Structural evolution of quiescent galaxies at intermediate redshifts at UV and red rest-frame wavelengths

We derived structural parameters for a mass-limited sample (M★ > 1010 M⊙) of 27 000 quiescent galaxies with 0.2 < z < 0.6 using grizy photometry from Subaru/Hyper Suprime-Cam and dense spectroscopy from the HectoMAP survey. Based on Sérsic profile fits in all five bands, we modeled the wavelength dependence of the circularized half-light radius (Re, c, a proxy for size) and Sérsic index (n, a proxy for central concentration). We estimated the structural parameters in two rest frames: UV (3500 Å, tracing young and metal-poor stellar populations) and red (7000 Å, tracing the stellar mass distribution). Combined with the stellar mass, redshift, and Dn4000, the estimates of Re, c and n enable us to explore the evolution in the correlations between structural properties and stellar mass for quiescent galaxies with different stellar population ages. At intermediate redshift, quiescent galaxies at all stellar masses exhibit a systematic decline in size and a rise in central concentration with the rest-frame wavelength. Over the full redshift range of the sample, variations in the Sérsic index are larger for galaxies that recently joined the quiescent population (newcomers, with 1.5 < Dn4000 < 1.6) than for descendants of galaxies that are already quiescent at the survey limit, z ∼ 0.6 (aging, or resident, population). Variations in size with the rest-frame wavelength are greater for newcomers than for the resident population, with z < 0.3. The combined evidence supports inside-out quenching as the dominant mechanism halting star formation during this epoch. The normalization of the size-stellar mass relation (the typical size of a M★ ∼ 1011 M⊙ quiescent galaxy) increases by ∼30% between z ∼ 0.6 and z ∼ 0.2 in the red rest frame and remains constant in the rest-frame UV. Size growth is age-independent, and newcomers are found to be ∼20% larger than the aging population across redshifts and rest frames. Based on the Sérsic index-stellar mass relation in the UV, we find that M★ ∼ 1011 M⊙ quiescent galaxies maintain a constant central concentration (n ∼ 4 for the aging population and n ∼ 2 for newcomers). In the red rest frame, both subpopulations exhibit de Vaucouleurs profiles after accounting for selection effects. The larger sizes and disk-like UV profiles of newcomers link them to their direct progenitors in the star-forming population. For the aging population, the steady red rest-frame growth and elevated central concentration in both red and UV rest frames suggest minor mergers with progressively redder satellites at lower redshifts as the primary driver of galaxy evolution in the quiescent phase. Large samples of low-surface-brightness galaxies available in forthcoming sensitive large-area imaging surveys are ideal testbeds for this prediction.