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

## TL;DR

This study investigates the color gradients and dichotomy in bulges and disks of EFIGI galaxies along the Hubble sequence, providing insights into galaxy evolution mechanisms.

## Contribution

It offers a detailed analysis of color profiles and gradients in galaxy components, highlighting the role of dust and internal processes in galaxy evolution.

## Key findings

- Bulges and disks in early-type galaxies are similarly red and uniform in color.
- Disks become bluer with increasing radius, especially in intermediate-type spirals.
- Color gradients suggest dust effects and in-situ stellar growth influence galaxy evolution.

## Abstract

One of the most outstanding questions in contemporary astrophysics is: What is the significance of galaxy morphology? What physical processes underlier the morphologies we observe and is a galaxy's internal structure driving its evolution (nature) or is it a sign of the external processes which drive galaxy evolution (nurture)? We aim to understand the color dichotomy and gradients in bulges and disks along the Hubble sequence. We fit S\'ersic functions to the 2D light distributions in the $g$, $r$, $i$ bands to bulges and disks of the large EFIGI sample of galaxies with high quality morphological classification. 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 S\'ersic profiles in $g$ versus $i$. The impact is not uniform with Hubble type and the bulges of intermediate-type spirals (Sb-Sc) have the reddest mean colors, the largest scatter in their colors, and show the largest mean and scatter in their color gradients. Disks of the intermediate-type galaxies show the strongest color gradients. 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 disk. These results favor secular evolution (nature) as the primary driver of galaxy evolution in the local Universe.