Building a Red Sequence in Cosmological Hydrodynamic Simulations

Abstract

Despite years of study, the origins of red galaxies are not fully understood in a cosmological context. We develop new models for quenching star-formation and producing red galaxies in cosmological hydrodynamic simulations. We start with phenomenological models applied in post-processing to previously-run simulations. We focus separately on mergers and hot haloes – akin to “quasar mode” and “radio mode” feedback – as the drivers shutting down star-formation. With appropriate parameter choices, each model can produce a reasonably good match observed color-magnitude diagrams and red galaxy luminosity functions at redshift zero. We uncover some difficulties with these models in general, including red galaxy stellar populations that appear too blue by 0.1 magnitudes in g − r due to a metallicity deficit. Building on the post-processing models, we develop quenching models for simulations that run on-the-fly. Again, we test merger quenching and hot halo quenching separately. We model merger feedback as > 1000 km s⁻¹ winds motivated by observations of post-starburst galaxies, and wemodel hot halo feedback by continuously adding thermal energy to circum-galactic gas in haloes dominated by gas above 250, 000 K. Merger quenching temporarily shuts down starformation, butmerger-remnant galaxies typically resume star-formation with 1− 2 Gyrs thanks to accretion of newfuel fromthe IGM.Hot halo quenching successfully produces a realistic red sequence, providing a good match to the observed red galaxy luminosity function. Despite some minor difficulties with hot halo quenching, we examine its effects in more detail. Specifically, we study the evolution of the simulated red sequence over time. We find that galaxies with stellar mass ∼ 10¹¹M⊙ are the first to populate the red sequence at z ≳ 2, with significantly fewer red galaxies around 10¹⁰M⊙ until z ≈ 0. We show that massive galaxies grow substantially after moving onto the red sequence, primarily through minor mergers. We also examine the relationship between quenching and environment.