2006ApJ...649L..67L

Using Spitzer IRAC mid-infrared imaging from the Great Observatories Origins Deep Survey, we study z₈₅₀-dropout sources in the Hubble Ultra Deep Field. After carefully removing contaminating flux from foreground sources, we clearly detect two z₈₅₀dropouts at 3.6 and 4.5 µm, while two others are marginally detected. The mid-infrared fluxes strongly support their interpretation as galaxies at z~7, seen when the universe was only 750 Myr old. The IRAC observations allow us for the first time to constrain the rest-frame optical colors, stellar masses, and ages of the highest redshift galaxies. Fitting stellar population models to the spectral energy distributions, we find photometric redshifts in the range 6.7-7.4, rest-frame colors U-V=0.2-0.4, V-band luminosities L_V=(0.6-3)x10¹⁰ L_☉, stellar masses (1-10)x10⁹ M_☉, stellar ages 50-200 Myr, star formation rates up to ∼25 M_☉/yr, and low reddening A_V<0.4. Overall, the z=7 galaxies appear substantially less massive and evolved than Lyman break galaxies or distant red galaxies at z=2-3, but fairly similar to recently identified systems at z=5-6. The stellar mass density inferred from our z=7 sample is ρ_*=1.6^+1.6_–0.8x10⁶ M_☉/Mpc3 (to 0.3L^*_z=3), in apparent agreement with recent cosmological hydrodynamic simulations, but we note that incompleteness and sample variance may introduce larger uncertainties. The ages of the two most massive galaxies suggest that they formed at z≳8, during the era of cosmic reionization, but the star formation rate density derived from their stellar masses and ages is not nearly sufficient to reionize the universe. The simplest explanation for this deficiency is that lower mass galaxies beyond our detection limit reionized the universe.