Evidence for Solar Metallicities in Massive Star-forming Galaxies at z>~2

Abstract

We present results of near-IR spectroscopic measurements of 7 star-forming galaxies at 2.1<z<2.5. Drawn from a large spectroscopic survey of galaxies photometrically pre-selected by their U_nGR colors to lie at z~2, these galaxies were chosen for their bright rest-frame optical luminosities (K_s<=20.0). Most strikingly, the majority of the sample of 7 galaxies exhibit [NII]/Ha nebular emission line ratios indicative of at least solar HII region metallicities, at a lookback time of 10.5 Gyr. The broadband colors of the K_s-bright sample indicate that most have been forming stars for more than a Gyr at z~2, and have already formed stellar masses in excess of 10^11 Msun. The descendants of these galaxies in the local universe are most likely metal-rich and massive spiral and elliptical galaxies, while plausible progenitors for them can be found among the population of z~3 Lyman Break Galaxies. While the K_s-bright z~2 galaxies appear to be highly evolved systems, their large Ha luminosities and uncorrected Ha star-formation rates of 24-60 Msun/yr indicate that active star formation is still ongoing. The luminous UV-selected objects presented here comprise more than half of the high-redshift (z>1.5) tails of current K-band-selected samples such as the K20 and Gemini Deep Deep surveys.