Evolved galaxies at z>1.5 from the Gemini Deep Deep survey:: The formation epoch of massive stellar systems

We present spectroscopic evidence from the Gemini Deep Deep Survey for a significant population of color-selected red galaxies 1.3<z<2.2 at whose integrated light is dominated by evolved stars. Unlike radio-selected objects, the z>1.5 old galaxies have a sky density greater than 0.1 arcmin(-2). Conservative age estimates for 20 galaxies with z>1.3, <z>=1.49 give a median age of 1.2 Gyr and <z(f)>=2.4. One-quarter of the galaxies have inferred z(f)>4. Models restricted to [Fe/H]less than or equal to0 give median ages and z(f) of 2.3 Gyr and 3.3, respectively. These galaxies are among the most massive and contribute similar to50% of the stellar mass density at 1<z<2. The derived ages and most probable star formation histories suggest a high star formation rate (similar to300-500 M-circle dot yr(-1)) phase in the progenitor population. We argue that most of the red galaxies are not descendants of the typical zsimilar to3 Lyman break galaxies. Galaxies associated with luminous submillimeter sources have the requisite star formation rates to be the progenitor population. Our results point toward early and rapid formation for a significant fraction of present-day massive galaxies.