Dissecting the size evolution of elliptical galaxies since z∼ 1: puffing-up versus minor-merging scenarios

By Ignacio Trujillo, I. Ferreras, I. G. D. L. Rosa. 2011

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At a fixed stellar mass, the size of low-redshift early-type galaxies is found to be a factor of 2 larger than that of their counterparts at z∼ 1, a result with important implications for galaxy formation models. In this paper, we have explored the buildup of the local mass–size relation of elliptical galaxies using two visually classified samples. At low redshift, we compiled a subsample of 2656 elliptical galaxies from the Sloan Digital Sky Survey, whereas at higher redshift (up to z∼ 1), we extracted a sample of 228 objects from the Hubble Space Telescope/Advanced Camera for Surveys images of the Great Observatories Origins Deep Survey. All the galaxies in our study have spectroscopic data, allowing us to determine the age and mass of the stellar component. Contrary to previous claims in the literature, using the fossil record information contained in the stellar populations of our local sample, we do not find any evidence for an age segregation at a given stellar mass, depending on the size of the galaxies. At a fixed dynamical mass, there is only a ≲9 per cent size difference in the two extreme age quartiles of our sample. Consequently, the local evidence does not support a scenario whereby the present-day mass–size relation has been progressively established via a bottom-up sequence, where older galaxies occupy the lower part of this relation, remaining in place since their formation. We do not find any age-segregation difference in our high-z sample either. Therefore, we find a trend in size that is insensitive to the age of the stellar populations, at least since z∼ 1. This result supports the idea that the stellar mass–size relation is formed at z∼ 1, with all galaxies populating a region which roughly corresponds to 1/2 of the present size distribution. We have explored two possible scenarios for size growth: puffing up or minor merging. The fact that the evolution in size is independent of the stellar age, together with the absence of an increase in the scatter of the relationship with redshift does not support the puffing-up mechanism. The observational evidence, however, cannot reject at this stage the minor-merging hypothesis. We have made an estimation of the number of minor-merger events necessary to bring the high-z galaxies into the local relation compatible with the observed size evolution. Since z= 0.8, if the mass ratio of the merger is 1:3, then we estimate ∼3 ± 1 minor mergers and if the ratio is 1:10, then we obtain ∼8 ± 2 events.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/91622546ba80ac053d48e20bef816a308c34ea0e

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Citations in the corpus

FORMING COMPACT MASSIVE GALAXIES (2015, background): “…dominated by minor mergers: such mergers are expected, and other mechanisms cannot easily produce the observedṙe/Ṁstars≈ 2 scaling between size growth and mass growth (Bezanson et al. 2009; Naab, Johansson, & Ostriker 2009; Hopkins et al. 2010; Trujillo et al. 2011; Hilz, Naab, & Ostriker 2013).”
How do minor mergers promote inside-out growth of ellipticals, transforming the size, density profile and dark matter fraction? (2012, background): “…(Malin & Carter 1983; Schweizer & Seitzer 1992; Tran et al. 2005; van Dokkum 2005; Bell et al. 006a,b; Faber 2007; McIntosh et al. 2008; Whitaker & van Dokkum 2008; Robaina et al. 2010; Duc 2011; Lotz et al. 2011; Trujillo et al. 2011; Weinzirl et al. 2011; Tal et al. 2012; López-Sanjuan 2012).”
The sizes, masses and specific star formation rates of massive galaxies at 1.3 < z < 1.5: strong evidence in favour of evolution via minor mergers (2012, result): “Recent results on this issue in the literature have been controversial, with different studies finding apparently contradictory claims (e.g. Saracco et al. 2009; Trujillo et al. 2011).”
CAN MINOR MERGING ACCOUNT FOR THE SIZE GROWTH OF QUIESCENT GALAXIES? NEW RESULTS FROM THE CANDELS SURVEY (2011, background): “On the other hand, some recent studies at higher redshift have found no sign of such a correlation (Trujillo et al. 2011; Whitaker et al. 2012; but see Saracco et al. 2011).”
THE COSMOLOGICAL SIZE AND VELOCITY DISPERSION EVOLUTION OF MASSIVE EARLY-TYPE GALAXIES (2011, background): “This process seems to be disfavored by observations (e.g. Hopkins et al. 2010b; Trujillo et al. 2011) and the absence of a significant young stellar population would indicate that such hypothesized secular processes would need to occur without significant star formation.”
SUPERDENSE GALAXIES AND THE MASS–SIZE RELATION AT LOW REDSHIFT (2012, methodology): “Since there are previous low-z studies that have used de Vaucoulers DR7 radii (Taylor et al. 2010, Trujillo et al. 2011), in Fig.”
The size evolution of star-forming and quenched galaxies in the IllustrisTNG simulation (2017, result): “Other studies concluded however that ‘progenitor bias’ is sub-dominant compared to the size growth of individual galaxies (Hopkins et al. 2010; Bernardi et al. 2011; Trujillo et al. 2011; Huang et al. 2013).”