2021ApJ...919...57H

We use ZFIRE and ZFOURGE observations with the spectral energy distribution fitting tool PROSPECTOR to reconstruct the star formation histories (SFHs) of protocluster and field galaxies at z ∼ 2 and compare our results to the TNG100 run of the IllustrisTNG cosmological simulation suite. In the observations, we find that massive protocluster galaxies (log[M_*/M_☉] > 10.5) form 45% ± 8% of their total stellar mass in the first 2 Gyr of the universe, compared to 31% ± 2% formed in the field galaxies. In both observations and simulations, massive protocluster galaxies have a flat/declining SFH with decreasing redshift compared to rising SFH in their field counterparts. Using IllustrisTNG, we find that massive galaxies (log[M_*/M_☉]≥ 10.5) in both environments are on average ≃190 Myr older than low-mass galaxies (log[M_*/M_☉]=9–9.5). However, the difference in mean stellar ages of cluster and field galaxies is minimal when considering the full range in stellar mass (log[M_*/M_☉]≥ 9). We explore the role of mergers in driving the SFH in IllustrisTNG and find that massive cluster galaxies consistently experience mergers with low gas fraction compared to other galaxies after 1 Gyr from the big bang. We hypothesize that the low gas fraction in the progenitors of massive cluster galaxies is responsible for the reduced star formation.