The Young, the Old, and the Dusty: Stellar Populations of AGN Hosts

TL;DR

This study models the stellar populations of AGN host galaxies across different redshifts, revealing similarities at low redshift and intrinsic differences at high redshift, challenging simple unified models.

Contribution

It introduces a comprehensive spectral energy distribution model including AGN, host stars, and dust, and analyzes their properties across cosmic time.

Findings

At z < 1, type 1 and type 2 AGN hosts have similar stellar populations.

At z > 1, type 2 AGN hosts are intrinsically different from type 1.

If Compton thick AGN evolve like less obscured AGN, their hosts are similar; otherwise, they are in specific evolutionary stages.

Abstract

Studying the average properties of active galactic nuclei (AGN) host stellar populations is an important step in understanding the role of AGN in galaxy evolution and the processes which trigger and fuel AGN activity. Here we calculate model spectral energy distributions (SEDs) that include emission from the AGN, the host galaxy stellar population, and dust enshrouded star formation. Using the framework of cosmic X-ray background population synthesis modeling, the model AGN hosts are constrained using optical (B band) and near infrared (J band, 3.6 um, 5.7 um, 8.0 um, and 24 um) luminosity functions and number counts. It is found that at z < 1, type 1 and type 2 AGN hosts have similar stellar populations, in agreement with the orientation based unified model and indicative of secular evolution. At z > 1, type 2 AGN hosts are intrinsically different from type 1 AGN hosts, suggesting that the simple orientation based unified model does not hold at z > 1. Also, it is found that if Compton thick (CT) AGN evolve like less obscured type 2 AGN, then, on average, CT AGN hosts are similar to type 2 AGN hosts; however, if CT obscuration is connected to an evolutionary stage of black hole growth, then CT AGN hosts will also be in specific evolutionary stages. Multi-wavelength selection criteria of CT AGN are discussed.