The dramatic size evolution of elliptical galaxies and the quasar feedback

TL;DR

This paper proposes that quasar feedback causes significant size growth in massive elliptical galaxies over cosmic time, explaining the observed size evolution from high redshift to the present.

Contribution

It introduces a model linking quasar feedback to galaxy size evolution, highlighting different mechanisms for massive and less massive galaxies.

Findings

Massive galaxies increase in size by a factor >~ 3 due to quasar feedback.

Less massive galaxies grow in size by up to ~2 times through stellar winds.

Significant velocity dispersion evolution is predicted for both galaxy types.

Abstract

Observations have evidenced that passively evolving massive galaxies at high redshift are much more compact than local galaxies with the same stellar mass. We argue that the observed strong evolution in size is directly related to the quasar feedback, which removes huge amounts of cold gas from the central regions in a Salpeter time, inducing an expansion of the stellar distribution. The new equilibrium configuration, with a size increased by a factor >~ 3, is attained after ~ 40 dynamical times, corresponding to ~ 2 Gyr. This means that massive galaxies observed at z >~ 1 will settle on the Fundamental Plane by z ~ 0.8-1. In less massive galaxies (M_star <~ 2 10^10 M_sun), the nuclear feedback is subdominant, and the mass loss is mainly due to stellar winds. In this case, the mass loss timescale is longer than the dynamical time and results in adiabatic expansion that may increase the effective radius by a factor of up to ~ 2 in 10 Gyr, although a growth by a factor of ~ 1.6 occurs within the first 0.5 Gyr. Since observations are focused on relatively old galaxies, with ages >~ 1 Gyr, the evolution for smaller galaxies is more difficult to perceive. Significant evolution of velocity dispersion is predicted for both small and large galaxies.