Must Starforming Galaxies Rapidly Get Denser Before They Quench?

Abstract

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 ($\Sigma_e\equiv M_*/2\pi r_{e}^{2}$). Results from two fully empirical analyses and one quenching-free model calculation support this claim at all $z\leq3$: (1) Qualitatively, galaxies' mean $U-V$ colors at $6.5\lesssim\log\Sigma_e/{\rm M_\odot}\,{\rm kpc}^{-2}\lesssim10$ have reddened at rates/times correlated with $\Sigma_e$, implying that there is no density threshold at which galaxies turn red but that $\Sigma_e$ 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 $\Sigma_e$, 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 $\log M_*\geq10$. 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 $\Sigma_e$ 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.