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 CO are used to obtain column densities and masses of hydrogen. By taking into account the effects of variations in molecular cloud parameters on conversion factors between integrated CO intensity and molecular hydrogen column density, it is shown that conversion factors are very sensitive to the kinetic temperature of the emitting gas. Results indicate that the gas temperatures in systems with high star formation rates can be quite high, and it is suggested that use of a standard conversion factor will lead to systematic overestimation of the amount of molecular gas.