HIghMass -- High HI Mass, HI-rich Galaxies at z~0: Sample Definition,
  Optical and Halpha Imaging, and Star Formation Properties
  We present first results of the study of a set of exceptional HI sources
identified in the 40% ALFALFA extragalactic HI survey catalog alpha.40 as being
both HI massive (M_HI > 10^10 Msun) and having high gas fractions for their
stellar masses: the HIghMass galaxy sample. We analyze UV- and
optical-broadband and Halpha images to understand the nature of their
relatively underluminous disks in optical and to test whether their high gas
fractions can be tracked to higher dark matter halo spin parameters or late gas
accretion. Estimates of their star formation rates (SFRs) based on SED-fitting
agree within uncertainties with the Halpha luminosity inferred SFRs. The HII
region luminosity functions have standard slopes at the luminous end. The
global SFRs demonstrate that the HIghMass galaxies exhibit active ongoing star
formation (SF) with moderate SF efficiency, but relative to normal spirals, a
lower integrated SFR in the past. Because the SF activity in these systems is
spread throughout their extended disks, they have overall lower SFR surface
densities and lower surface brightness in the optical bands. Relative to normal
disk galaxies, the majority of HIghMass galaxies have higher Halpha equivalent
widths and are bluer in their outer disks, implying an inside-out disk growth
scenario. Downbending double exponential disks are more frequent than upbending
disks among the gas-rich galaxies, suggesting that SF thresholds exist in the
downbending disks, probably as a result of concentrated gas distribution.
