The Arecibo Legacy Fast ALFA Survey: The Galaxy Population Detected by ALFALFA

Making use of H I 21 cm line measurements from the ALFALFA survey (α.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 α.40-SDSS- GALEX sample. In addition to their H I 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 α.40-SDSS- GALEX galaxies belong to the blue cloud, with the average gas fraction f H I ≡ M H I / M * ~ 1.5. A transition in star formation (SF) properties is found whereby below M * ~ 10 9.5 M ☉ , 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 H I content; below this transition mass, the SF is regulated strongly by the H I . Comparison of H I and optically selected samples over the same restricted volume shows that the H I -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 H I -to-H 2 conversion or that the process of SF in the very H I -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 H I -bearing galaxies at z ~ 0, the scaling relations and fundamental planes derived for the ALFALFA population can be used to assess the H I detection rate by future blind H I surveys and intensity mapping experiments at higher redshift.