Searching for the Lowest Metallicity Galaxies
  We report a method of identifying candidate low-metallicity blue compact
dwarf galaxies (BCDs) from the Sloan Digital Sky Survey (SDSS) imaging data and
present 3-m Lick Observatory and 10-m W.M. Keck Observatory optical
spectroscopic observations of 94 new systems that have been discovered with
this method. The candidate BCDs are selected from Data Release 12 (DR12) of
SDSS based on their photometric colors and morphologies. Using the Kast
spectrometer on the 3-m telescope, we confirm that the candidate
low-metallicity BCDs are emission-line galaxies and we make metallicity
estimates using the empirical R and S calibration methods. Follow-up
observations on a subset of the lowest-metallicity systems are made at Keck
using the Low Resolution Imaging Spectrometer (LRIS), which allow for a direct
measurement of the oxygen abundance. We determine that 45 of the reported BCDs
are low-metallicity candidates with 12 + log(O/H) <= 7.65, including six
systems which are either confirmed or projected to be among the
lowest-metallicity galaxies known, at 1/30 of the solar oxygen abundance, or 12
+ log(O/H) ~ 7.20.
