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 similar to 10 in a highly overdense region of the universe. These objects could represent the high-redshift galaxies recently observed by the Hubble's Wide Field Camera 3 and could as well be possible precursors of QSOs at z similar to 6-7. To overcome the sampling and resolution problems in cosmological simulations of these rare regions, we have used the constrained realizations method. Our main result for z similar to 10 shows the high-resolution central region of 3.5 h(-1) Mpc radius in comoving coordinates being completely dominated by disk galaxies in the total mass range of >= 10(9) h M-1(circle dot). 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 compared to z = 0, and the dark matter halos baryon-rich, by a factor of similar to 2-3 above the average fraction of baryons in the universe. The dominance of disk galaxies in the high-density peaks during the epoch of re-ionization is contrary to the morphology-density trend observed at low redshifts.