X-ray observations of highly obscuredτ_9.7μm > 1 sources: an efficient method for selecting Compton-thick AGN?

Abstract

Observations with the IRS spectrograph onboard Spitzer have found many sources with very deep Si features at 9.7 μm, that have optical depths of τ > 1. Since it is believed that a few of these systems in the local Universe are associated with Compton-thick active galactic nuclei (hereafter AGN), we set out to investigate whether the presence of a strong Si absorption feature is a good indicator of a heavily obscured AGN. We compile X-ray spectroscopic observations available in the literature on the optically-thick (τ 9.7 μm > 1) sources from the 12 μm IRAS Seyfert sample. We find that the majority of the high-τ optically confirmed Seyferts (six out of nine) in the 12 μm sample are probably Compton-thick. Thus, we provide direct evidence of a connection between mid-IR optically-thick galaxies and Compton-thick AGN, with the success rate being close to 70% in the local Universe. This is at least comparable to, if not better than, other rates obtained with photometric information in the mid to far-IR, or even mid-IR to X-rays. However, this technique cannot provide complete Compton-thick AGN samples, i.e., there are many Compton-thick AGN that do not display significant Si absorption, with the most notable example being NGC 1068. After assessing the validity of the high 9.7 μm optical-depth technique in the local Universe, we attempt to construct a sample of candidate Compton-thick AGN at higher redshifts. We compile a sample of seven high-τ Spitzer sources in the Great Observatories Origins Deep Survey (GOODS) and five in the Spitzer First-Look Survey. All these have been selected to have no PAH features (EW 6.2 μm < 0.3 μm) to maximise the probability that they are bona-fide AGN. Six out of the seven GOODS sources have been detected in X-rays, while for the five FLS sources only X-ray flux upper limits are available. The high X-ray luminosities (L X > 10 42 erg s −1 ) of the detected GOODS sources corroborates that these are AGN. For FLS, ancillary optical spectroscopy reveals hidden nuclei in two more sources. SED fitting can support the presence of an AGN in the vast majority of sources. Owing to the limited photon statistics, we cannot derive useful constraints from X-ray spectroscopy on whether these sources are Compton-thick. However, the low L X /L 6 μm luminosity ratios, suggest that at least four out of the six detected sources in GOODS may be associated with Compton-thick AGN.

Deep Silicate Absorption Features in Compton-Thick Active Galactic Nuclei Predominantly Arise Due to Dust in the Host Galaxy

“…In turn, this suggests that selection of sources with strong Si-absorption is a good method to find the most heavily obscured AGNs, i.e., Compton-thick sources with N H > 1.5 × 10 24 cm −2 (e.g., Georgantopoulos et al 2011). To first order, these results would appear to agree with those predicted by simple uniform torus models.…”

“…Throughout this manuscript, we define S 9.7 as the depth of the Si-absorption feature at λ ∼ 9.7 μm where S 9.7 = ln(f 9.7,intrinsic /f 9.7,observed ). Following many previous studies, we use S 9.7 as a good proxy for the apparent optical depth and dust extinction in our sample (e.g., Spoon et al 2007;Shi et al 2006;Levenson et al 2007;Deo et al 2007Deo et al , 2009Georgantopoulos et al 2011). Though we note that the relation between S 9.7 and true optical depth is most likely nonlinear and somewhat model and orientation dependent (see Levenson et al 2007;Nenkova et al 2008;Schartmann et al 2008).…”

“…Previous investigations have often employed simple extrapolation methods to estimate the depth of the silicate feature in mid-IR spectroscopy (e.g., Spoon et al 2007;Levenson et al 2007;Georgantopoulos et al 2011). This involves measurement of the observed continuum blueward and redward of the silicate feature, extrapolating a power law between the continuum points and comparing this to the observed flux at λ ∼ 9.7 μm.…”

“…Second, we employ the widely used spectral-fitting package, PAHFIT (Smith et al 2007) to fit a range of heated dust continua combined with PAH and emission-line features to derive S 9.7 (Column 16 of Table 1). Following Georgantopoulos et al (2011), we change the default dust continuum temperatures in PAHFIT and additionally allow hotter dust temperatures (400-1400 K) to be fitted to the spectra; these are expected to arise from an accretion-heated AGN torus. In general, we find very good agreement for S 9.7 derived using DecompIR and PAHFIT (see Columns 15 and 16 of Table 1).…”

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

“…AGNs in U/LIRGs can be identified in the IR by several means: (i) with the detection of high-excitation MIR emission lines (e.g., Sturm et al 2002) Lutz et al 1999;Petric et al 2011); (iii) with the EW of the PAH features, which tend to be lower in the presence of a bright AGN, since it can destroy PAH molecules (e.g., Imanishi et al 2010b); (iv) studying the slope of the 2.5-5 μm continuum (Γ 2.5-5 , e.g., Imanishi et al 2010b) or the continuum 30 μm/15 μm flux density ratio (e.g., Stierwalt et al 2013), which tend to be red in the presence of an AGN; (v) using the depth of absorption features (e.g., Imanishi & Dudley 2000; Risaliti et al 2006;Georgantopoulos et al 2011a), with large depths pointing toward AGNs obscured by dust; and/or (vi) from deviations of the well-known correlation between the far-IR (FIR) and the radio luminosity (Helou et al 1985;Condon et al 1991;Condon 1992) using the radio-FIR flux ratio q (e.g., Yun et al 2001). We find that all these proxies ( …”