Everything Every Band All at Once II: The Relationship Between Optical Size and Stellar Mass Over Eight Billion Years of Cosmic History

While the size-mass relation provides insight into the structural evolution of galaxies, the data available and methods employed have hindered our ability to study a detailed and comprehensive description of this key relation across cosmic history. The first paper in this series presents a morphology catalog based on 20 band JWST data in the field of Abell 2744. In this paper we utilize this catalog to measure the size-mass relation from $0.5<z<8$ and $0.5<z<3$ for star-forming and quiescent galaxies respectively. We perform a global fit to our sample using B-splines to flexibly model the redshift evolution which enforces smooth evolution and can account for all observational uncertainties. Symbolic regression is used to derive simple and portable expressions that describe the redshift evolution of the size-mass relation. Analyzing the size evolution of star-forming galaxies in the context of previous work at $z\sim0$ and $z>10$, we discuss three distinct phases: Rapid growth at $z>5$, growth that mimics dark matter halos at $5< z <1$ and a late plateau at $0.5<z<1$. For quiescent galaxies we confirm previous findings that the size-mass relation flattens at $\log\ M_/M_\odot < 10$, which inverts at $z>1$. Our results imply that quiescent galaxies are smaller than their star-forming counterparts only at around $\log M_/M_\odot = 10$; the two populations have similar sizes at lower and higher masses.

💡 Research Summary

This paper presents the second installment of the “Everything Every Band All at Once” series, delivering a comprehensive analysis of the optical size–stellar mass relation from redshift 0.5 to 8 for star‑forming galaxies and from 0.5 to 3 for quiescent galaxies. The authors exploit the unprecedented depth and wavelength coverage of the JWST UNCOVER and MegaScience surveys, which together provide imaging in 20 NIRCam filters over the lensing cluster Abell 2744. Building on the morphology catalog introduced in the first paper, they fit single‑Sérsic models to every source with signal‑to‑noise > 10 using the Bayesian code pysersic, achieving reliable structural parameters and full covariance matrices for > 90 % of the sample.

After rigorous quality cuts (χ²/N < 5, well‑constrained photometric redshifts, sufficient wavelength coverage for rest‑frame 5000 Å, and modest lensing magnifications), the final sample comprises 8,524 star‑forming galaxies and 137 quiescent galaxies. Stellar masses and photometric redshifts are derived with Prospector‑β, and galaxies are separated into star‑forming versus quiescent using synthetic u‑g‑i colors calibrated on the same SED fits. Completeness limits are computed in redshift bins and applied to ensure that only galaxies above the 95 % mass completeness threshold are retained.

The core methodological advance is the replacement of traditional discrete redshift‑bin fitting with a continuous model of the size–mass relation. The authors employ B‑splines to describe the redshift evolution of the relation’s slope, intercept, and intrinsic scatter, allowing the model to enforce smooth evolution while fully propagating uncertainties in redshift, mass, and size (including their covariances). This approach yields a robust estimate of the intrinsic scatter, a quantity highly sensitive to measurement errors and previously difficult to constrain.

Results for star‑forming galaxies reveal three distinct evolutionary phases. (1) Rapid growth at z > 5, where the effective radius increases steeply with cosmic time. (2) A halo‑like growth regime between 5 > z > 1, in which the size evolution mirrors the expected growth of dark‑matter halos (∝ (1+z)⁻¹). (3) A late‑time plateau at 0.5 < z < 1, where size growth slows dramatically. These phases align with theoretical expectations of gas accretion, feedback, and the diminishing role of major mergers at lower redshift.

For quiescent galaxies, the analysis confirms a flattening of the size–mass relation below log M*/M⊙ ≈ 10, consistent with earlier findings, but also uncovers an inversion at z > 1: low‑mass quiescent galaxies become relatively larger than their higher‑mass counterparts at early times. Consequently, the size difference between star‑forming and quiescent populations is pronounced only near log M* ≈ 10; at both lower and higher masses the two populations exhibit comparable effective radii.

To make the results easily usable, the authors apply symbolic regression to distill the B‑spline parameterizations into compact analytic expressions. For example, the mean size of star‑forming galaxies can be expressed as
log R_eff = a log M* + b log (1+z) + c