A comparison of CO and H I properties of Virgo cluster galaxies reveals a population of H I-rich, CO-poor, lower mass Sbc-Sm spiral galaxies undergoing prodigious massive star formation. Some of these galaxies exhibit radial H I surface density profiles which increase continuously toward the center, and have central H I surface densities as high as 5 x 10^21^ cm^-2^, which is several times greater than the H I surface densities in more massive, H_2_-dominated spirals. This implies that the weak CO emission in these systems is due, at least in part, to a lack of H_2_. Two tracers of massive star formation, the Hα luminosity and the far-infrared luminosity, show excellent correlations with the total gas mass (H I + H_2_) within the optical diameter. Thus, within this sample of 31 Virgo Sbc-Sm galaxies, the efficiency of massive star formation does not depend strongly on whether gas is in the molecular or atomic phase. It is also concluded that a column density of 10^21^ nuclei cm^-2^ is a necessary, but insufficient condition for the creation of an H_2_-dominated interstellar medium. Some other parameter(s) related to galaxy mass influences whether gas-rich galaxies form and maintain large steady-state supplies of molecular gas.
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Kenney et al. (1988) studied this question.