Must Star-forming Galaxies Rapidly Get Denser before They Quench?

Abstract

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 ( ). Results from two fully empirical analyses and one quenching-free model calculation support this claim at all z ≤ 3: (1) qualitatively, galaxies’ mean U–V colors at have reddened at rates/times correlated with , implying that there is no density threshold at which galaxies turn red but that sets the pace of maturation; (2) quantitatively, the abundance of red galaxies never exceeds that of the total population a quenching time earlier at any , implying that galaxies need not transit from low to high densities before quenching; (3) applying to a suite of lognormal star formation histories reproduces the evolution of the size–mass relation at . All results are consistent with evolutionary rates being set ab initio by global densities, with denser objects evolving faster than less-dense ones toward a terminal quiescence induced by gas depletion or other ∼Hubble-timescale phenomena. Unless stellar ages demand otherwise, observed 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.