  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 305m 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 10**7.5 solar masses and similarly large numbers greater
than 10**10.3 Msun. 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 HI masses
> 10**9.5 Msun cannot be identified with a stellar counterpart. The hundreds of
HI sources with observed line widths of 20-30 km/s 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.
