# MUFASA: The assembly of the red sequence

## TL;DR

Mufasa simulations effectively reproduce the observed red sequence of galaxies, revealing its dependence on stellar metallicity and illustrating galaxy quenching processes across cosmic time.

## Contribution

This study introduces a cosmological hydrodynamic simulation with an evolving halo mass-based quenching model that accurately captures the red sequence's properties and evolution.

## Key findings

- Red sequence slope driven by stellar metallicity relation.
- High-mass galaxies quench early and grow via dry mergers.
- Simulation over-quenches low-mass satellites at z<1.

## Abstract

We examine the growth and evolution of quenched galaxies in the Mufasa cosmological hydrodynamic simulations that include an evolving halo mass-based quenching prescription, with galaxy colours computed accounting for line-of-sight extinction to individual star particles. Mufasa reproduces the observed present-day red sequence reasonably 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-defined green valley at M*>10^10.5 Mo. 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 inflows more effectively penetrate hot halos. To higher redshifts, the red sequence becomes increasingly contaminated with massive dusty star-forming galaxies; UVJ selection subtly but effectively separates these populations. We then examine the evolution of the mass functions of central and satellite galaxies split into passive and star-forming via UVJ. Massive quenched systems show good agreement with observations 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's current scheme fails at the latter.