Size and velocity-dispersion evolution of early-type galaxies in a Λ cold dark matter universe

ABSTRACT

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 ∼1.8 Gyr time span between z∼ 2.2 and 1.3, which poses a serious challenge to hierarchical galaxy formation models where mergers occurring on a similar time-scale are the main mechanism for galaxy growth. We compute the merger-driven redshift evolution of stellar mass , half-mass radius and velocity dispersion predicted by concordance Λ 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 −1.5 ≲a_M≲−0.6, −1.9 ≲a_R≲−0.7 and 0.06 ≲a_σ≲ 0.22, under the assumption that the accreted satellites are spheroids. It follows that the predicted z∼ 2.2 progenitors of z∼ 1.3 ETGs are significantly less compact (on average a factor of ∼2 larger R_e at given M_*) than the quiescent galaxies observed at z≳ 2. 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.