The Gentle Growth of Galaxies at High Redshifts in Overdense Environments

TL;DR

This study uses high-resolution simulations to show that galaxies at high redshifts in overdense regions grow mainly through smooth accretion from cosmic filaments, with mergers playing a minor role, leading to rapid formation of massive galaxies.

Contribution

It reveals that galaxy growth at high redshifts is dominated by smooth accretion rather than mergers, especially in overdense environments, highlighting environment-dependent growth mechanisms.

Findings

Galaxies in overdense regions grow mainly by filamentary accretion.

Merger contribution to galaxy mass is significantly lower than accretion-based growth.

Massive galaxies (>10^{10} Mo) form rapidly in overdense regions, absent in normal regions.

Abstract

We have explored prevailing modes of galaxy growth for redshifts z ~ 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 ~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 X 10^9 Mo 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 regions lies in the amplified growth of massive galaxies in massive dark matter halos. This leads to the formation of >= 10^{10} Mo galaxies due to the ~100-fold increase in mass during the above time period. Such galaxies are basically absent in the normal regions at these redshifts.