GOODS spitzer IRAC observations of high- z galaxies - Implications for reionization

A significant number of Lyman-break galaxies (LBGs) at z ≈ 6 and 5 in the Hubble Ultradeep Field (HUDF) have been detected by the Great Observatories Origins Deep Survey (GOODS) observations with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We find that the SEDs for many of these IRAC-detected high- z galaxies are best fitted by models of stellar populations with masses of a few × 10 ¹⁰ M_⊙, and with ages of a few hundred million years. This implies that at least some of the galaxies that are responsible for finishing reionization at z ≈ 6 started forming their stars at z > 7 and earlier. The lower limits to the space density for high- z galaxies in this mass range are consistent with predictions from recent hydrodynamic simulations of structure formation in a ΛCDM universe. All objects in our samples are consistent with having solar metallicity, suggesting that the sources that contributed ionizing photons at z ≈ 6 might not be comprised of "first stars". Some of these galaxies have rest-frame UV colors that are bluer than those predicted by the stellar population models to which we compare them, and can be explained by the presence of very massive stars (>100 M_⊙). This suggests that very massive stars might play an important role in producing ionizing photons in the early stage of the universe. The analysis of z ≈ 6 LBGs has been extended to the entire GOODS fields, HDF-N and CDF-S, and we report the preliminary results.