The post-Herschel view of intrinsic AGN emission: constructing templates for galaxy and AGN emission at IR wavelengths

TL;DR

This paper develops new infrared templates for AGN and galaxy emission using Herschel data, enabling more accurate separation of star formation and AGN activity in IR observations, and introduces the IRAGNSEP fitting code.

Contribution

It presents a new set of IR templates for AGN and galaxies, and the IRAGNSEP code, to measure host galaxy properties free of AGN contamination, based on a sample of local AGNs with Herschel and Spitzer data.

Findings

AGN contribution to far-IR is generally lower than previously thought.

Three main AGN IR templates suffice to describe intrinsic AGN SEDs.

Two IR AGN templates relate to obscuration and polar dust emission.

Abstract

Measuring the star-forming properties of AGN hosts is key to our understanding of galaxy formation and evolution. However, this topic remains debated, partly due to the difficulties in separating the infrared (i.e. 1--1000 $\mu$m) emission into AGN and star-forming components. Taking advantage of archival far-infrared data from Herschel, we present a new set of AGN and galaxy infrared templates, and introduce the spectral energy distribution fitting code IRAGNSEP. Both can be used to measure infrared host galaxy properties, free of AGN contamination. To build these, we used a sample of 100 local ($z$ < 0.3), low-to-high luminosity AGNs (i.e. $L_{\rm bol}~\sim~10^{42--46}~\rm erg~s^{-1}$), selected from the 105-month Swift - BAT X-ray survey, which have archival Spitzer - IRS spectra and Herschel photometry. We first built a set of seven galaxy templates using a sample of 55 star-forming galaxies selected via infrared diagnostics. Using these templates, combined with a flexible model for the AGN contribution, we extracted the intrinsic infrared emission of our AGN sample. We further demonstrate that we can reduce the diversity in the intrinsic shapes of AGN spectral energy distributions down to a set of three AGN templates, of which two represent AGN continuum, and one represents silicate emission. Our results indicate that, on average, the contribution of AGNs to the far-infrared ($\lambda~\gtrsim$ 50 $\mu$m) is not as high as suggested by some recent work. We further show that the need for two infrared AGN continuum templates could be related to nuclear obscuration, where one of our templates appears dominated by the emission of the extended polar dust.