Resolving the AGN and host emission in the mid-infrared using a model-independent spectral decomposition

TL;DR

This paper introduces a robust, model-independent spectral decomposition method for separating AGN and host galaxy emissions in mid-infrared spectra, enabling unbiased AGN studies in distant galaxies using archival Spitzer data.

Contribution

It presents a novel spectral decomposition technique using template spectra that accurately isolates AGN emission without modeling extinction, validated against high-resolution ground-based data.

Findings

Decomposition closely matches nuclear spectra with 0.12 dex luminosity dispersion.

Method reliably removes host galaxy emission from IRS spectra.

Templates and code are publicly available for community use.

Abstract

We present results on the spectral decomposition of 118 Spitzer Infrared Spectrograph (IRS) spectra from local active galactic nuclei (AGN) using a large set of Spitzer/IRS spectra as templates. The templates are themselves IRS spectra from extreme cases where a single physical component (stellar, interstellar, or AGN) completely dominates the integrated mid-infrared emission. We show that a linear combination of one template for each physical component reproduces the observed IRS spectra of AGN hosts with unprecedented fidelity for a template fitting method, with no need to model extinction separately. We use full probability distribution functions to estimate expectation values and uncertainties for observables, and find that the decomposition results are robust against degeneracies. Furthermore, we compare the AGN spectra derived from the spectral decomposition with sub-arcsecond resolution nuclear photometry and spectroscopy from ground-based observations. We find that the AGN component derived from the decomposition closely matches the nuclear spectrum, with a 1-sigma dispersion of 0.12 dex in luminosity and typical uncertainties of ~0.19 in the spectral index and ~0.1 in the silicate strength. We conclude that the emission from the host galaxy can be reliably removed from the IRS spectra of AGN. This allows for unbiased studies of the AGN emission in intermediate and high redshift galaxies -currently inaccesible to ground-based observations- with archival Spitzer/IRS data and in the future with the Mid-InfraRed Instrument of the James Webb Space Telescope. The decomposition code and templates are available at http://www.denebola.org/ahc/deblendIRS.