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

Abstract

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 LambdaCDM framework. The effective radius R_e depends on stellar mass M as R_e(M) \propto M}^{alpha} with alpha ~ 0.5 at all redshifts. The redshift evolution of the mass- or SDSS-normalized size can be reproduced as \propto (1+z)^beta with beta ~ -1, with the most massive ETGs possibly showing the fastest evolutionary rate (beta ~ -1.4). This size evolution slows down significantly to beta ~ -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 LambdaCDM 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.

MNRAS (Letters), in press, 5 pages, 3 figures

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