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

• Published in: Monthly notices of the Royal Astronomical Society Vol. 422; no. 2; pp. 1714 - 1731
• Main Authors: Nipoti, Carlo, Treu, Tommaso, Leauthaud, Alexie, Bundy, Kevin, Newman, Andrew B., Auger, Matthew W.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.05.2012
• Subjects: galaxies: elliptical and lenticular, cD; galaxies: evolution; galaxies: formation; galaxies: kinematics and dynamics; galaxies: structure; galaxies: elliptical and lenticular, cD; galaxies: formation; galaxies: structure; galaxies: kinematics and dynamics; galaxies: evolution
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2012.20749.x

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.