Abstract
We develop and test a model for the cosmological role of mergers in the formation and quenching of red, early-type galaxies. By combining theoretically well-constrained halo and subhalo mass functions as a function of redshift and environment with empirical halo occupation models, we predict the distribution of mergers as a function of redshift, environment, and physical galaxy properties. Making the simple 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 and baryon fractions in galaxies at ~ -->L* (without any parameters tuned to this value), as well as the observed mass functions and mass density of red galaxies as a function of redshift, the formation times of early-type galaxies as a function of mass, and the fraction of quenched galaxies as a function of galaxy and halo mass, environment, and redshift. Comparing our model to a variety of semianalytic models in which quenching is primarily driven by halo mass considerations or secular/disk instabilities, we demonstrate that our model makes unique and robust qualitative predictions for a number of observables, including the bivariate red fraction as a function of galaxy and halo mass, the density of passive galaxies at high redshifts, the emergence/evolution of the color-morphology-density relations at high redshift, and the fraction of disky/boxy (or cusp/core) spheroids as a function of mass. In each case, the observations favor a model in which some mechanism quenches future star formation after a major merger builds a massive spheroid. Models where quenching is dominated by a halo mass threshold fail to match the behavior of the bivariate red fractions, predict too low a density of passive galaxies at high redshift, and overpredict by an order of magnitude the mass of the transition from disky to boxy ellipticals. Models driven by secular disk instabilities also qualitatively disagree with the bivariate red fractions, fail to predict the observed evolution in the color-density relations, and predict order-of-magnitude incorrect distributions of kinematic types in early-type galaxies. We make specific predictions for how future observations, for example, quantifying the red fraction as a function of galaxy mass, halo mass, environment, or redshift, can break the degeneracies between a number of different assumptions adopted in present galaxy formation models. We discuss a variety of physical possibilities for this quenching and propose a mixed scenario in which traditional quenching in hot, quasi-static massive halos is supplemented by the strong shocks and feedback energy input associated with a major merger (e.g., tidal shocks, starburst-driven winds, and quasar feedback), 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 (below the traditional threshold for accretion shocks).
Highlights
Recent, large galaxy surveys such as SDSS, 2dFGRS, COMBO-17, and DEEP have demonstrated that the local distribution of galaxies is bimodal with respect to a number of physical properties, including color, morphology, star formation, concentration, and surface brightness, (e.g. Strateva et al 2001), and that this bimodality extends at least to moderate redshifts, z ∼ 1.5 (e.g., Bell et al 2004; Willmer et al 2006) with a significant population of massive, red, passively evolving galaxies at even higher redshifts (Labbé et al 2005; Kriek et al 2006)
Making the simple 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 and baryon fractions in galaxies at ∼ L∗, as well as the observed mass functions and mass density of red galaxies as a function of redshift, the formation times of early-type galaxies 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 semianalytic models in which quenching is primarily driven by halo mass considerations or secular/disk instabilities, we demonstrate that our model makes unique and robust qualitative predictions for a number of observables, including the bivariate red fraction as a function of galaxy and halo mass, the density of passive galaxies at high redshifts, the emergence/evolution of the color-morphology-density relations at high redshift, and the fraction of disky/boxy spheroids as a function of mass
Summary
Large galaxy surveys such as SDSS, 2dFGRS, COMBO-17, and DEEP have demonstrated that the local distribution of galaxies is bimodal with respect to a number of physical properties, including color, morphology, star formation, concentration, and surface brightness, (e.g. Strateva et al 2001), and that this bimodality extends at least to moderate redshifts, z ∼ 1.5 (e.g., Bell et al 2004; Willmer et al 2006) with a significant population of massive, red, passively evolving galaxies at even higher redshifts (Labbé et al 2005; Kriek et al 2006). In many prescriptions (such as the “halo quenching” models to which we refer in § 3.2), it is assumed that the development of a hot halo at this mass threshold is the dominant criterion for quenching Both numerical simulations and analytic calculations (Kauffmann & Haehnelt 2000; Benson et al 2003; Kereš et al 2005, and references therein) argue that this transition alone cannot solve the “cooling flow” problem – namely that the high densities at the core of the pressure-supported hot halo will allow rapid cooling onto the central galaxy, producing large galaxies which are much too massive, gas-rich, disk-dominated, actively star-forming, young and blue relative to the observations. UBV magnitudes are in the Vega system, and SDSS ugriz magnitudes are AB
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