H2 and its relation to CO in the LMC and other magellanic irregulars

H2 column densities towards CO clouds in the LMC and SMC are estimated from their far-infrared surface brightness and HI column density. N(H2)/I(CO) conversion factors are X(LMC) = 1.3 and X(SMC) = 12 (unit: 10**21 mol cm**-2 (K km/s)**-1). LMC and SMC contain total (warm) H2 masses of 1.0 and 0.75 x 10**8 M(sun) respectively. Similar local H2/HI mass ratios are found in the magellanic irregulars NGC 1569 (X = 16), NGC 55, 4214 and 6822 (X = 3--6) and in NGC 4449 as well as the extragalactic HII regions NGC 604, 595 and 5461 in M 33 and M 101 (X = 1--2). The CO--H2 conversion factor X depends on both the ambient radiation field intensity per nucleon s(FIR)/N(H) and metallicity [O]/[H]. The results are 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' molecular gas. Although locally H2 may be the dominant ISM-component, the average global H2/HI mass ratio is 0.2 and the average H2 gas mass fraction is 0.12. Magellanic irregulars thus have warm molecular gas fractions very similar to those of our Galaxy, whereas other global properties (mass, luminosity, metallicity, CO luminosity) are very different.