Gas rich galaxies are typically star-forming. We make use of HI-MaNGA, a
program 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, 21cm) rich galaxies which have low Star
Formation Rates (SFRs); using infra-red color from the Wide-field Infrared
Survey Explorer (WISE) as a proxy for specific SFR. Out of a set of 1575 MaNGA
galaxies with HI-MaNGA detections, we find 83 (5%) 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 AGN feedback (from a high cLIER 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.Comment: 15 pages, in press MNRAS. v2 following corrections noticed in proof