We present a homogeneous compilation of H i spectral parameters extracted from global 21 cm line spectra for some 9000 galaxies in the local universe ( heliocentric velocity À200 < V < 28;000 km s À1 ) obtained with a variety of large single-dish radio telescopes but reanalyzed using a single set of parameter extraction algorithms. Corrections to the observed H i line flux for source extent and pointing offsets and to the H i line widths for instrumental broadening and smoothing are applied according to model estimates to produce a homogenous catalog of derived properties with quantitative error estimates. Where the redshift is available from optical studies, we also provide flux measurements for an additional 156 galaxies classified as marginal H i detections and rms noise limits for 494 galaxies classified as nondetections. Given the diverse nature of the observing programs contributing to it, the characteristics of the combined data set are heterogeneous, and as such, the compilation is neither integrated H i line flux nor peak flux limited. However, because of the large statistical base and homogenous reprocessing, the spectra and spectral parameters of galaxies in this optically targeted sample can be used to complement data obtained at other wavelengths to characterize the properties of galaxies in the local universe and to explore the large-scale structures in which they reside.

The Arecibo Legacy Fast ALFA (ALFALFA) survey has completed source extraction for 40% of its total sky area, resulting in the largest sample of H i-selected galaxies to date. We measure the H i mass function from a sample of 10,119 galaxies with 6.2 < log(M H i /M ) < 11.0 and with well-described mass errors that accurately reflect our knowledge of low-mass systems. We characterize the survey sensitivity and its dependence on profile velocity width, the effect of large-scale structure, and the impact of radio frequency interference in order to calculate the H i mass function with both the 1/V max and 2DSWML methods. We also assess a flux-limited sample to test the robustness of the methods applied to the full sample. These measurements are in excellent agreement with one another; the derived Schechter function parameters are φ * (h 3 70 Mpc −3 dex −1 ) = 4.8 ± 0.3 × 10 −3 , log(M * /M ) + 2 log h 70 = 9.96 ± 0.02, and α = −1.33 ± 0.02. We find70 , 16% larger than the 2005 HIPASS result, and our Schechter function fit extrapolated to log(M H i /M ) = 11.0 predicts an order of magnitude more galaxies than HIPASS. The larger values of Ω H i and of M * imply an upward adjustment for estimates of the detection rate of future large-scale H i line surveys with, e.g., the Square Kilometer Array. A comparison with simulated galaxies from the Millennium Run and a treatment of photoheating as a method of baryon removal from H i-selected halos indicate that the disagreement between dark matter mass functions and baryonic mass functions may soon be resolved.

The recently initiated Arecibo Legacy Fast ALFA (ALFALFA) survey aims to map $7000 deg 2 of the high Galactic latitude sky visible from Arecibo, providing a H i line spectral database covering the redshift range between À1600 and 18,000 km s À1 with $5 km s À1 resolution. Exploiting Arecibo's large collecting area and small beam size, ALFALFA is specifically designed to probe the faint end of the H i mass function in the local universe and will provide a census of H i in the surveyed sky area to faint flux limits, making it especially useful in synergy with wide-area surveys conducted at other wavelengths. ALFALFA will also provide the basis for studies of the dynamics of galaxies within the Local Supercluster and nearby superclusters, allow measurement of the H i diameter function, and enable a first wide-area blind search for local H i tidal features, H i absorbers at z < 0:06, and OH megamasers in the redshift range 0:16 < z < 0:25. Although completion of the survey will require some 5 years, public access to the ALFALFA data and data products will be provided in a timely manner, thus allowing its application for studies beyond those targeted by the ALFALFA collaboration. ALFALFA adopts a two-pass, minimum intrusion, drift scan observing technique that samples the same region of sky at two separate epochs to aid in the discrimination of cosmic signals from noise and terrestrial interference. Survey simulations, which take into account large-scale structure in the mass distribution and incorporate experience with the ALFA system gained from tests conducted during its commissioning phase, suggest that ALFALFA will detect on the order of 20,000 extragalactic H i line sources out to z $ 0:06, including several hundred with H i masses M H i < 10 7:5 M .

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