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

We present results of near-IR spectroscopic measurements of seven star-forming galaxies at 2.1 < z < 2.5. Drawn from a large spectroscopic survey of galaxies photometrically preselected by their UnGℛ colors to lie at z ~ 2, these galaxies were chosen for their bright rest-frame optical luminosities (Ks ≤ 20.0). Most strikingly, the majority of the sample of seven galaxies exhibit [N II]/Hα nebular emission-line ratios indicative of at least solar H II region metallicities, at a look-back time of 10.5 Gyr. The broadband colors of the Ks-bright sample indicate that most have been forming stars for more than a gigayear at z ~ 2 and have already formed stellar masses in excess of 1011 M☉. 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 Ks-bright z ~ 2 galaxies appear to be highly evolved systems, their large Hα luminosities and uncorrected Hα star formation rates of 24-60 M☉ yr-1 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.