Photometric identification and MMT spectroscopy of new extremely metal-poor galaxies: towards a better understanding of young stellar populations at low metallicity

Abstract

Extremely metal-poor star-forming galaxies (XMPs) represent one of our only laboratories for study of the low-metallicity stars we expect to encounter at early epochs. But as our understanding of the z > 6 universe has improved, it has become clear that the majority of known XMPs within 100 Mpc host significantly less prominent massive star populations than their reionization-era counterparts, severely limiting their utility as testbeds for interpreting spectral features found at the highest redshifts. Here we present a new photometric selection technique designed to identify nearby XMPs dominated by young stellar populations comparable to those expected in the reionization era. We apply our technique to uncover candidate XMPs in SDSS imaging at magnitudes 16 < i < 23, extending significantly below the completeness limits of the SDSS spectroscopic survey. Spectroscopic observations with the MMT confirm that 32 of the 53 uniformly metal-poor and high specific star formation rate targets we observed have gas-phase oxygen abundances 12 + log O/H < 7.7 (Z/Z(circle dot) < 0.1), including two in the range of the lowest metallicity galaxies known, Z/Z(circle dot) < 0.05. Our observations shed new light on to the long-standing mystery of He II emission in star-forming galaxies: we find that the equivalent width of the He II lambda 4686 high-ionization emission line does not scale with that of H beta in our sample, suggesting that binary evolution or other processes on >10 Myr time-scales contribute substantially to the He+-ionizing photon budget in this metallicity regime. Applying such selection techniques coupled with deep spectroscopy to next-generation photometric surveys like Large Synoptic Survey Telescope may eventually provide a basis for an empirical understanding of metal-poor massive stars.