Spectroscopic identification of massive galaxies at z∼2.3 with strongly suppressed star formation

We present first results of a spectroscopic survey targeting K-selected galaxies at using the Gemini z = 2.0 - 2.7 near-infrared spectrograph (GNIRS). We obtained near-infrared spectra with a wavelength coverage of 1.0 - 2.5 mm for 26 K-bright galaxies (K < 19.7) selected from the Multi-wavelength Survey by Yale-Chile (MUSYC) using photometric redshifts. We successfully derived spectroscopic redshifts for all 26 galaxies using rest-frame optical emission lines or the redshifted Balmer/4000 angstrom break. Twenty galaxies have spectroscopic redshifts in the range 2.0 < z < 2.7, for which bright emission lines like H alpha and [O III] fall in atmospheric windows. Surprisingly, we detected no emission lines for nine of these 20 galaxies. The median 2 sigma upper limit on the rest-frame equivalent width of H alpha for these nine galaxies is similar to 10 angstrom. The stellar continuum emission of these same nine galaxies is best fitted by evolved stellar population models. The best-fit star formation rate (SFR) is zero for five out of nine galaxies and is consistent with zero within 1 sigma for the remaining four. Thus, both the H alpha measurements and the independent stellar continuum modeling imply that 45% of our K-selected galaxies are not forming stars intensely. This high fraction of galaxies without detected line emission and low SFRs may imply that the suppression of star formation in massive galaxies occurs at higher redshift than is predicted by current cold dark matter (CDM) galaxy formation models. However, obscured star formation may have been missed, and deep mid-infrared imaging is needed to clarify this situation.