CfA Colloquium Schedule Fall 2007
 8 November 2007

8 November 2007

Speaker: Martha Haynes (Cornell University)

Title: Cecilia Payne-Gaposchkin Lecture:
HI Cosmology in the Local Universe with ALFALFA

Abstract: 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 4-5 years, ALFALFA will detect more than 25,000 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 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 "harassment" or tidal 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 near-field cosmology and galaxy evolution studies, setting the stage for future extension of these investigations to higher redshifts with the Square Kilometer Array.

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