THE RELATIVE ABUNDANCE OF COMPACT AND NORMAL MASSIVE EARLY-TYPE GALAXIES AND ITS EVOLUTION FROM REDSHIFTz∼ 2 TO THE PRESENT

Abstract

We report on the evolution of the number density and size of early-type galaxies (ETGs) from z ∼ 2 to z ∼ 0. We select a sample of 563 massive (M > 10 10 M ), passively evolving (specific star formation rate <10 −2 Gyr −1 ), and morphologically spheroidal galaxies at 0 < z < 2.5, using the panchromatic photometry and spectroscopic redshifts available in the Great Observatories Origins Deep Surveys fields. We combine Advanced Camera for Surveys and Wide Field Camera 3 Hubble Space Telescope images to study the morphology of our galaxies in their optical rest frame in the entire 0 < z < 2.5 range. We find that throughout the explored redshift range the passive galaxies selected with our criteria have weak morphological K-correction, with size being slightly smaller in the optical than in the UV rest frame (by ∼20% and ∼10% at z > 1.2 and z < 1.2, respectively). We measure a significant evolution of the mass-size relation of ETGs, with a fractional increment that is almost independent of the stellar mass. ETGs formed at z > 1 appear to be preferentially small, and the evolution of the mass-size relation at z < 1 is driven by both the continuous size growth of the compact galaxies and the appearance of new ETGs with large sizes. We also find that the number density of all passive ETGs increases rapidly, by a factor of five, from z ∼ 2 to z ∼ 1, and then more mildly by another factor of 1.5 from z ∼ 1 to z ∼ 0. We interpret these results as evidence that the bulk of the ETGs are formed at 1 < z < 3 through a mechanism that leaves very compact remnants. At z < 1 the compact ETGs grow gradually in size, becoming normal-size galaxies, and at the same time new ETGs with normal-large sizes are formed.

The dominant role of mergers in the size evolution of massive early-type galaxies sincez ~ 1

“…The fact that the more compact galaxies at z ∼ 0.1 ) and z ∼ 1 (Martinez-Manso et al 2011) have similar young ages (∼1−2 Gyr), combined with their paucity in the local universe Taylor et al 2010;Cassata et al 2011) also support the size evolution of these systems along cosmic time. Mergers, specially the minor ones, have been proposed to explain this evolution (e.g., Naab et al 2009;Bezanson et al 2009;Hopkins et al 2010b;Weinzirl et al 2011).…”

“…The number density of massive (red) galaxies at z = 2 is ∼15−30% of that in the local universe (e.g., Arnouts et al 2007;Pérez-González et al 2008;Williams et al 2010;Ilbert et al 2010), and those ETGs that have reached the red sequence at later times are systematically more extended than those which did it at high redshift. This effect is called the progenitor bias and mimic a size growth (see van der Wel et al 2009a;Valentinuzzi et al 2010a,b;Cassata et al 2011, for further datails). Both van der Wel et al (2009a) and Saglia et al (2010) estimate that the progenitor bias of massive ETGs accounts for a factor 1.25 (δr e = 0.8) of the size evolution since z = 1, and we assume this value in the following.…”

“…Massive ETGs as compact as observed at high redshifts are rare in the local universe Taylor et al 2010;Cassata et al 2011), suggesting that they must evolve since z ∼ 2 to the present. It has been proposed that high redshift compact galaxies are the cores of present day ellipticals, and that they increased their size by adding stellar mass in the outskirts of the galaxy Hopkins et al 2009a;van Dokkum et al 2010).…”

Ultramassive dense early-type galaxies: Velocity dispersions and number density evolution sincez= 1.6

“…Daddi et al 2005;Trujillo et al 2006Trujillo et al , 2007Longhetti et al 2007;Zirm et al 2007;Toft et al 2007;Franx et al 2008;Cimatti et al 2008;Buitrago et al 2008;van der Wel et al 2008van der Wel et al , 2014Bernardi 2009;Damjanov et al 2009;Bezanson et al 2009;Mancini et al 2009Mancini et al , 2010Cassata et al 2011;Damjanov et al 2011;Bruce et al 2012;Szomoru et al 2012;Newman et al 2012;Cassata et al 2013;Belli et al 2014). For example, van der Wel et al (2014) find that the mean effective radius R e of passive galaxies selected with a colour−colour criterion increases as R e ∝ (1 + z) −1.48 over the redshift range 0 < z < 3.…”

“…Buitrago et al 2013) includes in the sample a significant portion (>30%) of disk galaxies (e.g. van der Wel et al 2011;Ilbert et al 2010;Cassata et al 2011;Tamburri et al 2014) and, concurrently, does not include all of the galaxies that are visually classified as ellipticals. At the same time, a selection based on a cut in sSFR that is constant with time (usually sSFR < 10 −11 yr −1 ) selects predominantly red spheroids at z ∼ 2, but includes a large contamination of disk/spiral galaxies in the local Universe (Szomoru et al 2011).…”