2006ApJ...651...24Y

Using the deep Spitzer Infrared Array Camera (IRAC) observations of the Great Observatories Origins Deep Survey (GOODS), we study the stellar masses and star formation histories of galaxies at z~6 based on the i₇₇₅-band dropout sample selected from the GOODS fields. In total, we derive stellar masses for 53 i₇₇₅-band dropouts that have robust IRAC detections. These galaxies have typical stellar masses of ∼10¹⁰ M_☉and typical ages of a couple of hundred million years, consistent with earlier results based on a smaller sample of z~6 galaxies. The existence of such massive galaxies at z~6 can be explained by at least one set of N-body simulations of the hierarchical paradigm. We also study 79 i₇₇₅-band dropouts that are invisible in the IRAC data and find that they are typically less massive by a factor of 10. These galaxies are much bluer than those detected by the IRAC, indicating that their luminosities are dominated by stellar populations with ages ≲40 Myr. Based on our mass estimates, we derive a lower limit to the global stellar mass density at z~6, which is 1.1-6.7x10⁶ M_☉/Mpc3. The prospect of detecting the progenitors of the most massive galaxies at yet higher redshifts is explored. We also investigate the implication of our results for reionization and find that the progenitors of the galaxies comparable to those in our sample, even in the most optimized (probably unrealistic) scenario, cannot sustain the reionization for a period longer than ∼2 Myr. Thus most of the photons required for reionization must have been provided by other sources, such as the progenitors of the dwarf galaxies that are far below our current detection capability.