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

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 factors to such cloud para- meters as density, temperature, and element abundances is examined quantitatively. The 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, with the result that substantial errors in the inferred molecular gas distributions and abundances are possible. In particular, the amounts of molecular gas in the exceptional starburst galaxies detected by IRAS have probably been overestimated by factors of 4-5, while the molecular contents of metal-poor systems (such as many irregular galaxies) may have been underestimated.