Molecular Hydrogen Deficiency in H I-poor Galaxies and its Implications for Star Formation

We use a sample of 47 homogeneous and high-sensitivity CO images taken from the Nobeyama and BIMA surveys to demonstrate that, contrary to common belief, a significant number (~40%) of H I-deficient nearby spiral galaxies are also depleted in molecular hydrogen. While H I deficiency by itself is not a sufficient condition for molecular gas depletion, we find that H₂ reduction is associated with the removal of H I inside the galaxy optical disk. Those H I-deficient galaxies with normal H₂ content have lost H I mainly from outside their optical disks, where the H₂ content is low in all galaxies. This finding is consistent with theoretical models in which the molecular fraction in a galaxy is determined primarily by its gas column density. Our result is supported by indirect evidence that molecular deficient galaxies form stars at a lower rate or have dimmer far infrared fluxes than gas rich galaxies, as expected if the star formation rate is determined by the molecular hydrogen content. Our result is consistent with a scenario in which, when the atomic gas column density is lowered inside the optical disk below the critical value required to form molecular hydrogen and stars, spirals become quiescent and passive evolving systems. We speculate that this process would act on the timescale set by the gas depletion rate and might be a first step for the transition between the blue and red sequence observed in the color-magnitude diagram.