The CO to H-2 conversion factor in normal late-type galaxies

The molecular gas mass in nearby galaxies is generally estimated using (CO)-C-12(1-0) line intensities and assuming the X conversion factor between I(CO) and N(H-2) measured in the solar neighborhood. It is however known that this X conversion factor is not universal since it changes with metallicity, cosmic ray density and UV radiation field. Far-IR data in the spectral range 100-1000 mum can be used to estimate the molecular gas content of late-type galaxies in an independent way of CO line measurements once a metallicity-dependent dust to gas ratio is assumed, allowing a direct estimate of X. This exercise is presented here for a large sample of galaxies with available multifrequency data. X spans from similar to 10(20) mol cm(-2) (K km s(-1))(-1) in giant spirals to similar to 10(21) mol cm(-2) (K km s(-1))(-1) in dwarf irregulars.