Metal Enrichment in Near-Infrared Luminous Galaxies at z ~ 2: Signatures of Proto-elliptical Galaxies?^*

We present the analysis of the co-added rest-frame UV spectrum (1200 Å < z < 2000 Å) of five K-luminous galaxies at z ~2 from the K20 survey. The composite spectrum is characterized by strong absorption lines over the UV continuum from C, N, O, Al, Si, and Fe in various ionization stages. While some of these lines are interstellar, several among the strongest absorptions are identified with stellar photospheric lines. Most of the photospheric and interstellar features are stronger in the K-luminous composite spectrum than in Lyman break galaxies at z ~3. This suggests higher metallicity and possibly also larger interstellar velocity dispersion caused by macroscopic motions. The absorption lines and the slope of the UV continuum are well matched by the spectrum of the nearby luminous infrared galaxy NGC 6090, which is in the process of merging. A metallicity higher than solar is suggested by comparing the pure photospheric lines (Si III, C III, and Fe V) with starburst models. The evidence of high metallicity, together with the high masses, high star formation rates, and possibly strong clustering, well qualify these galaxies as progenitors of local massive elliptical galaxies.