Infrared Contributions of X-Ray Selected Active Galactic Nuclei in Dusty Star-Forming Galaxies

TL;DR

This study analyzes the infrared emission from X-ray selected active galactic nuclei in dusty star-forming galaxies, using spectral energy distribution decomposition to distinguish AGN and host galaxy contributions across a large sample.

Contribution

It presents the largest sample to date of X-ray AGN with IR detections and provides new insights into the IR contributions of AGN versus host galaxy emission.

Findings

Weak correlation between star formation activity and AGN activity.

Average dust covering factor of 33%, indicating a significant obscured AGN population.

Major IR contribution (~80%) from AGN in the mid-IR, decreasing at longer wavelengths.

Abstract

We investigate the infrared contribution from supermassive black hole activity versus host galaxy emission in the mid to far-infrared (IR) spectrum for a large sample of X-ray bright active galactic nuclei (AGN) residing in dusty, star-forming host galaxies. We select 703 AGN with L_X = 10^42-46 ergs/s at 0.1 < z < 5 from the Chandra XBootes X-ray Survey with rich multi-band observations in the optical to far-IR. This is the largest sample to date of X-ray AGN with mid and far-IR detections that uses spectral energy distribution (SED) decomposition to determine intrinsic AGN and host galaxy infrared luminosities. We determine weak or nonexistent relationships when averaging star-formation activity as a function of AGN activity, but see stronger positive trends when averaging L_X in bins of star-forming activity for AGN at low redshifts. We estimate an average dust covering factor of 33% based on infrared SEDs and bolometric AGN luminosity, corresponding to a Type 2 AGN population of roughly a third. We also see a population of AGN that challenge the inclination based unification model with individual dust covering factors that contradict the nuclear obscuration expected from observed X-ray hardness ratios. We see no strong connection between AGN fractions in the IR and corresponding total infrared, 24 um, or X-ray luminosities. The average rest-frame AGN contribution as a function of IR wavelength shows significant (~80%) contributions in the mid-IR that trail off at lambda > 30 um. Additionally, we provide a relation between observed L_X and pure AGN IR output for high-z AGN allowing future studies to estimate AGN infrared contribution using only observed X-ray flux density estimates.