Resolved HI Imaging of a Population of Massive HI-Rich Galaxies with
  Suppressed Star Formation
  Despite the existence of well-defined relationships between cold gas and star
formation, there is evidence that some galaxies contain large amounts of HI
that do not form stars efficiently. By systematically assessing the link
between HI and star formation within a sample of galaxies with extremely high
HI masses (log M_HI/M_sun > 10), we uncover a population of galaxies with an
unexpected combination of high HI masses and low specific star formation rates
that exists primarily at stellar masses greater than log M_*/M_sun ~ 10.5. We
obtained HI maps of 20 galaxies in this population to understand the
distribution of the HI and the physical conditions in the galaxies that could
be suppressing star formation in the presence of large quantities of HI. We
find that all of the galaxies we observed have low HI surface densities in the
range in which inefficient star formation is common. The low HI surface
densities are likely the main cause of the low sSFRs, but there is also some
evidence that AGN or bulges contribute to the suppression of star formation.
The sample's agreement with the global star formation law highlights its
usefulness as a tool for understanding galaxies that do not always follow
expected relationships.
