The Ratio of Molecular to Atomic Gas in Spiral Galaxies as a Function of Morphological Type

As part of the FCRAO Extragalactic CO Survey, we have observed λ2.6 mm CO emission along the major axis in 142 galaxies. We find that the ratio of molecular to atomic gas among spiral galaxies decreases by a factor of ~ 20 as a function of morphological type; the mean M(H_2_)/M(H I) ratio is 4.0 +/- 1.9 for S0-Sa galaxies and 0.2 +/- 0.1 for Sd-Sm galaxies. The decrease in the M(H_2_)/M(H I) ratio with type is not due to a Malmqulst bias, since a volume-limited subset of the galaxies also displays a decrease in the M(H_2_)/M(H I) ratio. Furthermore, we argue that the observed M(H_2_)/M(H I) decrease is not due to variations in molecular cloud properties as a function of type. For all of the spiral galaxies in our sample, we find the mean value of the ratio of total ISM gas mass to optical area to be 17 M_sun_ pc^-2^, within a factor of 2 of the mean value for each type. We find only factor of 2 increase in the mean gas surface density of spiral galaxies from early to late morphological type. The principal effect we find is that the dominant phase of the gas changes as a function of type. This may indicate that atomic gas can become molecular at a lower critical column density in early- than late-type spiral galaxies.