Must Starforming Galaxies Rapidly Get Denser Before They Quench?

Using the deepest data yet obtained, we find no evidence preferring compaction-triggered quenching—where rapid increases in galaxy density truncate star formation—over a null hypothesis in which galaxies age at constant surface density (${\mathrm{\Sigma }}_e\equiv M_{\ast }\mathrm{/}2\pi r_e^2$). Results from two fully empirical analyses and one quenching-free model calculation support this claim at all $z\le 3$: (1) Qualitatively, galaxies’ mean $U-V$ colors at $6.5\lesssim \log {\mathrm{\Sigma }}_e\mathrm{/}{\mathrm{M}}_{\odot }{\mathrm{k}\mathrm{p}\mathrm{c}}^{-2}\lesssim 10$ have reddened at rates/times correlated with ${\mathrm{\Sigma }}_e$, implying that there is no density threshold at which galaxies turn red but that ${\mathrm{\Sigma }}_e$ sets the pace of maturation; (2) Quantitatively, the abundance of $\log M_/{\rm M_\odot}\geq9.4$ red galaxies never exceeds that of the total population a quenching time earlier at any ${\mathrm{\Sigma }}_e$, implying that galaxies need not transit from low to high densities before quenching; (3) Applying $d\log r_{e}/dt =1/2,d\log M_/dt$ to a suite of lognormal star formation histories reproduces the evolution of the size–mass relation at $\log M_{\ast }\ge 10$. All results are consistent with evolutionary rates being set ab initio by global densities, with denser objects evolving faster than less-dense ones towards a terminal quiescence induced by gas depletion or other $\sim$Hubble-timescale phenomena. Unless stellar ages demand otherwise, observed ${\mathrm{\Sigma }}_e$ thresholds need not bear any physical relation to quenching beyond this intrinsic density–formation epoch correlation, adding to Lilly & Carollo’s arguments to that effect.