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

TL;DR

The paper investigates how the structural parameters of quiescent galaxies at intermediate redshifts depend on rest-frame wavelength and stellar population age. By combining dense MMT/Hectospec spectroscopy from the HectoMAP survey with Subaru/HSC multi-band imaging, the authors derive rest-frame UV ($3500$ Å) and red ($7000$ Å) half-light radii $R_{e,c}$ and Sérsic indices $n$ for a mass-limited sample spanning $0.2<z<0.6$, and examine how these parameters vary with $M_\star$, $z$, and $D_n4000$. They uncover systematic, wavelength-dependent trends: galaxies appear larger and less concentrated in UV than in red, with stronger wavelength sensitivity for newcomers to the quiescent population than for aging members; this supports inside-out quenching and minor-merger-driven growth in the late-time evolution. The analysis yields two rest-frame scaling relations (size–mass and $n$–mass) and reveals pivot masses and redshift trends, showing, for example, a near $30\%$ red-light size growth for $M_\star\sim10^{11} M_\odot$ galaxies from $z\sim0.55$ to $z\sim0.25$, while UV sizes remain largely unchanged. The results provide a statistically robust framework for testing quenching and assembly scenarios and establish a baseline for upcoming wide-area surveys to validate these predictions at higher redshift.

Abstract

We model the wavelength dependence of structural parameters for a mass-limited sample ($M_\star>10^{10}M_\odot$) of $\sim27,000$ 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 estimate the circularized half-light radius $R_{e,c}$ and Sérsic index $n$ in two rest-frames: UV (3500 Å) and red (7000 Å). Combined with $M_\star$, $z$, and D$_n4000$, $R_{e,c}$ and $n$ enable exploration of the evolution in the structural properties - stellar mass correlations for quiescent galaxies with different stellar population ages. At intermediate redshift, quiescent galaxies at all stellar masses show a systematic decline in $R_{e,c}$ and rise in $n$ with rest-frame wavelength. These structural variations are stronger for galaxies that recently joined the quiescent population (newcomers) than for the descendants of galaxies that are already quiescent at the survey limit, $z \sim 0.6$ (aging population). The combined evidence supports inside-out quenching as the dominant mechanism halting star formation during this epoch. The typical size of a $M_\star\sim10^{11}M_\odot$ quiescent galaxy increases by $\sim30\%$ between $z \sim 0.6$ and $z \sim 0.2$ in the red and remains constant in the UV; newcomers are $\sim20\%$ larger than the aging population. In the UV, quiescent galaxies maintain a constant $n\sim4$ for the aging population and $n\sim2$ for newcomers; in the red, both subpopulations have de Vaucouleurs profiles. Our findings link newcomers to their direct progenitors in the star-forming population. For the aging population, we suggest minor mergers with progressively redder satellites at lower redshifts as the primary driver of quiescent galaxy evolution. Forthcoming sensitive large-area imaging surveys will allow testing this prediction.

Figures (13)

• Figure 1: Stellar mass and redshift of HectoMAP galaxies. Blue and orange points in the central panel show $M_\star$ as a function of $z$ for the HectoMAP subsample of color-selected galaxies with measured $M_\star$. The orange points show HectoMAP quiescent galaxies ($D_n4000>1.5$). The blue line indicates the stellar mass limit as a function of redshift based on the survey magnitude limit. Quiescent (orange) galaxies within the region outlined by green solid lines are included in the mass-complete quiescent sample. The upper and right panels respectively show the marginal distributions in $z$ and $M_\star$ for the full (black), quiescent (orange), and mass-complete (green) HectoMAP samples. In these panels, the black dash-dotted, orange dotted, and green solid lines indicate medians of the correspondingly colored distributions. To improve readability in the central panel we show only a randomly selected $30\%$ of the total galaxy sample. Marginal distributions refer to the total samples.
• Figure 2: Half-light radius, $R_e$, and Sérsic index, $n$, in five HSC bands for 26,912 quiescent galaxies. These galaxies have morphological measurements in all five bands and $R_e$ exceeds half of the seeing FWHM. The central panel shows the distributions of $r\text{-}$ band in the $R_e-n$ plane. Galaxies (points) are segregated into two classes of $D_n4000$: $1.5<D_n4000<1.6$ (dark blue) and $D_n4000>1.6$ (green). Black contours show the probability density of HectoMAP galaxies in the $R_e-n$ plane derived from a Gaussian kernel estimator (KDE). The largest to the smallest contour enclose regions where the probability density exceeds the 10th to the 90th percentiles of the distribution. The upper and right panels show the marginal distributions in $R_e$ and $n$, respectively, color-coded by the HSC band. The equivalently color-coded dashed lines indicate the medians of these distributions. To improve readability, the central panel displays a randomly selected $30\%$ of the total sample. Histograms and isocurves are based on the total samples.
• Figure 3: Rate of change in $R_{e,c}$ with rest-frame wavelength. Blue, orange, green, and red circles show the median $\frac{\Delta \log R_{e,c}}{\Delta \log \lambda^{RF}}$ in four bins of increasing redshift listed in the legend. Shadowed bands in matching colors indicate the interquartile ranges of the distributions.
• Figure 4: Same as Fig. \ref{['fig:gradR']} but for the Sérsic index.
• Figure 5: $\frac{\Delta \log R_{e,c}}{\Delta \log \lambda^{RF}}$ as a function of stellar mass and $D_n4000$ for two quiescent subpopulations. Circles show the median rate of change in size with rest-frame wavelength for the subpopulations of aging galaxies (shades of purple and red) and newcomers (blue) in $\Delta\left[\log(M_{\star}/M_\odot)\right]=0.25$ bins of stellar mass. The color of the symbols corresponds to the median $D_n4000$ index of the subpopulation. The panels show the four $\Delta z=0.1$ intermediate redshift bins (as indicated in the panel legends). Error bars denote the interquartile range of the correspondingly colored population. Colors indicate $D_n4000$. The black curve in each panel shows the median rate of change in size with rest-frame wavelength of the total quiescent population (same as Fig. \ref{['fig:gradR']}) and the gray area shows the corresponding interquartile range.