The 12CO (1-0) to H2 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 call be used to estimate the molecular gas content of late-type galaxies in an independent, way of CO line measurements once a constant 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 using ISOPHOT and IRAS far-IR data. This analysis, which is extremely useful to study the properties of the ISM of galaxies spanning a large range in luminosity and morphological type, is however limited by the. lack of photometric data in the range. 100 mum less than or equal to lambda less than or equal to 1000 mum, the spectral domain observed by FIRST.