Galaxy Formation in Heavily Overdense Regions at z~10: the Prevalence of Disks in Massive Halos

Using a high-resolution cosmological numerical simulation, we have analyzed the evolution of galaxies at z~10 in a highly overdense region of the universe. These objects could represent the high redshift galaxies recently observed by the Hubble's WFC3, and be as well possible precursors of QSOs at z~6-7. To overcome the sampling and resolution problems in cosmological simulations, we have used the Constrained Realizations method. Our main result for z~10 shows the region of 3.5h^{-1}Mpc radius in comoving coordinates completely dominated by disk galaxies in the total mass range of >=10^9h^{-1}Mo. We have verified that the gaseous and stellar disks we identify are robust morphological features, capable of surviving the ongoing merger process at these redshifts. Below this mass range, we find a sharp decline in the disk fraction to negligible numbers. At this redshift, the disks appear to be gas-rich and the dark matter halos baryon-rich, by a factor of ~2-3 above the average fraction of baryons in the universe. The prevalence of disk galaxies in the high density peaks during the epoch of reionization is contrary to the morphology-density trend observed at low redshifts.
To be published by the Astrophysical Journal Letters, 5 pages, 3 figures. Small corrections following the referee report