The ¹²CO(1—0) to H₂ conversion factor in normal late-type galaxies

¹ Laboratoire d'Astrophysique de Marseille, BP. 8, traverse du Siphon, 13376 Marseille, France
² DEMIRM, Observatoire de Paris, 61 avenue de l'Observatoire, 75014 Paris, France
³ Universitá degli Studi di Milano-Bicocca, Dipartimento di Fisica, Piazza dell'Ateneo Nuovo 1, 20126 Milano, Italy

Abstract

The molecular gas mass in nearby galaxies is generally estimated using ¹²CO(1-0) line intensities and assuming the X conversion factor between I(CO) and N(H₂) 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 μm 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 using ISOPHOT and IRAS far-IR data. X spans from ~10²⁰ mol cm^-2 (K km s^-1)^-1 in giant spirals to ~10²¹ mol cm^-2 (K km s^-1)^-1 in dwarf irregulars. The relationships between X and the UV radiation field (as traced by the H_α[NII]E.W.), the metallicity and the H band luminosity are analysed.