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 (${\Sigma }_e\equiv M_{\ast }/2\pi r_e^2$). 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 $6.5\lesssim \log {\Sigma }_e/{\mathrm{M}}_{\odot }{\mathrm{kpc}}^{-2}\lesssim 10$ 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 $\log M_{\ast }/{\mathrm{M}}_{\odot }\ge 9.4$ 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 $d\log r_e/dt=1/2d\log M_{\ast }/dt$ 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 ${\Sigma }_e$ 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.

arXiv comment11 pages, 9 figures; Accepted to ApJ. This version replaces "How Strong is the Evidence for Density-Triggered Quenching?" (posted August 2016) to reflect revisions during refereeing. All results are unchanged, but the text now includes modeling, data, and appendices not present in the original manuscript. A movie of the size--mass evolution (Figure 4) is available upon request

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