Fast evolving size of early-type galaxies at z > 2 and the role of dissipationless (dry) merging




We present the analysis of a large sample of early-type galaxies (ETGs) at 0 < z < 3 aimed at tracing the cosmic evolution of their size and compare it with a model of pure dissipationless (dry) merging in the Λ cold dark matter (ΛCDM) framework. The effective radius Re depends on stellar mass inline image as inline image with α∼ 0.5 at all redshifts. The redshift evolution of the mass- or SDSS-normalized size can be reproduced as ∝(1 +z)β with β∼−1, with the most massive ETGs possibly showing the fastest evolutionary rate (β∼−1.4). This size evolution slows down significantly to β∼−0.6 if the ETGs at z > 2 are removed from the sample, suggesting an accelerated increase of the typical sizes at z > 2, especially for the ETGs with the largest masses. A pure dry merging ΛCDM model is marginally consistent with the average size evolution at 0 < z < 1.7, but predicts descendants too compact for z > 2 progenitor ETGs. This opens the crucial question on what physical mechanism can explain the accelerated evolution at z > 2, or whether an unclear observational bias is partly responsible for that.