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Resolving the AGN and Host Emission in the Mid-Infrared Using A Draft version February 23, 2015 Preprint typeset using LATEX style emulateapj v. 5/2/11 RESOLVING THE AGN AND HOST EMISSION IN THE MID-INFRARED USING A MODEL-INDEPENDENT SPECTRAL DECOMPOSITION Antonio Hernan´ -Caballero,1 Almudena Alonso-Herrero,1 Evanthia Hatziminaoglou,2 Henrik W. W. Spoon,3 Cristina Ramos Almeida,4,5 Tanio D´ıaz Santos,6,7 Sebastian F. H¨onig,8 Omaira Gonzalez´ -Mart´ın,9 and Pilar Esquej10 Draft version February 23, 2015 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 distribu- tion 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 spec- tral 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-σ 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. Subject headings: galaxies: active - infrared: galaxies - infrared:spectroscopy 1. INTRODUCTION Shang et al. 2011; Shi et al. 2013). In the 5.5 years duration of the Spitzer (Werner et al. Sub-arcsecond resolution observations with ground-based 2004) cryogenic mission, the Infrared Spectrograph (IRS; MIR spectrographs on 8-m and 10-m class telescopes probe Houck et al. 2004) obtained mid-infrared (MIR) spectra with nuclear regions in scales <100 pc for local AGN, effectively unprecedented sensitivity for more than one thousand AGN resolving away most of the emission from the host (see (Lebouteiller et al. 2011), including quasars, Seyferts, LIN- Figure 1). This is evidenced by the very small equivalent ERs, radio-galaxies, blazars, etc. However, the limited spatial widths (EW) of the PAH features in the nuclear spectra resolution of IRS observations (3.6” and 10.5” for the Short- (Roche et al. 2006; H¨onig et al. 2010; Gonz´alez-Mart´ın et al. Low (SL) and Long-Low (LL) modules, respectively) implies 2013; Esquejetal. 2014; Alonso-Herreroetal. 2014; that the extended emission from the host galaxy usually con- Ramos Almeida et al. 2014) and the tight correlation between tributes a significant fraction of the flux within the IRS aper- the MIR luminosity of the nuclear point source and the ture. Because of the difficulty in disentangling the emission bolometric luminosity of the AGN derived from hard X-ray from the AGN and its host galaxy, the shape of the AGN spec- observations (e.g. Krabbeet al. 2001; Horst et al. 2008; trum at MIR wavelenghts usually has large uncertainties, ex- Gandhi et al. 2009; Asmus et al. 2014). However, ground- cept for some quasar-luminosity AGN (Mullaney et al. 2011; based MIR observations are limited to specific windows of atmospheric transmission and suffer from much lower [email protected] sensitivity. Current samples of AGN with ground-based MIR 1 / arXiv:1502.05820v1 [astro-ph.GA] 20 Feb 2015 Instituto de F´ısica de Cantabria, CSIC-UC, Avenida de los Castros s n, observations are limited to a few dozens of local sources with 39005, Santander, Spain ∼8–13µm spectroscopy (e.g. Gonz´alez-Mart´ın et al. 2013; 2 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei M¨unchen, Germany Esquej et al. 2014) and a few hundreds with N-band and/or 3 Cornell University, CRSR, Space Sciences Building, Ithaca, NY Q-band photometry (Asmus et al. 2014). 14853, USA An alternative approach is to separate the spectral 4 Instituto de Astrof´ısica de Canarias, V´ıa L´actea s/n, E-38205 La La- components corresponding to emission from the AGN guna, Tenerife, Spain and the host in the integrated spectrum. Multiple 5 Departamento de Astrof´ısica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain spectral decomposition methods have been applied to 6 N´ucleo de Astronom´ıa de la Facultad de Ingenier´ıa, Universidad Diego MIR spectra (e.g. Laurentetal. 2000; Tranetal. 2001; Portales, Av. Ej´ercito Libertador 441, Santiago, Chile Lutz et al. 2004; Sajina et al. 2007; Hern´an-Caballero et al. 7 Spitzer Science Center, California Institute of Technology, MS 220-6, 2009; Alonso-Herreroet al. 2012a), broadband spectral Pasadena, CA 91125, USA energy distributions (SEDs) (e.g. Hatziminaoglouet al. 8 School of Physics & Astronomy, University of Southampton, Southampton SO18 1BJ, United Kingdom 2008; Mateos et al. 2015), and combinations of both (e.g. 9 Centro de Radioastronom´ıa y Astrof´ısica (CRyA-UNAM), 3-72 (Xan- Mullaney et al. 2011; Feltre et al. 2013). gari), 8701, Morelia, Mexico Libraries of theoretical models abound in the literature both 10 Departamento de Astrof´ısica, Facultad de CC. F´ısicas, Universidad for the IR emission of AGN (e.g. Pier & Krolik 1992, 1993; Complutense de Madrid, E-28040 Madrid, Spain 2 Hern´an-Caballero et al. Granato & Danese 1994; Efstathiou & Rowan-Robinson 1995; Fritz et al. 2006; Nenkovaet al. 2008) and galaxies (e.g. Silva etal. 1998; Popescu etal. 2000; Gordonetal. 