The Outer Stellar Populations and Environments of Unusually HI-rich
  Galaxies
  We investigate the nature of HI-rich galaxies from the ALFALFA and GASS
surveys, which are defined as galaxies in the top 10th percentile in atomic gas
fraction at a given stellar mass. We analyze outer (R>1.5 Re) stellar
populations for a subset of face-on systems using optical g-r versus r-z
colour/colour diagrams. The results are compared with those from control
samples that are defined without regard to atomic gas content, but are matched
in redshift, stellar mass and structural parameters. HI-rich early-type (C>2.6)
and late-type (C<2.6) galaxies are studied separately. When compared to the
control sample, the outer stellar populations of the majority of HI-rich
early-type galaxies are shifted in the colour/colour plane along a locus
consistent with younger stellar ages, but similar metallicities. The outer
colours of HI-rich late-type galaxies are much bluer in r-z than the HI-rich
early types, and we infer that they have outer disks which are both younger and
more metal-poor. We then proceed to analyze the galaxy environments of HI-rich
galaxies on scales of 500 kpc. HI-rich early-type galaxies with low (log M* <
10.5) stellar masses differ significantly from the control sample in that they
are more likely to be central rather than satellite systems. Their satellites
are also less massive and have younger stellar populations. Similar, but weaker
effects are found for low mass HI-rich late-type galaxies. In addition, we find
that the satellites of HI-rich late-types exhibit a greater tendency to align
along the major axis of the primary. No environmental differences are found for
massive (log M* > 10.5) HI-rich galaxies, regardless of type.
