021mol/cm2/(K.km/s). The post-starburst galaxy NGC 1569 has a very high value similar to that of the SMC. The CO-H2 conversion factor X is found to depend on both the ambient radiation field intensity per nucleon σFIR/NH and metallicity [O]/[H]: logX∝0.9±0.1log(σFIR)/(NH)-3.5±0.2log([O])/([H]). Neglecting dependency on radiation field, a reasonable approximation is also provided by logX∝-2.7±0.3log([O])/([H]). Milky Way values are consistent with these relations. This result is interpreted as the consequence of selective photodissociation of CO subjected to high radiation field energy densities and poor (self)shielding in low-metallicity environments, and especially the preferential destruction of diffuse CO in `interclump' gas. Although locally H2 may be the dominant ISM-component, the average global H2/HI mass ratio is 0.2±0.04 and the average H2 gas mass fraction is 0.12±0.02. Magellanic irregulars have warm molecular gas fractions very similar to those of our Galaxy, whereas other global properties (mass, luminosity, metallicity, CO luminosity) are very different.