Spitzer IRAC confirmation of z850-dropout galaxies in the Hubble Ultra Deep Field:: Stellar masses and ages at z≈7

Using Spitzer IRAC mid-infrared from the Great Observations Origins Deep Survey, we study z(850)-dropout sources in the Hubble Ultra Deep Field. After carefully removing contaminating flux from foreground sources, we clearly detect two z(850) dropouts at 3.6 and 4.5 mu m, while two others are marginally detected. The mid-infrared fluxes strongly support their interpretation as galaxies at z approximate to 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) x 10(10) L-circle dot, stellar masses (1-10) x 10(9) M-circle dot, stellar ages 50-200 Myr, star formation rates up to similar to 25 M-circle dot yr(-1,) and low reddening A(p) < 0.4. Overall, the z = 7 galaxies appear substantially less massive and envolved 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 p, = 1.6(-0.8)(+1.6) x 10(6) M-circle dot MPC-3 (to 0.3L*(z=3)), in apparent agreement with recent cosmological hydrodynamical 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 greater than or similar to 8, during the era of cosmic reiconization, 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.