Halo Histories vs. Galaxy Properties at z=0, I: The Quenching of Star Formation

TL;DR

This study investigates the relationship between dark matter halo formation history and galaxy quenching at z=0, finding a small but significant correlation at high masses and little to no correlation at low masses, challenging some theoretical models.

Contribution

It demonstrates that halo formation history influences galaxy quenching at high masses but not at low masses, highlighting the importance of halo age definitions and environmental effects.

Findings

At high stellar masses, quenched fraction increases by about 5% from low to high density environments.

Halo formation history has a minor but significant impact on quenching in high-mass galaxies.

No significant correlation between halo formation history and quenching in low-mass galaxies.

Abstract

We test whether halo age and galaxy age are correlated at fixed halo and galaxy mass. The formation histories, and thus ages, of dark matter halos correlate with their large-scale density $\rho$, an effect known as assembly bias. We test whether this correlation extends to galaxies by measuring the dependence of galaxy stellar age on $\rho$. To clarify the comparison between theory and observation, and to remove the strong environmental effects on satellites, we use galaxy group catalogs to identify central galaxies and measure their quenched fraction, $f_Q$, as a function of large-scale environment. Models that match halo age to central galaxy age predict a strong positive correlation between $f_Q$ and $\rho$. However, we show that the amplitude of this effect depends on the definition of halo age: assembly bias is significantly reduced when removing the effects of splashback halos---those halos that are central but have passed through a larger halo or experienced strong tidal encounters. Defining age using halo mass at its peak value rather than current mass removes these effects. In SDSS data, at M$_{\rm gal}\gtrsim 10^{10.0}$ M_sol/h$^2$, there is a $\sim 5\%$ increase in $f_Q$ from low to high densities, which is in agreement with predictions of dark matter halos using peak halo mass. At lower stellar mass there is little to no correlation of $f_Q$ with $\rho$. For these galaxies, age-matching is inconsistent with the data across the wide range the halo formation metrics that we tested. This implies that halo formation history has a small but statistically significant impact on quenching of star formation at high masses, while the quenching process in low-mass central galaxies is uncorrelated with halo formation history.