The Arecibo Legacy Fast ALFA Survey: The ALFALFA Extragalactic HI Source Catalog

We present the catalog of similar to 31,500 extragalactic H I line sources detected by the completed Arecibo Legacy Fast ALFA (ALFALFA) survey out to z < 0.06, including both high signal-to-noise ratio (>6.5) detections and ones of lower quality that 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 H I line spectra associated with the cataloged sources. In addition to the extragalactic H I line detections, we report nine confirmed OH megamasers (OHMs) and 10 OHM 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.