Properties and Distribution of the Darkest Dwarf Galaxies in the ALFALFA Survey

Amélie Saintonge (Cornell) and the ALFALFA Consortium

We present results from the Arecibo Legacy Fast ALFA Survey (ALFALFA). Using early data from this large blind extragalactic HI survey, we have identified a population of gas-rich, low-mass dwarf galaxies. These HI detections are matched with very low optical surface brightness counterparts, and as such represent a previously poorly studied population of objects. We have obtained high quality long-slit optical spectra to determine their chemical abundances and assess their star formation histories. With large mass-to-light ratios, these are among the "darkest" galaxies detected by ALFALFA. We compare the properties of these galaxies found in groups and on the outskirts of clusters to those found in very low density cosmic environments.