Testing Galaxy Quenching Theories with Scatter in the Stellar to Halo Mass Relation

TL;DR

This paper uses the scatter in the stellar-to-halo mass relation to test galaxy quenching models, finding that low observed scatter constrains the quenching mechanisms and suggests a tight correlation with galaxy properties.

Contribution

It introduces a framework to test galaxy quenching models based on scatter in stellar mass at fixed halo mass, ruling out many models and highlighting the importance of mass thresholds.

Findings

Most quenching models are inconsistent with observed low scatter.

A stellar mass threshold model remains viable under current observational constraints.

Tight correlations between galaxy properties and formation history are predicted.

Abstract

We use the scatter in the stellar-to-halo mass relation to constrain galaxy evolution models. If the efficiency of converting accreted baryons into stars varies with time, halos of the same present-day mass but different formation histories will have different z=0 galaxy stellar mass. This is one of the sources of scatter in stellar mass at fixed halo mass, $\sigma_{\log M\ast}$. For massive halos that undergo rapid quenching of star formation at z~2, different mechanisms that trigger this quenching yield different values of $\sigma_{\log M\ast}$. We use this framework to test various models in which quenching begins after a galaxy crosses a threshold in one of the following physical quantities: redshift, halo mass, stellar mass, and stellar-to-halo mass ratio. Our model is highly idealized, with other sources of scatter likely to arise as more physics is included. Thus, our test is whether a model can produce scatter lower than observational bounds, leaving room for other sources. Recent measurements find $\sigma_{\log M\ast}=0.16$ dex for 10^11 Msol galaxies. Under the assumption that the threshold is constant with time, such a low value of $\sigma_{\log M\ast}$ rules out all of these models with the exception of quenching by a stellar mass treshold. Most physical quantities, such as metallicity, will increase scatter if they are uncorrelated with halo formation history. Thus, to decrease the scatter of a given model, galaxy properties would correlate tightly with formation history, creating testable predictions for their clustering. Understanding why $\sigma_{\log M\ast}$ is so small may be key to understanding the physics of galaxy formation.