Properties and Expected Number Counts of Active Galactic Nuclei and their Hosts in the Far Infrared

TL;DR

This paper models the spectral energy distribution of active galactic nuclei in the far infrared to predict their counts, contributions to the cosmic infrared background, and insights into their star formation history and evolution.

Contribution

It introduces a method combining X-ray data and photoionization simulations to predict FIR properties of AGN and their hosts, including differential counts and background contributions.

Findings

FIR observations at ~500 μm effectively trace star formation in AGN hosts.

AGN contribution to the CIRB can reveal different star formation evolution in AGN hosts.

Bright end counts of Compton thick AGN test quasar triggering models.

Abstract

Telescopes like Herschel and the Atacama Large Millimeter/submillimeter Array (ALMA) are creating new opportunities to study sources in the far infrared (FIR), a wavelength region dominated by cold dust emission. Probing cold dust in active galaxies allows for study of the star formation history of active galactic nuclei (AGN) hosts. The FIR is also an important spectral region for observing AGN which are heavily enshrouded by dust, such as Compton thick AGN. By using information from deep X-ray surveys and cosmic X-ray background synthesis models, we compute Cloudy photoionization simulations which are used to predict the spectral energy distribution (SED) of AGN in the FIR. Expected differential number counts of AGN and their host galaxies are calculated in the Herschel bands. The expected contribution of AGN and their hosts to the cosmic infrared background (CIRB) and the infrared luminosity density are also computed. Multiple star formation scenarios are investigated using a modified blackbody star formation SED. It is found that FIR observations at ~500 um are an excellent tool in determining the star formation history of AGN hosts. Additionally, the AGN contribution to the CIRB can be used to determine whether star formation in AGN hosts evolves differently than in normal galaxies. The contribution of Compton thick AGN to the bright end differential number counts and to the bright source infrared luminosity density is a good test of AGN evolution models where quasars are triggered by major mergers.