AGN Unification at z ~ 1: u - R Colors and Gradients in X-ray AGN Hosts

TL;DR

This study investigates the colors and gradients of X-ray-selected AGN host galaxies at z ~ 1, finding that obscuration is likely due to geometry rather than star formation activity, challenging feedback-driven quenching models.

Contribution

It provides the first detailed analysis of rest-frame u - R colors and color gradients in AGN hosts at z ~ 1, using advanced calibration to correct for AGN contamination.

Findings

Obscured and unobscured AGN hosts have similar colors beyond 6 kpc.

Color gradients show minimal difference between obscured and unobscured hosts.

Results support a geometric unification model over feedback-driven quenching.

Abstract

We present uncontaminated rest-frame u - R colors of 78 X-ray-selected AGN hosts at 0.5 < z < 1.5 in the Chandra Deep Fields measured with HST/ACS/NICMOS and VLT/ISAAC imaging. We also present spatially-resolved NUV - R color gradients for a subsample of AGN hosts imaged by HST/WFC3. Integrated, uncorrected photometry is not reliable for comparing the mean properties of soft and hard AGN host galaxies at z ~ 1 due to color contamination from point-source AGN emission. We use a cloning simulation to develop a calibration between concentration and this color contamination and use this to correct host galaxy colors. The mean u - R color of the unobscured/soft hosts beyond ~6 kpc is statistically equivalent to that of the obscured/hard hosts (the soft sources are 0.09 +/- 0.16 magnitudes bluer). Furthermore, the rest-frame V - J colors of the obscured and unobscured hosts beyond ~6 kpc are statistically equivalent, suggesting that the two populations have similar distributions of dust extinction. For the WFC3/IR sample, the mean NUV - R color gradients of unobscured and obscured sources differ by less than ~0.5 magnitudes for r > 1.1 kpc. These three observations imply that AGN obscuration is uncorrelated with the star formation rate beyond ~1 kpc. These observations favor a unification scenario for intermediate-luminosity AGNs in which obscuration is determined geometrically. Scenarios in which the majority of intermediate-luminosity AGN at z ~ 1 are undergoing rapid, galaxy-wide quenching due to AGN-driven feedback processes are disfavored.