Defining the intrinsic AGN infrared spectral energy distribution and measuring its contribution to the infrared output of composite galaxies

TL;DR

This paper empirically defines the intrinsic infrared spectral energy distribution (SED) of typical active galactic nuclei (AGNs) and develops a method to measure their contribution to the infrared output of composite galaxies, aiding large-scale galaxy analysis.

Contribution

It provides the first empirical intrinsic AGN IR SED from 6-100 um, compares it with torus models, and introduces a new fitting procedure for galaxy IR data to quantify AGN contributions.

Findings

AGN IR SED is a broken power-law, steepening beyond 40 um.

At least 3 of 11 AGNs are AGN-dominated at 60 um.

Intrinsic AGN IR SED becomes bluer with higher X-ray luminosity.

Abstract

We use infrared spectroscopy and photometry to empirically define the intrinsic, thermal infrared spectral energy distribution (i.e., 6-100 um SED) of typical active galactic nuclei (i.e., 2-10 keV luminosity, Lx=10^{42}-10^{44} ergs/s AGNs). On average, the infrared SED of typical AGNs is best described as a broken power-law at <40 um that falls steeply at >40um (i.e., at far-infrared wavelengths). Despite this fall-off at long wavelengths, at least 3 of the 11 AGNs in our sample have observed SEDs that are AGN-dominated even at 60 um, demonstrating the importance of accounting for possible AGN contribution even at far-infrared wavelengths. Our results also suggest that the average intrinsic AGN 6-100 um SED gets bluer with increasing X-ray luminosity, a trend seen both within our sample and also when we compare against the intrinsic SEDs of more luminous quasars (i.e., Lx>10^{44} ergs/s). We compare our intrinsic AGN SEDs with predictions from dusty torus models and find they are more closely matched by clumpy, rather than continuous, torus models. Next, we use our intrinsic AGN SEDs to define a set of correction factors to convert either monochromatic infrared or X-ray luminosities into total intrinsic AGN infrared (i.e., 8-1000 um) luminosities. Finally, we outline a procedure that uses our newly defined intrinsic AGN infrared SEDs, in conjunction with a selection of host-galaxy templates, to fit the infrared photometry of composite galaxies and measure the AGN contribution to their total infrared output. We verify the accuracy of our SED fitting procedure by comparing our results to two independent measures of AGN contribution. Our SED fitting procedure opens up the possibility of measuring the intrinsic AGN luminosities of large numbers of galaxies with well-sampled infrared data (e.g., IRAS, ISO, Spitzer and Herschel).