The Arecibo Legacy Fast ALFA Survey: The Galaxy Population Detected by
  ALFALFA
  Making use of HI 21 cm line measurements from the ALFALFA survey (alpha.40)
and photometry from the Sloan Digital Sky Survey (SDSS) and GALEX, we
investigate the global scaling relations and fundamental planes linking stars
and gas for a sample of 9417 common galaxies: the alpha.40-SDSS-GALEX sample.
In addition to their HI properties derived from the ALFALFA dataset, stellar
masses (M_*) and star formation rates (SFRs) are derived from fitting the
UV-optical spectral energy distributions. 96% of the alpha.40-SDSS-GALEX
galaxies belong to the blue cloud, with the average gas fraction f_HI =
M_HI/M_* ~ 1.5. A transition in SF properties is found whereby below M_* ~
10^9.5 M_sun, the slope of the star forming sequence changes, the dispersion in
the specific star formation rate (SSFR) distribution increases and the star
formation efficiency (SFE) mildly increases with M_*. The evolutionary track in
the SSFR-M_* diagram, as well as that in the color magnitude diagram are linked
to the HI content; below this transition mass, the star formation is regulated
strongly by the HI. Comparison of HI- and optically-selected samples over the
same restricted volume shows that the HI-selected population is less evolved
and has overall higher SFR and SSFR at a given stellar mass, but lower SFE and
extinction, suggesting either that a bottleneck exists in the HI to H_2
conversion, or that the process of SF in the very HI-dominated galaxies obeys
an unusual, low efficiency star formation law. A trend is found that, for a
given stellar mass, high gas fraction galaxies reside preferentially in dark
matter halos with high spin parameters. Because it represents a full census of
HI-bearing galaxies at z~0, the scaling relations and fundamental planes
derived for the ALFALFA population can be used to assess the HI detection rate
by future blind HI surveys and intensity mapping experiments at higher
redshift.
