Measuring the Evolution of Bulge, Disk and Colour Gradients in HST Observations of Galaxies with 3D Modelling

Abstract

We measure galaxy structural properties and colour gradients using HST images to trace the evolution of galaxy components. We jointly fit 3D bulge and disk models to 2505 galaxies in GOODS-South across seven bands (bvizYJH) to IAB = 25.5, accounting for different component ellipticities and inclination-dependent dust extinction. Extinction strongly affects structural parameters and colour gradients in ~26% of the sample - primarily edge-on galaxies with central obscuration (B-band face-on optical depth tau ~ 4) that reveal clear bulge components in the near-infrared. Despite irregular morphologies, the model captures observed colour gradients well. Bulges at z ~ 1 differ markedly from z ~ 0, with typical Sersic index n ~ 1.0 and bulge-to-disc size ratio Re/hd ~ 0.15, suggesting most galaxies host pseudo-bulges formed via secular evolution. Galaxy ellipticity correlates strongly with disk scale-length and absolute magnitude, partly driven by dust extinction variations. We trace bulge and disk evolution from z ~ 0 to z ~ 2.5: bulges are redder than disks (observed-frame) at z < 1.4, but colours converge at higher redshifts and fainter magnitudes. Redder galaxies show redder cores relative to their outskirts, and brighter galaxies have redder cores.