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. 2-10keV luminosity, L2-10keV∼ 1042-1044ergs-1) active galactic nuclei (hereafter AGNs). The average infrared SED of typical AGNs is best described as a broken power law at ≲40μ 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 60μ m, demonstrating the importance of accounting for any AGN contribution when calculating galaxy infrared luminosities. We find that the average intrinsic AGN 6-100μ 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. L2-10keV≳ 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). © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.