The Gentle Growth of Galaxies at High Redshifts in Overdense Environments

We have explored prevailing modes of galaxy growth for redshifts z similar to 6-14, comparing substantially overdense and normal regions of the universe, using high-resolution zoom-in cosmological simulations. Such rare overdense regions have been projected to host high-z quasars. We demonstrate that galaxies in such environments grow predominantly by a smooth accretion from cosmological filaments which dominates the mass input from major, intermediate, and minor mergers. We find that by z similar to 6, the accumulated galaxy mass fraction from mergers falls short by a factor of 10 of the cumulative accretion mass for galaxies in the overdense regions, and by a factor of 5 in the normal environments. Moreover, the rate of the stellar mass input from mergers also lies below that of an in situ star Formation (SF) rate. The fraction of stellar masses in galaxies contributed by mergers in overdense regions is similar to 12%, and similar to 33% in the normal regions, at these redshifts. Our median SF rates for similar to few x 10(9) M-circle dot galaxies agrees well with the recently estimated rates for z similar to 7 galaxies from Spitzer's SURF-UP survey. Finally, we find that the main difference between the normal and overdense regions lies in the amplified growth of massive galaxies in massive dark matter halos. This leads to the Formation of similar to 10(10) M-circle dot galaxies due to the similar to 100 fold increase in mass during the above time period. Such galaxies are basically absent in the normal regions at these redshifts.