THE DRAMATIC SIZE EVOLUTION OF ELLIPTICAL GALAXIES AND THE QUASAR FEEDBACK

Observations have shown 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 greater than or similar to 3, is attained after similar to 40 dynamical times, corresponding to similar to 2 Gyr. This means that massive galaxies observed at z >= 1 will settle on the fundamental plane by z similar to 0.8-1. In less massive galaxies (M(*) less than or similar to 2 x 10(10) M(circle dot)), 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 similar to 2 in 10 Gyr, although a growth by a factor of similar or equal to 1.6 occurs within the first 0.5 Gyr. Since observations are focused on relatively old galaxies, with ages greater than or similar to 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.