Cosmic Evolution of Size and Velocity Dispersion for Early-Type Galaxies

Abstract

Massive (stellar mass M ⋆ 3 × 10 10 M ⊙ ), passively evolving galaxies at redshifts z 1 exhibit on the average physical sizes smaller by factors ≈ 3 than local early type galaxies (ETGs) endowed with the same stellar mass. Small sizes are in fact expected on theoretical grounds, if dissipative collapse occurs. Recent results show that the size evolution at z 1 is limited to less than 40%, while most of the evolution occurs at z 1, where both compact and already extended galaxies are observed and the scatter in size is remarkably larger than locally. The presence at high redshift of a significant number of ETGs with the same size as their local counterparts as well as of ETGs with quite small size ( 1/10 of the local one), points to a timescale to reach the new, expanded equilibrium configuration of less than the Hubble time t H (z). We demonstrate that the projected mass of compact, high redshift galaxies and that of local ETGs within the same physical radius, the nominal half−luminosity radius of high redshift ETGs, differ substantially, in that the high redshift ETGs are on the average significantly denser. This result suggests that the physical mechanism responsible for the size increase should also remove mass from central galaxy regions (r 1 kpc). We propose that quasar activity, which peaks at redshift z ∼ 2, can remove large amounts of gas from central galaxy regions on a timescale shorter than, or of order of the dynamical one, triggering a puffing up of the stellar component at constant stellar mass; in this case the size increase goes together with a decrease of the central mass. The size evolution is expected to parallel that of the quasars and the inverse hierarchy, or downsizing, seen in the quasar evolution is mirrored in the size evolution. Exploiting the virial theorem, we derive the relation between the stellar velocity dispersion of ETGs and the characteristic velocity of their hosting halos at the time of formation and collapse. By combining this relation with the halo formation rate at z 1 we predict the local velocity dispersion distribution function. On comparing it to the observed one, we show that velocity dispersion evolution of massive ETGs is fully compatible with the observed average evolution in size at constant stellar mass. Less massive ETGs (with stellar masses M ⋆ 3 × 10 10 M ⊙ ) are expected to evolve less both in size and in velocity dispersion, because their evolution is ruled essentially by supernova feedback, which cannot yield winds as powerful as those triggered by quasars. The differential evolution is expected to leave imprints in the size vs. luminosity/mass, velocity dispersion vs. luminosity/mass, central black hole mass vs. velocity dispersion relationships, as observed in local ETGs.

The Coevolution of Supermassive Black Holes and Massive Galaxies at High Redshift

“…The results of Maiolino et al (2012) suggest that the central regions of the host are deprived by about half of their mass in 10 7 yr. So these outflows must affect the inner structure of the host galaxies, as pointed out by several authors (see Fan et al 2008Fan et al , 2010Fan et al , 2013Damjanov et al 2009;.…”

The dominant role of mergers in the size evolution of massive early-type galaxies sincez ~ 1

“…We assume a = 0.4 ± 0.1 in the following (δσ = 1.32 at z = 1, Cappellari et al 2009;Saglia et al 2010;van de Sande et al 2011). When we apply our simple model using the prescriptions of Fan et al (2010) for the evolution of σ in merger events, μ ≥ 1/10 mergers are only able to explain 15% of the observed evolution, δσ m (1) = 1.05. Hopkins et al (2009b) propose another prescription to trace the evolution in σ from the evolution in size that takes into account the dark matter component of the galaxy,…”

Sizes, colour gradients and resolved stellar mass distributions for the massive cluster galaxies in XMMUJ2235-2557 atz= 1.39

“…If we assume this observed size evolution is genuine, in the sense that it is not completely an effect of progenitor bias, additional physical processes must be in place over redshift to increase the size of the population but not significantly their stellar mass, and at the same time cannot severely disrupt the existing stellar population gradients. The "puffing-up" scenario (or adiabatic expansion) is one of the candidates to explain the size evolution of passive galaxies (Fan et al 2008(Fan et al , 2010. While it may work for increasing the size, it is yet unclear whether it can sufficiently explain the observed evolution of colour gradients.…”