H_2 and its relation to CO in the LMC and other magellanic irregular galaxies

H_2 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 H_2 column densities imply N(H_{2)}/I(CO) conversion factors (in units of 10(21) mol cm(-2) (K km s(-1) )(-1) ) X_LMC = 1.3+/-0.2 and X_SMC = 12+/-2. LMC and SMC contain total (warm) H_2 masses of 1.0+/-0.3 x 10(8) M_sun and 0.75+/-0.25 x 10(8) M_sun respectively. Local H_2/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 10(21) mol cm(-2) (K km s(-1) )(-1) . The post-starburst galaxy NGC 1569 has a very high value similar to that of the SMC. The CO-H_2 conversion factor X is found to depend on both the ambient radiation field intensity per nucleon {sigma _FIR/{N_H and metallicity [O]/[H]: log X ~ 0.9+/-0.1 logfrac {sigma _FIR{N_H - 3.5+/-0.2 log([O])/([H]). Neglecting dependency on radiation field, a reasonable approximation is also provided by log X ~ -2.7+/-0.3 log([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 H_2 may be the dominant ISM-component, the average global H_2/HI mass ratio is 0.2+/-0.04 and the average H_2 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.