pop-cosmos : Galaxy size evolution across structural and star-formation classifications in COSMOS-Web

Abstract Galaxy sizes are correlated with stellar mass and redshift, as characterised by size scaling relations. The inferred forms of these scaling relations are sensitive to how galaxies are classified – either by their star formation activity (e.g. specific star-formation rate, sSFR) or by their morphology markers (e.g. bulge-to-total ratio, Sérsic index). We combine stellar mass and sSFR estimates from pop-cosmos (a generative model trained on COSMOS2020 Spitzer IRAC Ch. 1 < 26) with size and morphology measurements from COSMOS-Web, obtaining 99 369 galaxies. By investigating the size–mass and the size–redshift relations, we show that: (i) the sSFR/morphology splits give quantitatively different slopes, intercepts, and intrinsic scatter behaviour; (ii) intrinsic scatter depends on structural morphology but not on sSFR, which constrains the galaxy–halo connection; (iii) the quiescent and bulge-dominated size–mass relations both show double-power law breaks, but at different pivot masses, indicating that quenching and structural transformation occur on different time-scales; (iv) the morphology-dependent trends are only recoverable from space-based imaging. Further, the quiescent pivot mass M* ~ 1010.7 M⊙ coincides with the mass scale at which the active galactic nuclei (AGN, infrared torus) bolometric luminosity fraction peaks in transitioning galaxies, while the bulge-dominated pivot mass M* ~ 1011.1 M⊙ coincides with the halo mass at which AGN-driven baryonic redistribution peaks, which may trace the interval over which AGN feedback ramps from quenching onset to structural transformation.