Galaxies on the Blue Edge

Abstract

We have successfully constructed a catalog of H I–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 H I–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 305 m dish detected over 50% of the observed candidates. The detected galaxies are H I–rich, with H I 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 H I 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.