Optical spectroscopy of K-selected extremely red galaxies

We have obtained spectroscopic redshifts for 24 sources from a sample of bright, K-selected extremely red objects (EROs) using the Keck I Telescope. These EROs have high-resolution morphologies from the Hubble Space Telescope and were selected with a median K-s magnitude of 18.7 and F814W - K-s > 4 mag (Yan & Thompson 2003). Among the 24 redshifts, the majority (92%) are at 0.9 < z < 1.5. We derived the rest-frame J-band luminosity function at z(median) = 1.14. Our result suggests that the luminosity evolution between bright EROs at z similar to 1 and the present-day greater than L* massive galaxies is at most about 0.7 mag. Combining the morphologies and deep spectroscopy revealed that the 24 EROs have the following properties: ( 1) Eighty-six percent of the spectra have absorption features from old stars, suggesting that the dominant stellar populations seen in the rest-frame UV are old stars. Fifty percent of the sources have pure absorption lines, while the remaining 50% have emission lines, indicating recent star formation. We conclude that the color criterion for EROs is very effective in selecting old stellar populations at z similar to 1, and a large fraction of these systems with prominent old stellar populations also have recent star formation. ( 2) The 12 emission-line systems have the same number of disk and bulge galaxies as in the remaining 12 pure absorption-line systems. We conclude that spectral classes do not have a simple, direct correspondence with morphological types. ( 3) Three EROs could be isolated, pure passively evolving early-type galaxies at z similar to 1. This implies that only a small fraction (10% - 15%) of early-type galaxies are formed in a rapid burst of star formation at high redshifts and evolved passively since then. ( 4) Three EROs have very red continua and pure emission-line spectra. Their redshifts and star formation rates are similar to that of HR 10. These three sources are potential candidates for dusty starbursts. ( 5) We identified three active galactic nuclei ( 13%), which is consistent with what has been found by deep Chandra observations.