Abstract
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 Λ cold dark matter (ΛCDM) framework. The effective radius Re depends on stellar mass as 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.
Highlights
Early-type galaxies (ETGs) are important probes of structure formation and massive galaxy evolution
At 0 < z < 1, the ETG stellar mass function shows a downsizing evolution apparently difficult to reproduce with the current models of galaxy formation, with the majority of massive ETGs (M > 1011 M⊙) already in place at z ≈ 0.7, (Pozzetti et al 2010 and references therein)
The predicted number of major mergers per unit time dNm/dt decreases for decreasing z: for our model ETGs we get, on average, dNm/dt ∼ 0.13(0.23) Gyr−1 at z ∼ 0.55(1.15). These rates are higher by a factor of ∼ 2 than estimated observationally at similar z by Bundy et al (2009; see Lotz et al 2011), indicating that the considered model might be extreme in this respect. Both major and minor mergers contribute significantly to the growth of stellar mass, so, at least within ΛCDM, massive ETGs do not accrete most of their mass in very minor mergers, which would be more effective in increasing the galaxy size
Summary
Early-type galaxies (ETGs) are important probes of structure formation and massive galaxy evolution. ETGs with the same mass (e.g. Daddi et al 2005; Trujillo et al 2006; van der Wel et al 2008; Cimatti et al 2008; Buitrago et al 2008; Saracco, Longhetti & Andreon 2009; Williams et al 2010; Cassata et al 2011; Damjanov et al 2011; Newman et al 2011, and references therein). It is not clear yet whether the environment plays (e.g. Papovich et al 2011; Cooper et al 2011) or not (e.g. Rettura et al 2010) a role in the ETG size evolution.
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