Mufasa: The assembly of the red sequence

Abstract

We examine the growth and evolution of quenched galaxies in the Mufasa cosmo- logical hydrodynamic simulations that include an evolving halo mass-based quench- ing prescription, with galaxy colours computed accounting for line-of-sight extinc- tion to individual star particles. Mufasa reproduces the observed present-day red sequence quite well, including its slope, amplitude, and scatter. In Mufasa, the red sequence slope is driven entirely by the steep stellar mass{stellar metallicity relation, which independently agrees with observations. High-mass star-forming galaxies blend smoothly onto the red sequence, indicating the lack of a well-de ned green valley at M & 1010:5M . The most massive galaxies quench the earliest and then grow very little in mass via dry merging; they attain their high masses at earlier epochs when cold in ows more e ectively penetrate hot halos. To higher redshifts, the red sequence becomes increasingly contaminated with massive dusty star-forming galaxies; UVJ selection subtly but e ectively separates these populations. We then examine the evo- lution of the mass functions of central and satellite galaxies split into passive and star-forming via UVJ. Massive quenched systems show good agreement with obser- vations out to z 2, despite not including a rapid early quenching mode associated with mergers. However, low-mass quenched galaxies are far too numerous at z . 1 in Mufasa, indicating that Mufasa strongly over-quenches satellites. A challenge for hydrodynamic simulations is to devise a quenching model that produces enough early massive quenched galaxies and keeps them quenched to z = 0, while not being so strong as to over-quench satellites; Mufasa only succeeds at the former.