The z=0.8 precursors of today's bulges

TL;DR

This study investigates the color structure of disk galaxies at intermediate redshifts to understand bulge formation timing relative to disks, finding evidence for coeval evolution and diverse bulge properties.

Contribution

It provides observational evidence that bulges and disks in galaxies at z~0.8 evolve roughly simultaneously, challenging models where bulges form significantly before or after disks.

Findings

Bulges are not distinctly redder than disks.

Bulge colors strongly correlate with global galaxy colors.

Approximately 50% of galaxy nuclei with central light excesses are passively evolving red populations.

Abstract

We study the color structure of disk galaxies in the Groth strip at redshifts 0.1<z<1.2. Our aim is to test formation models in which bulges form before/after the disk. We find smooth color distributions with gentle outward blueing across the galaxy image: bulges are not distinctly redder than their disks; and bulge colors strongly correlate with global colors. The results suggest a roughly coeval evolution of bulges and disks. About 50% of the nuclei of galaxies with central light excesses above the outer exponential profile hold passively evolving red populations. The remainder 50% are galaxies with central blue colors similar to their disks. They may be bulges in formation, or the central parts of disks with non-exponential surface brightness profiles.