HI Cosmology in the Local Universe with ALFALFA

The Arecibo Legacy Fast ALFA (ALFALFA) survey is an on-going second generation blind extragalactic HI survey 
exploiting Arecibo's superior sensitivity, angular resolution and digital technology to conduct a census of the 
local HI universe over a cosmologically significant volume. When complete in 3-4 years, ALFALFA will detect some 
30,000 extragalactic HI line sources out to z~0.06. Its catalog will be especially useful in synergy with wide 
area surveys conducted at other wavelengths. ALFALFA is detecting HI masses as low as 10**6 solar masses and as 
high as 10**10.8 solar masses with positional accuracies typically better than 20 arcsec, allowing immediate
identification of the most probable optical counterpart to each HI detection. In the region of the Virgo Cluster
of galaxies, a number of optically dark HI sources have been found. These all lie in the outskirts of the 
cluster and could be tidal or "harassment" debris, the result of high speed gravitational encounters. First 
ALFALFA results already indicate, in agreement with the suggestions of previous, more limited studies, that 
there is not a cosmologically significant population of optically dark but HI rich galaxies.

However, the majority of ALFALFA detections are too optically faint or of low surface brightness to have been 
included in previous targeted HI surveys. ALFALFA promises a wealthy dataset for the exploration of many issues
in local universe cosmology and galaxy evolution studies, setting the stage for future extension of these 
investigations to higher redshifts with the Square Kilometer Array.