Making use of HI 21 cm line measurements from the ALFALFA survey (α.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 α.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 α.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 ⊙ , 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.
We present the catalog of ∼31500 extragalactic HI line sources detected by the completed ALFALFA survey out to z < 0.06 including both high signal-to-noise ratio (> 6.5) detections and ones of lower quality which coincide in both position and recessional velocity with galaxies of known redshift. We review the observing technique, data reduction pipeline, and catalog construction process, focusing on details of particular relevance to understanding the catalog's compiled parameters. We further describe and make available the digital HI line spectra associated with the catalogued sources. In addition to the extragalactic HI line detections, we report nine confirmed OH megamasers and ten OH megamaser candidates at 0.16 < z < 0.22 whose OH line signals are redshifted into the ALFALFA frequency band. Because of complexities in data collection and processing associated with the use of a feed-horn array on a complex single-dish antenna in the terrestrial radio frequency interference environment, we also present a list of suggestions and caveats for consideration by users of the ALFALFA extragalactic catalog for future scientific investigations.
We present a current catalog of 21 cm HI line sources extracted from the Arecibo Legacy Fast Arecibo L-band Feed Array (ALFALFA) survey over ∼2800 deg 2 of sky: the α.40 catalog. Covering 40% of the final survey area, the α.40 catalog contains 15855 sources in the regionsOf those, 15041 are certainly extragalactic, yielding a source density of 5.3 galaxies per deg 2 , a factor of 29 improvement over the catalog extracted from the HI Parkes All Sky Survey. In addition to the source centroid positions, HI line flux densities, recessional velocities and line widths, the catalog includes the coordinates of the most probable optical counterpart of each HI line detection, and a separate compilation provides a crossmatch to identifications given in the photometric and spectroscopic catalogs associated with the Sloan Digital Sky Survey Data Release 7. Fewer than 2% of the extragalactic HI line sources cannot be identified with a feasible optical counterpart; some of those may be rare OH megamasers at 0.16 < z <0.25. A detailed analysis is presented of the completeness, width dependent sensitivity function and bias inherent of the α.40 catalog. The impact of survey selection, distance errors, current volume coverage and local large scale structure on the derivation of the HI mass function is assessed. While α.40 does not yet provide a completely representative sampling of cosmological volume, derivations of the HI mass function using future data releases from ALFALFA will further improve both statistical and systematic uncertainties.
We use both an HI-selected and an optically-selected galaxy sample to directly measure the abundance of galaxies as a function of their "baryonic" mass (stars + atomic gas). Stellar masses are calculated based on optical data from the Sloan Digital Sky Survey (SDSS) and atomic gas masses are calculated using atomic hydrogen (HI) emission line data from the Arecibo Legacy Fast ALFA (ALFALFA) survey. By using the technique of abundance matching, we combine the measured baryonic function (BMF) of galaxies with the dark matter halo mass function in a ΛCDM universe, in order to determine the galactic baryon fraction as a function of host halo mass. We find that the baryon fraction of lowmass halos is much smaller than the cosmic value, even when atomic gas is taken into account. We find that the galactic baryon deficit increases monotonically with decreasing halo mass, in contrast with previous studies which suggested an approximately constant baryon fraction at the low-mass end. We argue that the observed baryon fractions of low mass halos cannot be explained by reionization heating alone, and that additional feedback mechanisms (e.g. supernova blowout) must be invoked. However, the outflow rates needed to reproduce our result are not easily accommodated in the standard picture of galaxy formation in a ΛCDM universe.Stellar mass is not always the dominant baryonic component in a galaxy. In fact, the HI-
We examine the global properties of the stellar and HI components of 229 low HI mass dwarf galaxies extracted from the ALFALFA survey, including a complete sample of 176 galaxies with HI masses < 10 7.7 M ⊙ and HI line widths < 80 km s −1 . SDSS data are combined with photometric properties derived from GALEX to derive stellar masses (M * ) and star formation rates (SFRs) by fitting their UV-optical spectral energy distributions (SEDs). In optical images, many of the ALFALFA dwarfs are faint and of low surface brightness; only 56% of those within the SDSS footprint have a counterpart in the SDSS spectroscopic survey. 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 * 10 8 M ⊙ 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. The imposition of an upper HI mass limit yields the selection of a sample with lower gas fractions for their M * than found for the overall ALFALFA population. Many of the ALFALFA dwarfs, particularly the Virgo members, have HI depletion timescales shorter than a Hubble time. An examination of the dwarf galaxies within the full ALFALFA population in the context of global star formation laws is consistent with the general assumptions that gas-rich galaxies have lower star formation efficiencies than do optically selected populations and that HI disks are more extended than stellar ones.1 Based on observations made with the Arecibo Observatory and the NASA Galaxy Evolution Explorer (GALEX). The Arecibo Observatory is The 11HUGS sample is complete in HI mass only above 2 × 10 8 M ⊙ , becoming rapidly incomplete at smaller HI masses. To develop further the current understanding of how the gas supply regulates star formation in the lowest mass systems, a larger sample of extreme dwarf galaxies is needed. Making use of the Arecibo L-band Feed Array (ALFA), the on-going Arecibo Legacy Fast ALFA (ALFALFA) extragalactic HI line survey is specially designed to identify low mass, gas rich objects in the local universe . Because of its combination of wide areal coverage, sensitivity, and velocity resolution, ALFALFA has already detected more than 400 galaxies with HI masses M HI < 10 8 M ⊙ (Haynes et al. 2011). While star formation is more directly linked to the molecular interstellar component, the detection of CO in low-metallicity dwarfs is difficult (Leroy et al. 2005, and references therein), suggesting further that CO no longer traces H 2 well. Furthermore, in many gas-rich dwarf galaxies, the HI component dominates both the gas as well as the baryonic mass (e.g. Leroy et al. 2007). The combination of HI parameters from ALFALFA with complementary multi-wavelength data contributed by SDSS and GALEX provides an ideal dataset to investigate the abundance and distribution of gas-rich dwarfs and t...
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