The Arecibo Legacy Fast ALFA (ALFALFA) Extragalactic HI Survey

The Arecibo Legacy Fast ALFA (ALFALFA) survey is a program aimed at obtaining a census of HI-bearing objects over a cosmologically significant volume of the local universe. When complete in ∼3–4 years, it will cover 7000 square degrees of high latitude sky using the 305 m telescope and the seven-beam Arecibo L-band feed array (ALFA). As of May 1, 2008, almost 60% of the required observations are complete and a catalog exists in preliminary form for 25% of the final sky area. ALFALFA is detecting about twice as many HI sources as predicted based on previously published HI mass functions and should deliver a final catalog of > 25000 extragalactic HI sources. ALFALFA will detect hundreds of galaxies with HI masses less than 10M⊙ and similarly large numbers greater than 10M⊙. Its centroiding accuracy allows for the immediate identification of highly probably optical counterparts to each HI detection. Fewer than 3% of all extragalactic HI sources, and < 1% of ones with MHI > 10M⊙ cannot be identified with a stellar counterpart. The hundreds of HI sources with observed line widths of 20− 30 km s−1 include a population of optically faint dwarf galaxies. The objects with highest HI masses exhibit a range of morphologies, optical colors and surface brightnesses, but most appear to be massive disk systems. The latter represent the population likely to dominate future studies of HI at high redshift.