Abstract
ABSTRACT The mean size (effective radius Re) of massive galaxies (MGs; Mstar > 1011.2M⊙) is observed to increase steadily with cosmic time. It is still unclear whether this trend originates from the size growth of individual galaxies (via, e.g. mergers and/or AGN feedback) or from the inclusion of larger galaxies entering the selection at later epochs (progenitor bias). We here build a data-driven, flexible theoretical framework to probe the structural evolution of MGs. We assign galaxies to dark matter haloes via stellar mass–halo mass (SMHM) relations with varying high-mass slopes and scatters σSMHM in stellar mass at fixed halo mass, and assign sizes to galaxies using an empirically motivated, constant and linear relationship between Re and the host dark matter halo radius Rh. We find that (1) the fast mean size growth of MGs is well reproduced independently of the shape of the input SMHM relation; (2) the numbers of compact MGs grow steadily until z ≳ 2 and fall off at lower redshifts, suggesting a lesser role of progenitor bias at later epochs; (3) a time-independent scatter σSMHM is consistent with a scenario in which compact star-forming MGs transition into quiescent MGs in a few 108 yr with a negligible structural evolution during the compact phase, while a scatter increasing at high redshift implies significant size growth during the star-forming phase. A robust measurement of the size function of MGs at high redshift can set strong constraints on the scatter of the SMHM relation and, by extension, on models of galaxy evolution.
Highlights
There is substantial evidence that galaxies of a given stellar mass are smaller at higher redshift than in the local Universe (e.g. Daddi et al 2005; Trujillo et al 2007; Buitrago et al 2008; van Dokkum et al 2010, 2015; Cassata et al 2011; Cimatti, Nipoti & Cassata 2012; Newman et al 2012; Huertas-Company et al 2013; Kawamata et al 2015; Shibuya, Ouchi & Harikane 2015)
To study the size distribution and evolution of massive galaxies (MGs) (Mstar > 1011.2 M ), at any redshift of interest we build a catalogue of dark matter haloes with mass Mh and size Rh, to which we assign a stellar mass Mstar and an effective radius Re
The degeneracy between δ and σ stellar mass–halo mass (SMHM) in producing the same abundances of MGs was already identified in previous studies (e.g. Behroozi, Conroy & Wechsler 2010; Shankar et al 2014)
Summary
There is substantial evidence that galaxies of a given stellar mass are smaller at higher redshift than in the local Universe (e.g. Daddi et al 2005; Trujillo et al 2007; Buitrago et al 2008; van Dokkum et al 2010, 2015; Cassata et al 2011; Cimatti, Nipoti & Cassata 2012; Newman et al 2012; Huertas-Company et al 2013; Kawamata et al 2015; Shibuya, Ouchi & Harikane 2015). The size evolution of the galaxy population in a given stellar mass bin is well fitted by a relation of the type. It is found that in general star-forming galaxies follow shallower trends (lower values of α) than quiescent galaxies (e.g. van der Wel et al 2014). The size growth rate of star-forming galaxies increases with stellar mass, becoming comparable to that of quiescent galaxies (with α ∼ 1) for Mstar > 1011.2 M (Faisst et al 2017; Mowla et al 2019b). In this mass regime, as pointed out in several studies We focus on the structural evolution of galaxies in this high mass regime, which we label in what follows as ‘MGs’
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