The CO to H₂ Conversion Factor in Normal Late-Type Galaxies

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. 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.