2011MNRAS.414.1082M

We use infrared spectroscopy and photometry to empirically define the intrinsic mid- to far-infrared spectral energy distribution (i.e. 6–100 µm SED) of thermal emission produced by typical (i.e. 2–10 keV luminosity, L_2-10 keV_∼ 10⁴²–10⁴⁴ erg/s) active galactic nuclei (hereafter AGNs). The average infrared SED of typical AGNs is best described as a broken power law at ≲ 40 µ m that falls steeply at ≳ 40 µm (i.e. at far-infrared wavelengths). Despite this fall-off at long wavelengths, at least three of the 11 AGNs in our sample have demonstrated SEDs that are AGN dominated even at 60 µ m, demonstrating the importance of accounting for any AGN contribution when calculating galaxy infrared luminosities. We find that the average intrinsic AGN 6–100 µ m SED gets bluer with increasing X-ray luminosity – a trend seen both within our sample and also when we compare it with the intrinsic SEDs of more luminous quasars (i.e. L_2-10 keV_ ≳ 10⁴⁴ erg/s). The range of intrinsic AGN SEDs is more closely matched by clumpy, rather than continuous, torus models. Using our intrinsic AGN SEDs we define a set of correction factors to convert either monochromatic infrared or X-ray luminosities into total (i.e. 8–1000 µm) AGN infrared luminosities. We outline a procedure that uses our newly defined intrinsic AGN infrared SEDs, in conjunction with a selection of host-galaxy templates, to measure the AGN contribution to the infrared output of composite galaxies from photometry alone. We verify the accuracy of our SED-fitting procedure by comparing our results to two independent measures of AGN contribution: (1) 12-µm luminosities obtained from high spatial resolution observations of nearby galaxies and (2) the equivalent width of the 11.25-µm PAH feature. Our SED-fitting procedure opens up the possibility of measuring the intrinsic AGN luminosities for large numbers of galaxies with well-sampled infrared data (e.g. IRAS, ISO, Spitzer and Herschel).

The intrinsic AGN infrared SEDs, host-galaxy templates and a procedure used to combine them to fit infrared photometry are available at