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 Lambda cold dark matter (Lambda CDM) framework. The effective radius R-e depends on stellar mass M as R-e(M) alpha M-alpha with alpha similar to 0.5 at all redshifts. The redshift evolution of the mass- or SDSS-normalized size can be reproduced as alpha(1 + z)(beta) with beta similar to -1, with the most massive ETGs possibly showing the fastest evolutionary rate (beta similar to -1.4). This size evolution slows down significantly to beta similar to -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 Lambda CDM 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.