DEEP SPECTROSCOPY OF ULTRA-STRONG EMISSION-LINE GALAXIES

Abstract

Ultra-strong emission-line galaxies (USELs) with extremely high equivalent widths (EW(Hβ) ≳ 30 Å) 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. Since most have detectable [O iii]λ4363, their metallicities can be determined using the direct method. These large samples hold out the possibility for determining whether there is a metallicity floor for the galaxy population. In this, the second of our papers on the topic, we describe the results of an extensive spectroscopic follow-up of the Kakazu et al. catalog of 542 USELs carried out with the DEIMOS spectrograph on Keck. We have obtained 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 relation between metallicity and the R23 parameter for our sample, and we compare this to the relationship for low-redshift galaxies. The determined metallicity–luminosity relation for this sample is compared with that 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 Hα luminosity function of the USELs as a function of redshift and use this to compute an upper bound on the Lyα emitter luminosity function over the z = 0–1 redshift range.