1997A&A...328..471I

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)) X_LMC=1.3±0.2 and X_SMC=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 σ_FIR/N_H and metallicity [O]/[H]: logX∝0.9±0.1log(σ_FIR)/(N_H)-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.