Hi-rich but low star formation galaxies in MaNGA: physical properties and comparison to control samples

Gas-rich galaxies are typically star forming. We make use of Hi-MaNGA, a programme of Hi follow-up for the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey of the Sloan Digital Sky Surveys, to construct a sample of unusual neutral hydrogen (Hi, 21 cm)-rich galaxies that have low star formation rates (SFRs), using infrared colour from the Wide-field Infrared Survey Explorer as a proxy for specific SFR. Out of a set of 1575 MaNGA galaxies with Hi-MaNGA detections, we find that 83 (5 percent) meet our selection criteria to be Hi rich with low SFR. We construct two stellar mass-matched control samples: Hi-rich galaxies with typical SFR (high SF control) and Hi-poor galaxies with low SFR (low Hi control). We investigate the properties of each of these samples, comparing physical parameters such as ionization state maps, stellar and ionized gas velocity and dispersion, environment measures, metallicity, and morphology to search for the reasons why these unusual Hi-rich galaxies are not forming stars. We find evidence for recent external accretion of gas in some galaxies (via high counter-rotating fractions), along with some evidence for active galactic nucleus (AGN) feedback (from a high central low-ionization emission-line region and/or red geyser fraction), and bar quenching (via an enhanced strong bar fraction). Some galaxies in the sample are consistent with simply having their Hi in a high angular momentum, large-radius, low-density disc. We conclude that no single physical process can explain all Hi-rich, low-SFR galaxies.