HIghMass - High HI Mass, HI-Rich Galaxies at z∼0: Combined HI and H₂ Observations

We present resolved  and CO observations of three galaxies from the HIghMass sample, a sample of H I-massive (M_{H I} > 10¹⁰ M_Θ), gas-rich (M_{H I} in the top 5% for their M _*) galaxies identified in the ALFALFA survey. Despite their high gas fractions, these are not low-surface-brightness galaxies and have typical specific star formation rates (SFR/M_*) for their stellar masses. The three galaxies have normal SFRs for their H₂ masses, but unusually short star formation efficiency scale lengths, indicating that the star formation bottleneck in these galaxies is in the conversion of H I to H₂, not in converting  to stars. In addition, their dark matter spin parameters (λ) are above average, but not exceptionally high, suggesting that their star formation has been suppressed over cosmic time but is now becoming active, in agreement with prior Hα observations.