I(CO)/N(H2) conversions and molecular gas abundances in spiral and irregular galaxies

Abstract

Observations of emission in the J = 1-0 rotational transition of interstellar carbon monoxide have been used to infer column densities and masses of hydrogen on the basis of relations of the form N(H2)/ICO = α, where α is a constant. Although the large-scale value of α appears to be accurately constant over most of the Milky Way, the underlying reason for this is not fully understood, nor is it reasonable to expect similar behavior in other galaxies. The sensitivity of the conversion factor to such cloud parameters as density, temperature, and element abundances is examined quantitatively. It is found that conditions under which the conversion factor will be constant and equal to the Galactic value are almost certainly violated on global scales in many galaxies.