TO BE PUBLISHED BY THE ASTROPHYSICAL JOURNAL LETTERS Preprint typeset using LATEX style emulateapj v. 04/17/13 THE GENTLE GROWTH OF GALAXIES AT HIGH REDSHIFTS IN OVERDENSE ENVIRONMENTS

To be published by the Astrophysical Journal Letters We have explored prevailing modes of galaxy growth for redshifts z ∼ 6 − 14, comparing substantially over-dense 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 environ-ments 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 ∼ 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 ∼ 12%, and ∼ 33 % in the normal regions, at these redshifts. Our median SF rates for ∼ few × 109 M galaxies agrees well with the recently estimated rates for z ∼ 7 galaxies from Spitzer’s SURF-UP survey. Finally, we find that the main difference between the normal and overdense re-gions lies in the amplified growth of massive galaxies in massive dark matter halos. This leads to the formation of> ∼ 1010 M galaxies due to the ∼ 100-fold increase in mass during the above time period. Such galaxies ar