Return to [Log-]Normalcy: Rethinking Quenching, The Star Formation Main Sequence, and Perhaps Much, Much More

Louis E. Abramson, Michael D. Gladders, Alan Dressler, Augustus Oemler, Bianca Poggianti, Benedetta Vulcani

Published 2016-03-31Version 1

Knowledge of galaxy evolution rests on cross-sectional observations of different objects at different times. Understanding of galaxy evolution rests on longitudinal interpretations of how these data relate to individual objects moving through time. The connection between the two is generally assumed to be clear, but we use a simple "physics-free" model to show that it is not and that exploring its nuances can lead to new insights. Comprising nothing more than $\sim2000$ loosely constrained lognormal star formation histories, the model faithfully reproduces the following data it was never designed to match: stellar mass functions at $z\leq8$; the slope of the star formation rate/stellar mass relation (the SF "Main Sequence") at $z\leq6$; the mean ${\rm sSFR}(\equiv{\rm SFR}/M_*)$ of low-mass galaxies at $z\leq7$; "fast-" and "slow-track quenching"; galaxy downsizing; and a correlation between formation timescale and ${\rm sSFR}(M_*,t)$ similar to recent results from simulations that provides a natural connection to bulge growth. We take these surprising findings -- which ultimately imply that quenching is the "peeling-off" of the densest tail of the starforming population at any epoch -- to mean either that: (1) models in which galaxies are diversified on long timescales by something akin to initial conditions rival the dominant "grow-and-quench" framework as good descriptions of the data; or (2) cornerstone metrics of galaxy evolution are too information-poor -- if not intrinsically misleading -- to confirm a unique explanation. We outline future tests of our model but stress that, even if it is shown to be incorrect, it illustrates how exploring different evolutionary paradigms can aid learning and, we hope, more detailed modeling efforts.