Does the AGN Unified Model Evolve with Redshift? Using the X‐Ray Background to Predict the Mid‐Infrared Emission of AGNs

Abstract

Deep X-ray surveys by Chandra and XMM-Newton have resolved about 80% of the 2-10 keV cosmic extragalactic X-ray background (CXRB) into point sources, the majority of which are obscured AGN. The obscuration might be connected to processes within the host galaxy, possibly the star-formation rate. Here, we use the results of CXRB synthesis calculations as input to detailed Cloudy simulations in order to predict the evolution of AGN properties at several mid-IR wavelengths. Computations were performed for three different evolutions of the AGN type 2/type 1 ratio between z=0 and 1: where the ratio increased as (1+z)^{0.9}, as (1+z)^{0.3} and one with no redshift evolution. Models were calculated with the inner radius of the absorbing gas and dust at 1 pc or at 10 pc. Comparing the results of the calculations to combined X-ray and Spitzer data of AGN shows that the predicted spectral energy distributions are a good description of average AGNs found in the deep surveys. The existing data indicates that the mid-IR emission from an average AGN is best described by models where the attenuating material is ~10 pc from the central engine. We present the expected Spitzer cumulative number count distributions and the evolution of the total AGN (type 1 + type 2) luminosity function (LF) between z=0 and 1 at rest-frame 8 microns and 30 microns for the three evolutionary scenarios. The mid-IR AGN LF will be an excellent tool to measure the evolution of the covering factor of the gas and dust from z~0 to 1.