Discovery of Massive Evolved Galaxies atz> 3 in the Hubble Ultra Deep Field

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Abstract
We have identified six early-type galaxies at z>2.8 in the central 5.76 arcmin^2 Hubble Ultra Deep Field NICMOS region based on a pronounced broad-band discontinuity between the NICMOS F110W and F160W bandpasses. These galaxies have red observed optical and near-infrared colors (AB_F775W - AB_F160W >= 2) that cannot be explained entirely by dust reddening (indicating advanced age), and their luminosities at rest-frame optical wavelengths suggest a substantial stellar mass. One of the galaxies is detected in X-ray and is likely to have an active nuclear region, while the rest are estimated to be at least 1 Gyr old at z ~ 3 and contain total stellar mass of (0.4 - 9.1) x 10^{10} h^{-2} M_\odot. We calculate a cumulative comoving stellar mass density of rho_star = 0.7 - 1.2 x 10^7 h M_\odot Mpc^{-3} for massive evolved galaxies of M > 10^9 h^{-2} M_\odot at z>2.5. Comparison of the stellar mass density confined in galaxies of different types shows that massive evolved galaxies contribute ~ 16 - 28% of total stellar mass density even at these early epochs. Finally, an analysis of their morphology observed in the ACS and NICMOS images shows that the active galaxy has compact structure, while the rest are less concentrated. The presence of massive evolved galaxies at z ~ 3, when the universe was only 2 Gyr old, suggests that early star formation may have been particularly efficient in massive halos.Comment: 14 pages, 3 figures, ApJ in pres