2001; Siebenmorgen& Kr¨ugel 2007; Draine & Li 2007; da Cunha et al. 2008; Galliano et al. 2008). While they are suitable for fitting spectral energy distributions spanning a wide wavelength range, in general they cannot reproduce accurately, for example, the diverse shapes of the 10µm silicate feature or the relative strength of individual PAH bands. More detailed models of the MIR spectrum con- taining dozens of physically motivated components, such as those produced by CAFE (Marshall et al. 2007) or PAHFIT (Smith etal. 2007) and its variants (e.g. Gallimore etal. 2010), allow to reproduce these features with high fidelity. However, the large number of free parameters involved (more than 100 in the case of PAHFIT) implies that solutions are often degenerate except in very high signal-to-noise ratio Figure 1. Sketch illustrating the influence of spatial resolution in the spectra (SNR) spectra. of local AGN. The red box represents the aperture size of a typical ground- Another approach that requires few free parameters yet based nuclear spectrum, while the green one represents the much larger aper- provides acceptable fits for many MIR spectra is spectral ture of the Spitzer/IRS Short-Low module. The spectrum observed by IRS is the sum of the ground-based spectrum and the spectrum in the area within decomposition with empirical templates. Most previous at- the red and green squares, which only contains emission from the host. tempts at decomposing MIR spectra with empirical templates have relied on two or three spectral components represented In this paper we take a new approach to spectral decompo- by universal templates (e.g. Laurent et al. 2000; Tran et al. sition that uses libraries of observed high SNR spectra as tem- 2001; Nardini et al. 2008; Hern´an-Caballero et al. 2009; plates. We show that the method produces excellent fits that Hern´an-Caballero & Hatziminaoglou 2011; Veilleux et al. accurately reproduce most features in high SNR spectra, yet it 2009; Ruiz et al. 2013). This technique usually models is robust against degeneracies even in poor SNR spectra. We the effects of extinction by assuming a dust screen with a obtain reliable expectation values and uncertainties for AGN particular extinction law that applies to at least the AGN properties by computing full probability distribution functions component, and uses the value of AV as an extra free pa- (PDFs) using a marginalisation method that overcomes the rameter. This simple approach has a caveat: while the MIR inhomogeneity in the template set. By comparing the de- emission from the host galaxy is relatively uniform across rived AGN properties with those obtained from ground-based galaxies, it is simply not possible to reproduce the full range high spatial resolution (0.4”–0.8”) MIR photometry and spec- of SEDs observed in AGN-dominated spectra by just varying troscopy, which exclude circumnuclear emission, we demon- the amount of extinction applied to a single AGN template. strate that this kind of spectral decomposition is accurate at This is not critical when only a narrow spectral range is separating the emission from the AGN and its host galaxy and considered (e.g. Nardini et al. 2008; Ruiz et al. 2013), but recovering the shape of the AGN spectrum. In §2 we present it quickly becomes an issue as the fitted spectral range the method of spectral decomposition and our approach to the grows. For example, Hern´an-Caballero et al. (2009) showed calculation of the expectation values of observables. §3 de- that AGN with a steeper 5–15µm slope compared to the scribes the set of templates and §4 the selection of the test chosen AGN template require increasingly high contributions sample.
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