Newly-quenched galaxies as the cause for the apparent evolution in average size of the population

TL;DR

This study investigates the apparent size evolution of quenched early-type galaxies from redshift 1.0 to 0.2, finding that the increase in median size is mainly due to new, larger galaxies joining the population, with minimal size change in individual galaxies.

Contribution

It demonstrates that the observed size growth is primarily caused by the addition of newly quenched, larger galaxies rather than individual galaxy growth, challenging previous interpretations.

Findings

No change in number density of compact Q-ETGs at 10^10.5<M<10^11 Msun.

Decrease in number density of large Q-ETGs at >10^11 Msun by 30%.

Median sizes of newly-quenched galaxies scale with (1+z)^-1.

Abstract

Abridged. We use COSMOS to study in a self-consistent way the change in the number densities of quenched early-type galaxies (Q-ETGs) of a given size over the interval 0.2 < z < 1.0 to study the claimed size evolution of these galaxies. At 10^10.5<Mgalaxy<10^11 Msun, we see no change in the number density of compact Q-ETGs, while at >10^11 Msun we find a 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 towards later epochs, with a (U-V) difference consistent with passive evolution of their stellar populations, indicating that they are a population that does not appreciably evolve in size. At all epochs, the larger Q-ETGs (at least in the lower mass bin) have average rest-frame colors systematically bluer than those of the more compact Q-ETGs, suggesting that the former are 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 supported by the sizes and number of the star-forming galaxies that are expected to be progenitors of the new Q-ETGs over the same period. In the low mass bin, the new Q-ETG have 30% smaller sizes than their star-forming progenitors. This is likely due to the fading of their disks after they cease star-formation. Comparison with higher z shows that the median size of newly-quenched galaxies roughly scales, at constant mass, as (1+z)^-1. The dominant cause of the size evolution seen in the Q-ETG population is thus that the average sizes of individual Q-ETGs scale with the average density of the Universe at the time when they were quenched, with subsequent size changes in individual objects through eg merging of secondary importance, especially at masses <10^11 Msun.