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

May 2018 • 2018ApJ...858...40A

Authors • Abramson, L. E. • Morishita, T.

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 ({{{Σ }}}_e\equiv {M}_*/2π {r}_e²). 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 6.5 ≲ {log}{{{Σ }}}_e/{\text{}}{M}_⊙ {kpc}}^-2≲ 10 have reddened at rates/times correlated with {{{Σ }}}_e, implying that there is no density threshold at which galaxies turn red but that {{{Σ }}}_e sets the pace of maturation; (2) quantitatively, the abundance of {log}{M}_*/{\text{}}{M}_⊙≥slant 9.4 red galaxies never exceeds that of the total population a quenching time earlier at any {{{Σ }}}_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}_*/{\text{}}{M}_⊙≥slant 10. 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 {{{Σ }}}_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.

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Takahiro Morishita

Assistant Scientist