The relation between SFHs, quenching times, and stellar-mass surface density (Σ₁) for early forming galaxies (﹩{z}_{50}\gt 2.9﹩). (a) shows a the mean SFH for galaxies stacked as a function of ﹩\mathrm{log}({{\rm{\Sigma }}}_{1})﹩ in bins of 0.2 dex. (b) shows the cumulative fraction of stellar mass formed. Both (a) and (b) show that galaxies with higher ﹩\mathrm{log}({{\rm{\Sigma }}}_{1})﹩ form more stellar mass earlier with higher peak SFRs, and experience a more rapid decline in their SFR compared to galaxies with lower Σ₁. (c) shows the quenching timescale (t_Q) defined as the time between when the galaxy had formed 50% and 90% of its stellar mass, as a function of Σ₁, with error bars derived from bootstrapping. Galaxies with higher Σ₁ have shorter quenching times. (d) shows the effects of mergers on the SFH timescales. We randomly merged simulated galaxies and measured the change in t_Q and t₅₀ from major mergers (mass ratios >1:4; red) and minor mergers (mass ratios <1:10; blue). The error bars show the inter-68%-tile scatter (68% of the simulations fall in this range).