The Role of Bulge Formation in the Homogenization of Stellar Populations at $z\sim2$ as revealed by Internal Color Dispersion in CANDELS

TL;DR

This study investigates how bulge formation influences the homogenization of stellar populations in galaxies at redshift 1.5-3.5, revealing that stellar population diversity peaks in disk-dominated galaxies with specific mass ranges before bulge development.

Contribution

It introduces the use of Internal Color Dispersion (ICD) to connect stellar population variation with galaxy morphology and mass at high redshift, highlighting the role of bulge formation in stellar homogenization.

Findings

High ICD correlates with disk-dominated, intermediate-mass galaxies.

ICD decreases with increasing stellar mass and Sersic index.

Maximum stellar population variation occurs before bulge formation in galaxies.

Abstract

We use data from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey to study how the spatial variation in the stellar populations of galaxies relate to the formation of galaxies at $1.5 < z < 3.5$. We use the Internal Color Dispersion (ICD), measured between the rest-frame UV and optical bands, which is sensitive to age (and dust attenuation) variations in stellar populations. The ICD shows a relation with the stellar masses and morphologies of the galaxies. Galaxies with the largest variation in their stellar populations as evidenced by high ICD have disk-dominated morphologies (with S\'{e}rsic indexes $< 2$) and stellar masses between $10 < \mathrm{Log~M/ M_\odot}< 11$. There is a marked decrease in the ICD as the stellar mass and/or the S\'ersic index increases. By studying the relations between the ICD and other galaxy properties including sizes, total colors, star-formation rate, and dust attenuation, we conclude that the largest variations in stellar populations occur in galaxies where the light from newly, high star-forming clumps contrasts older stellar disk populations. This phase reaches a peak for galaxies only with a specific stellar mass range, $10 < \mathrm{Log~M/ M_\odot} < 11$, and prior to the formation of a substantial bulge/spheroid. In contrast, galaxies at higher or lower stellar masses, and/or higher S\'{e}rsic index ($n > 2$) show reduced ICD values, implying a greater homogeneity of their stellar populations. This indicates that if a galaxy is to have both a quiescent bulge along with a star forming disk, typical of Hubble Sequence galaxies, this is most common for stellar masses $10 < \mathrm{Log~M/M_\odot} < 11$ and when the bulge component remains relatively small ($n<2$).