A Cosmological Framework for the Co-Evolution of Quasars, Supermassive Black Holes, and Elliptical Galaxies: II. Formation of Red Ellipticals

TL;DR

This paper presents a cosmological model linking galaxy mergers to the formation and quenching of red ellipticals, successfully explaining observed galaxy properties and favoring merger-driven quenching over halo mass or secular processes.

Contribution

It introduces a model where major mergers quench star formation, predicting galaxy evolution features and contrasting with other quenching mechanisms, supported by observational data.

Findings

Model reproduces the red galaxy mass functions and formation times.

Observations favor merger-driven quenching over halo mass or secular processes.

Predicts the evolution of color-morphology-density relations at high redshift.

Abstract

(Abridged) We develop and test a model for the cosmological role of mergers in the formation and quenching of red, early-type galaxies. Making the ansatz that star formation is quenched after a gas-rich, spheroid-forming major merger, we demonstrate that this naturally predicts the turnover in the efficiency of star formation at ~L_star, as well as the observed mass functions/density of red galaxies as a function of redshift, the formation times of spheroids as a function of mass, and the fraction of quenched galaxies as a function of galaxy and halo mass, environment, and redshift. Comparing to a variety of semi-analytic models in which quenching is primarily driven by halo mass considerations or secular/disk instabilities, we demonstrate that our model and different broad classes of models make unique and robust qualitative predictions for a number of observables, including the red fraction as a function of galaxy and halo mass, the density of passive galaxies and evolution of the color-morphology-density relations at high z, and the fraction of disky/boxy spheroids as a function of mass. In each case, the observations favor a model in which galaxies quench after a major merger builds a massive spheroid, and disfavor quenching via secular or pure halo processes. We discuss a variety of physical possibilities for this quenching, and propose a mixed scenario in which traditional quenching in hot, massive halos is supplemented by the feedback associated with star formation and quasar activity in a major merger, which temporarily suppress cooling and establish the conditions of a dynamically hot halo in the central regions of the host, even in low mass halos.