Galaxies on the Blue Edge

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Abstract
We have successfully constructed a catalog of HI-rich galaxies selected from the Minnesota Automated Plate Scanner Catalog of the Palomar Observatory Sky Survey (POSS I) based solely on optical criteria. We identify HI-rich candidates by selecting the bluest galaxies at a given apparent magnitude, those galaxies on the "blue edge" of POSS I color-magnitude parameter space. Subsequent 21-cm observations on the upgraded Arecibo 305m dish detected over 50% of the observed candidates. The detected galaxies are HI-rich with HI masses comparable to "normal" high surface brightness disk galaxies and they have gas mass-to-light ratios ranging from 0.1 to 4.8 (in solar units). Comparison of our candidate galaxies with known low surface brightness galaxies (hereafter LSBs) shows that they exhibit similar optical and HI properties to that population. We also show that previously identified LSBs, including several LSBs with red B-V colors, preferentially occupy the "blue edge" of POSS I color-magnitude parameter space. Their presence on the "blue edge" appears to be a selection effect due to differing plate limits in the two POSS I bandpasses. This suggests the POSS I is a good filter for separating galaxies on the higher surface brightness end of the LSB population from the general population of galaxies in the night sky.Comment: 56 pages, 19 figures, to be published in the Astronomical Journal (July 1, 2002