Color dichotomy and gradients in the bulges and disks of EFIGI galaxies along the Hubble sequence

¹ Université Lyon 1, ENS de Lyon, Centre de Recherche Astrophysique de Lyon (UMR5574), 69230 Saint-Genis-Laval, France
² Institut d’Astrophysique de Paris (UMR7095), Sorbonne Université, 98 bis boulevard Arago, 75014 Paris, France

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 21 August 2025
Accepted: 31 March 2026

Abstract

Context. One of the most outstanding questions in contemporary astrophysics is: What is the significance of galaxy morphology? What physical processes underlie the morphologies we observe and is a galaxy’s internal structure shaping its evolution (nature) or is it a sign of the external processes which drive galaxy evolution (nurture)?

Aims. We aim to understand the color dichotomy and gradients in bulges and disks along the Hubble sequence.

Methods. We fit Sérsic functions to the 2D light distributions in the g, r, and i bands to bulges and disks of the large EFIGI (Extraction de Formes Idéalisées de Galaxies en Imagerie) sample of galaxies with high-quality morphological classifications.

Results. In early-type galaxies, bulges and disks have similarly red and nearly uniform colors. Disks become significantly bluer with increasing lateness of their types and bulges get slightly redder because of patchy dust. Disks have increasingly blue colors with increasing radius, whereas dust extinction and scattering leads to smaller effective radii of the bulges and lower steepness of the best-fit Sérsic functions in g versus i. This impact depends on Hubble type, with the bulges of intermediate-type spirals (Sb-Sc) having the reddest mean colors, the largest scatter in their colors, and the largest mean and scatter in their color gradients. Similarly to bulges, disks of the intermediate-type galaxies show the strongest color gradients. The variations in bulge gradients appear to result from the change in the Hubble type rather than in the total galaxy stellar mass.

Conclusions. We interpret these properties of the bulges and disks of intermediate type spirals as being due to dust extinction and scattering, which we hypothesize to be an indicator of the gas content and inflow of gas. If early-type galaxies do evolve from massive spiral galaxies, typically intermediate-type spirals, these color gradients are signs of in situ stellar growth within their bulges, likely driven by morphological structure in their disks. These results favor secular evolution (nature) as the primary driver of galaxy evolution in the local Universe.