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).

NuSTAR UNVEILS A HEAVILY OBSCURED LOW-LUMINOSITY ACTIVE GALACTIC NUCLEUS IN THE LUMINOUS INFRARED GALAXY NGC 6286

“…---- , significantly lower than the value expected from pure AGNs (e.g., Mullaney et al 2011). Considering the observed 2-10 keV luminosity, the ratio is L L log 4.14 Using the relationship of Mullaney et al (2011) it is possible to convert the 2-10 keV luminosity into the expected IR ( ( ) ) = -we find that the IR luminosity of the AGN is between 0.1% and 0.6% of the total IR luminosity.…”

Observational evidence that positive and negative AGN feedback depends on galaxy mass and jet power

“…The Herschel Space Observatory (Pilbratt et al 2010), with its high FIR sensitivity and broad wavelength coverage, offers a powerful way of measuring the approximate SFR with minimal AGN contamination (e.g. Netzer et al 2007;Hatziminaoglou et al 2010;Mullaney et al 2011;Bonfield et al 2011;Hardcastle et al 2013;Virdee et al 2013). However, a drawback of previous works is that they are fundamentally limited by the strong correlation between redshift and luminosity, i.e., only the most powerful sources are observed at high redshifts and, due to the much smaller volume probed, only the less luminous, more abundant populations are found at lower redshifts.…”

TheSpitzerdiscovery of a galaxy with infrared emission solely due to AGN activity

“…Recently, Mullaney et al (2011) subtracted the host-galaxy contribution from the observed 6-100 μm IR SEDs of a sample of local galaxies hosting an AGN to derive the IR emission intrinsic to the AGN. When we compare the observed SED of SAGE1CJ053634 with the intrinsic SEDs presented in Mullaney et al (2010) we find a very good agreement.…”

“…We have investigated this possibility by comparing the observed PAH emission with the starburst galaxy templates of Mullaney et al (2011). These templates were derived by mean averaging similar SEDs from groups of galaxies that cover the full range of IR colours of starburst and Luminous InfraRed galaxies in the local Universe.…”