THE ARECIBO LEGACY FAST ALFA SURVEY: THE GALAXY POPULATION DETECTED BY ALFALFA

Making use of HI 21 cm line measurements from the ALFALFA survey (a. 40) and photometry from the Sloan Digital Sky Survey (SDSS) and Galaxy Evolution Explorer (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 data set, 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) equivalent to M-HI/M-* similar to 1.5. A transition in star formation (SF) properties is found whereby below M-* similar to 10(9.5) M-circle dot, 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, is linked to the HI content; below this transition mass, the SF 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 SF 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 similar to 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.