Gas-Rich Galaxies In The Alfalfa Survey: From Hi Dwarfs To Giants

Making use of galaxy catalog generated by the ALFALFA survey ([alpha].40) and photometry from the Sloan Digital Sky Survey (SDSS) and GALEX, etc., we investigate the gas, stellar, and star formation (SF) properties of HI-selected galaxies in the local universe. In addition to the HI 21 cm line measurements, stellar masses (M* ) and star formation rates (SFRs) are derived from fitting the UVoptical spectral energy distributions. We examine 229 low HI mass dwarf galaxies, including a complete sample of 176 galaxies with HI masses < 107.7 M[CIRCLED DOT OPERATOR] and HI line widths < 80 km s[-]1 . A large fraction of the dwarfs have high specific star formation rates (SSFRs) and estimates of their SFRs and M* obtained by SED fitting are systematically smaller than ones derived via standard formulae assuming a constant SFR. The increased dispersion of the SSFR distribution at M* 108 M[CIRCLED DOT OPERATOR] is driven by a set of dwarf galaxies that have low gas fractions and SSFRs; some of these are dE/dSphs in the Virgo cluster. 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. 96% of the [alpha].40-SDSS-GALEX galaxies belong to the blue cloud, with the average gas fraction fHI ≡ MHI /M* ~ 1.5. A transition in SF properties is found whereby below M* ~ 109.5 M[CIRCLED DOT OPERATOR] , 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 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 H2 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 ([lamda]s). We present an H[alpha] dataset of 29 HIghMass galaxies identified from the [alpha].40 catalog as HI massive disks with extraordinary fHI s. The high gas fractions may due to suppressed SF in the past as a result of the high [lamda] halos. The sample contains several low surface brightness galaxies, which are strong candidates of the galaxies in high [lamda] halos. Alternatively, some others may be attributed to late cold gas accretion given the extended UV disks or the centrally-peaked SF history. There is no evidence of abnormal behavior of massive star formation. The integrated SFRs demonstrate that the HIghMass galaxies exhibit healthy ongoing SF despite of potentially inactive SF in the past. However, the SF activities are spread throughout extended disks so that they have overall lower SFR surface densities and lower surface brightness in the R-band. The majority of HIghMass galaxies have higher EWs and being bluer in the outer disks, implying inside-out disk growth. Downbending double exponential disks are more frequent than upbending disks among the gas-rich galaxies. SF thresholds exist in the downbending disks probably as a result of concentrated gas distribution