ALFALFA: HI Cosmology in the Local Universe
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 2012, ALFALFA will have detected more than 30,000 extragalactic HI line sources out to z~0.06, and its catalog will be especially useful in synergy with wide area surveys conducted at other wavelengths. ALFALFA is detecting HI masses as low as 106 solar masses and as high as 1010.8 solar masses with positional accuracies typically better than 20 arcsec, allowing immediate identification of the most probable optical counterpart to each HI detection. ALFALFA results already suggest, in agreement with the suggestions of previous, more limited studies, that there does not appear to be a cosmologically significant population of optically dark but HI rich galaxies; fewer than 2% of the extragalactic HI line sources detected by ALFALFA cannot be identified with a feasible optical counterpart. 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. Additionally, ALFALFA has identified a intriguing population of massive galaxies with unusually high fractions of their baryons in the form of cool HI gas: how have these galaxies managed to maintain such massive HI disks without converting their gas into stars? At the low mass end, ALFALFA has already detected several hundred galaxies with HI masses less than 108 solar masses and we have identified a class of ultracompact high velocity clouds which match the predictions for gas-bearing Local Group mini-halos. The 2011 ALFALFA sample approaches homogeneity but is not yet a perfectly representative sample of the nearby volume; the full ALFALFA survey, when completed, will be. 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.
Professor Riccardo Giovanelli
Professor Martha Haynes