Gas-Rich Dwarf Galaxies from the PSS-II --- II. Optical Properties

We describe the optical properties of a sample of 101 gas-rich field dwarf galaxies found on PSS-II (Second Palomar Sky Survey) plates, most newly discovered as part of a survey to investigate the clustering properties of dwarf galaxies relative to giants. These galaxies have low surface brightnesses and are relatively distant, with recession velocities ranging up to 10,000 km/s. They have bluer V-I colors (median value of 0.75) than either actively star-forming giant galaxies or low metallicity globular clusters, implying that these dwarfs have both low metallicities and little past star formation. These galaxies are also extremely gas rich, with a median HI mass to V luminosity ratio of approximately 2 in solar units. We divide the sample into two groups: true dwarfs with diameters (at 25 I mag arcsec^-2) less than 7.5 kpc and Magellanic dwarfs with diameters greater than that value. The true dwarfs have greater HI mass to V luminosity ratios and slightly bluer V-I colors than the Magellanic dwarfs. Overall, the optical properties of our sample of dwarf galaxies point towards their being quiescent objects that have undergone little star formation over the age of the universe. They are not faded objects, but instead may be going through one of their first periods of weak star formation.