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

Abstract

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, L2-10 keV∼ 1042–1044 erg s−1) 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. L2-10 keV≳ 1044 erg s−1). 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).