A combined VANDELS and LEGA-C study: the evolution of quiescent galaxy size, stellar mass, and age from z=0.6 to z=1.3

We study the relationships between stellar mass, size, and age within the quiescent population, using two mass-complete spectroscopic samples with log(10) (M-*/M-circle dot) > 10.3, taken from VANDELS at 1.0 < z < 1.3, and LEGA-C at 0.6 < z < 0.8. Using robust D(n)4000 values, we demonstrate that the well-known 'downsizing' signature is already in place by z similar or equal to 1.1, with D(n)4000 increasing by similar or equal to 0.1 across a similar or equal to 1 dex mass interval for both VANDELS and LEGA-C. We then proceed to investigate the evolution of the quiescent galaxy stellar mass-size relation from z similar or equal to 1.1 to z similar or equal to 0.7. We find the median size increases by a factor of 1.9 +/- 0.1 at log(10)(M-*/M-circle dot) = 10.5, and see tentative evidence for flattening of the relation, finding slopes of alpha = 0.72 +/- 0.06 and alpha = 0.56 +/- 0.04 for VANDELS and LEGA-C, respectively. We finally split our sample into galaxies above and below our fitted mass-size relations, to investigate how size and D(n)4000 correlate. For LEGA-C, we see a clear difference, with larger galaxies found to have smaller D(n)4000 at fixed stellar mass. Due to the faintness and smaller numbers of the VANDELS sample, we cannot confirm whether a similar relation exists at z similar or equal to 1.1. We consider whether differences in stellar age or metallicity are most likely to drive this size-D(n)4000 relation, finding that any metallicity differences are unlikely to fully explain the observed offset, meaning smaller galaxies must be older than their larger counterparts. We find the observed evolution in size, mass, and D(n)4000 across the similar or equal to 2 Gyr from z similar to 1.1 to z similar to 0.7 can be explained by a simple toy model in which VANDELS galaxies evolve passively whilst experiencing a series of minor mergers.

Automated summary

We study the relationships between stellar mass, size, and age within the quiescent population and find evidence of the "downsizing" phenomenon already in place at a redshift of approximately 1.1. We observe an increase in size and tentative evidence for a flattening of the mass-size relation as redshift decreases. Furthermore, we find a correlation between size and D(n)4000, with larger galaxies having smaller D(n)4000 at fixed stellar mass. The observed evolution in size, mass, and D(n)4000 can be explained by passive evolution with minor mergers.