A Population of Massive and Evolved Galaxies at z gtrsim 5

Abstract

We report results from a search for massive and evolved galaxies at z gtrsim 5 in the GOODS southern field. Combining HST ACS, VLT ISAAC, and Spitzer IRAC broadband photometric data, we develop a color selection technique to identify candidates for being evolved galaxies at high redshifts. The color selection is primarily based on locating the Balmer break using the K and 3.6 μm bands. Stellar population synthesis models are fitted to the SEDs of these galaxies to identify the final sample. We find 11 candidates with photometric redshifts in the range 4.9 <= z < 6.5, dominated by an old stellar population, with ages 0.2-1.0 Gyr. The stellar masses are in the range (0.5-5) × 10¹¹ M_⊙. One candidate has a spectroscopically confirmed redshift, in good agreement with our photometric redshift. The galaxies are very compact, with half-light radii in the observed K band smaller than ~2 kpc. Seven of the 11 candidates are also detected at 24 μm with the MIPS instrument on Spitzer. While the observed 24 μm emission is consistent with an obscured AGN, we define a "no-MIPS" sample of candidates in addition to the full sample. Results will be quoted for both samples. If the stellar mass estimates are correct, the presence of these massive and evolved galaxies when the universe was ~1 Gyr old could suggest that conversion of baryons into stars proceeded more efficiently in the early universe than it does today.