Deep Spectroscopy of Ultra-Strong Emission Line Galaxies

Ultra strong emission-line galaxies (USELs) with extremely high equivalent widths (EW(H beta) > 30A) can be used to pick out galaxies of extremely low metallicity in the z=0-1 redshift range. Large numbers of these objects are easily detected in deep narrow band searches and, since most have detectable [OIII] 4363, their metallicities determined using the direct method. These large samples hold the possibility for determining if there is a metallicity floor for the galaxy population. Here we describe results of an extensive spectroscopic follow-up of the Kakazu et al. (2007) catalog of 542 USELs using the DEIMOS spectrograph on Keck, with high S/N spectra of 348 galaxies. The two lowest metallicity galaxies in our sample have 12+log(O/H)=6.97+/-0.17 and 7.25+/-0.03 -- values comparable to the lowest metallicity galaxies found to date. We determine an empirical metallicity-R23 parameter relation for our sample, and compare this to the relationship for low redshift galaxies. The determined metallicity-luminosity relation is compared with those of magnitude selected samples in the same redshift range. The emission line selected galaxies show a metal-luminosity relation where the metallicity decreases with luminosity and they appear to define the lower bound of the galaxy metallicity distribution at a given continuum luminosity. We also compute the H alpha luminosity function of the USELs as a function of redshift and use this to compute an upper bound on the Ly alpha emitter luminosity function over the z=0-1 redshift range.