Galaxy downsizing evidenced by hybrid evolutionary tracks

Abstract

The stellar-dark halo mass relation of galaxies at different redshifts, Ms(Mh, z), encloses relevant features concerning their physical processes and evolution. This sequence of relations, defined in the range 0 < z < 4, together with average λ cold dark matter (λCDM) halo mass aggregation histories (MAHs), is used here for inferring average galaxian hybrid evolutionary tracks (GHETs), where "hybrid" remarks on the combination of observational (Ms) and theoretical (Mh) ingredients. As a result of our approach, a unified picture of stellar and halo mass buildup, population migration, and downsizing of galaxies as a function of mass is presented. The inferred average Ms growth histories (GHETs) of the highest and lowest mass galaxies are definitively quite different from the average MAHs, Mh(z), of the corresponding dark halos. Depending on how a given Mh(z) compares with the mass at which the Ms-to-Mh ratio curve peaks at the epoch z, M hp(z), two evolutionary phases are evidenced: (1) galaxies in an active regime of Ms growth when Mh < Mhp and (2) galaxies in a quiescent or passive regime when Mh > M hp. The typical Ms at which galaxies transit from the active (star-forming) to the quiescent regime, Mtran, increases with z, log(Mtran/M⊙) ≈ 10.30 + 0.55z, making evident a population downsizing phenomenon. This result agrees with independent observational determinations based on the evolution of the galaxy stellar mass function decomposition into blue and red galaxy populations. The specific star formation rate (SSFR) predicted from the derivative of the GHET is consistent with direct measures of the SSFR for galaxies at different redshifts, though both sets of observational inferences are independent. The average GHETs of galaxies smaller than Mtran at z = 0 (Ms ≈ 1010.3M⊙, Mh ≈ 1011.8M⊙) did not reach the quiescent regime, and for them, the lower the mass, the faster the laterMs growth rate (downsizing in SSFR). The GHETs allow us to predict the transition rate in the number density of active to passive population; the predicted values agree with direct estimates of the growth rate in the number density for the (massive) red population up to z ∼ 1. We show that λCDM-based models of disk galaxy evolution, including feedback-driven outflows, are able to reproduce the low-mass side of the Ms-Mh relation at z ∼ 0, but at higher z strongly disagree with the GHETs: models do not reproduce the strong downsizing in SSFR and the high SSFR of low-mass galaxies. © 2010. The American Astronomical Society.