HI Cosmology in the Local Universe with ALFALFA

The Arecibo Legacy Fast ALFA (ALFALFA) survey is an ongoing 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. As of mid2007, ∼4500 good quality extragalactic HI line sources have been extracted in ∼15% of the final survey area. ALFALFA is detecting HI masses as low as 10M and as high as 10 M with positional accuracies typically better than 20, allowing immediate identification of the most probable optical counterparts. Only 3% of all extragalactic HI sources and fewer than 1% of detections with MHI > 10 M cannot be identified with a stellar component. First ALFALFA results already suggest, in agreement with previous studies, that there does not appear to be a cosmologically significant population of optically dark but HI rich galaxies. ALFALFA promises a wealthy dataset for the exploration of many issues in near-field cosmology and galaxy evolution studies, setting the stage for their extension to higher redshifts with the Square Kilometer Array (SKA).