Dust-Corrected Colors Reveal Bimodality in AGN Host Galaxy Colors at z~1

TL;DR

This study uses dust-corrected colors and accurate photometric redshifts to reveal that AGN host galaxies at z~1 are bimodal in color, with implications for understanding black hole growth and galaxy evolution.

Contribution

It introduces a method to distinguish dust-reddened star-forming galaxies from passive ones using near-infrared colors, revealing bimodality in AGN host galaxy colors at z~1.

Findings

AGN hosts are mostly massive, on the red sequence or green valley.

Approximately 25% of red AGN hosts are dust-reddened star-forming galaxies.

AGN activity occurs in both passive and dusty star-forming galaxies.

Abstract

Using new, highly accurate photometric redshifts from the MUSYC medium-band survey in the Extended Chandra Deep Field South (ECDF-S), we fit synthetic stellar population models to compare AGN host galaxies to inactive galaxies at 0.8 < z < 1.2. We find that AGN host galaxies are predominantly massive galaxies on the red sequence and in the green valley of the color-mass diagram. Because both passive and dusty galaxies can appear red in optical colors, we use rest-frame near-infrared colors to separate passively evolving stellar populations from galaxies that are reddened by dust. As with the overall galaxy population, ~25% of the `red' AGN host galaxies and ~75% of the `green' AGN host galaxies have colors consistent with young stellar populations reddened by dust. The dust-corrected rest-frame optical colors are the blue colors of star-forming galaxies, which implies that these AGN hosts are not passively aging to the red sequence. At z~1, AGN activity is roughly evenly split between two modes of black hole growth: the first in passively evolving host galaxies, which may be heating up the galaxy's gas and preventing future episodes of star formation, and the second in dust-reddened young galaxies, which may be ionizing the galaxy's interstellar medium and shutting down star formation.