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

TL;DR

The paper develops a 3D bulge–disk modelling framework to measure colour gradients in galaxies observed with the Hubble Space Telescope, incorporating inclination- and wavelength-dependent dust extinction through radiative-transfer-based weights. By jointly fitting seven-band HST images of 2505 GOODS-South galaxies, it derives bulge and disk structural parameters, ellipticities, and colour gradients, revealing that bulges at z~1 are typically pseudo-bulges with $n\,\approx\,1$ and $R_e/h_d\approx0.15$, and that extinction is significant for ~18–26% of the sample, especially edge-on systems. The results show colour gradients correlated with overall galaxy colour and, to a lesser extent, redshift, with redder cores in redder systems and a trend toward convergence of bulge/disk colours at higher redshift; the work also provides weak-lensing priors based on disk scale lengths and ellipticity relations. Practically, the study demonstrates the feasibility of forward-modelling colour-gradient biases for Euclid/LSST-like surveys and highlights the need to account for dust-extinction effects when interpreting high-redshift galaxy structure and evolution.

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.

Figures (20)

• Figure 1: Distributions for the magnitude-limited sample: (a) redshift, (b) magnitude $I_{AB}$, (c) absolute magnitude $M_I$ and (d) $V-I$ colour. In this work, we examine the dependence of the colour gradient (measured by a model) on each of these quantities.
• Figure 2: The distribution of the galaxy sample in the $z$-$M_I$ plane. The colour scale indicates the normalised density of galaxies.
• Figure 3: Data flow diagram showing the processes that are followed to translate the public HST images through to the output galaxy catalogs.
• Figure 4: Relation between inclination angle and ellipticity for different galaxy components, i.e., prolate bulges with different $n$ and unextincted disk models. The ellipticity is computed on the noise-free and PSF-free models for each component using the quadrupole moments described in Sect. \ref{['subsec:method_ellipticity']}.
• Figure 5: Distributions of key galaxy structural parameters, Sérsic index $n_{\rm g}$ (left) and half-light radius of galaxy $R_e^{\rm g}$ (right), from two methods: single component 3D multi-band Sérsic fits (this work) and single-band Sérsic fits with GALFIT by griffith_2012 for $I_{AB} < 25.5$ (see Sec. \ref{['method:single_sersic_fitting']}). The results of the single component fits are also used as starting points for the two-component fits.