1202.0971 (C. Nipoti et al.)

Size and velocity-dispersion evolution of early-type galaxies in a Lambda cold dark matter universe    [PDF]

C. Nipoti, T. Treu, A. Leauthaud, K. Bundy, A. B. Newman, M. W. Auger

Early-type galaxies (ETGs) are observed to be more compact at z>2 than in the local universe. Remarkably, much of this size evolution appears to take place in a short \sim1.8 Gyr time span between z\sim2.2 and z\sim1.3, which poses a serious challenge to hierarchical galaxy formation models where mergers occurring on a similar timescale are the main mechanism for galaxy growth. We compute the merger-driven redshift evolution of stellar mass Mstar\propto(1+z)^aM, half-mass radius Re\propto(1+z)^aR and velocity-dispersion sigma0\propto(1+z)^asigma predicted by concordance Lambda Cold Dark Matter for a typical massive ETG in the redshift range z=1.3-2.2. Neglecting dissipative processes, and thus maximizing evolution in surface density, we find -1.52. Furthermore, we find that the scatter introduced in the size-mass correlation by the predicted merger-driven growth is difficult to reconcile with the tightness of the observed scaling law. We conclude that -- barring unknown systematics or selection biases in the current measurements -- minor and major mergers with spheroids are not sufficient to explain the observed size growth of ETGs within the standard model.

View original: http://arxiv.org/abs/1202.0971