Newly Quenched Galaxies as the Cause for the Apparent Evolution in Average Size of the Population

Abstract

We use the large COSMOS sample of galaxies to study in an internally self-consistent way the change in the number densities of quenched early-type galaxies (Q-ETGs) of a given size over the redshift interval 0.2 < z < 1 in order to study the claimed size evolution of these galaxies. In a stellar mass bin at 10 10.5 < M galaxy < 10 11 M , we see no change in the number density of compact Q-ETGs over this redshift range, while in a higher mass bin at >10 11 M , where we would expect merging to be more significant, we find a small decrease, by ∼30%. In both mass bins, the increase of the median sizes of Q-ETGs with time is primarily caused by the addition to the size function of larger and more diffuse Q-ETGs. At all masses, compact Q-ETGs become systematically redder toward later epochs, with a (U − V ) color difference which is consistent with a passive evolution of their stellar populations, indicating that they are a stable population that does not appreciably evolve in size. We find furthermore, at all epochs, that the larger Q-ETGs (at least in the lower mass bin) have average rest-frame colors that are systematically bluer than those of the more compact Q-ETGs, suggesting that the former are indeed younger than the latter. The idea that new, large, Q-ETGs are responsible for the observed growth in the median size of the population at a given mass is also supported by analysis of the sizes and number of the star-forming galaxies that are expected to be the progenitors of the new Q-ETGs over the same period. In the low mass bin, the new Q-ETGs appear to have ∼30% smaller half-light radii than their star-forming progenitors. This is likely due to the fading of their disks after they cease star formation. Comparison with higher redshifts shows that the median size of newly quenched galaxies roughly scales, at constant mass, as (1 + z) −1 . We conclude that the dominant cause of the size evolution seen in the Q-ETG population is that the average sizes and thus stellar densities of individual Q-ETGs roughly scale with the average density of the universe at the time when they were quenched, and that subsequent size changes in individual objects, through merging or other processes, are of secondary importance, especially at masses below 10 11 M .

The impact of galactic properties and environment on the quenching of central and satellite galaxies: a comparison between SDSS, Illustris and L-Galaxies

“…This results in galaxies which are quenched earlier in the history of the Universe having smaller sizes for their masses, and hence higher central mass concentrations and presumably central velocity dispersions. Therefore, it is possible that the observed strong correlation between quenched fraction and central velocity dispersion is in some sense tautologous, when the size evolution is taken into account (as first argued for in Carollo et al 2013;Lilly & Carollo 2016). It remains to be seen in detail if size evolution alone can lead to the observed trends witnessed in this work.…”

The size evolution of star-forming and quenched galaxies in the IllustrisTNG simulation

“…Instead, given that quenched galaxies at a given mass are observed to be smaller than star-forming ones, the quenched population size growth can be achieved merely by the continuous addition of larger star-forming galaxies as more and more of them quench over time (van Dokkum & Franx 2001;Carollo et al 2013). The picture that emerges from the study presented here is that individual quenched galaxies do indeed grow in size over cosmic time, but as a population they do so at a rate that does not differ very substantially from individual main-sequence galaxies for most of cosmic time.…”

“…The typical size of galaxies with any given selection criteria (and in particular, quenched galaxies in a given mass bin) will evolve with time due to two factors: i) the size evolution of individual galaxies that correspond to the selection, and ii) the transition of galaxies into and outside of the selection. A particular scenario within this context that is extensively debated in the literature is 'progenitor bias', according to which the time evolution of the typical size of quenched galaxies is driven by the appearance of new, large quenched galaxies as a result of the ongoing quenching of (large) star-forming galaxies (Carollo et al 2013;Cassata et al 2013;Krogager et al 2014;Bruce et al 2014). 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).…”

Sizes, colour gradients and resolved stellar mass distributions for the massive cluster galaxies in XMMUJ2235-2557 atz= 1.39

“…It has been shown to have a non-negligible effect on inferred size evolution, and is able to explain part if not all of the observed size evolution (e.g. Saglia et al 2010;Valentinuzzi et al 2010b;Carollo et ρ=−0.26 p=0.14 Figure 13. Dependence of ratio of mass-weighted to light-weighted sizes on different galaxy parameters.…”