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Mid-infrared properties of local active galactic nuclei at high angular resolution

We present the largest mid-infrared atlas of active galactic nuclei at sub-arcsecond spatial scales containing 249 objects. It comprises all ground-based HR MIR observations performed to date. This catalog also includes a large number of new observations. The photometry in multiple filters allows for characterization of the properties of the dust emission for most objects. We derive upper limits on the star formation contribution toward the unresolved nuclear emission and assess whether the AGN dominates the MIR emission of the galaxies. Furthermore, the AGN MIR atlas is well-suited for AGN unification studies because of its size and characteristics. We discuss the enlarged MIR–X-ray correlation, which extends over six orders of magnitude in luminosity and potentially probes different physical mechanisms, as an example.


Introduction
Mid-infrared (MIR) observations enable the study of the astrophysical dust in active galactic nuclei (AGN). This dust plays a key role in our understanding of the central accreting supermassive black hole and the surrounding star formation (SF). During the last decade, an increasing number of articles have demonstrated the power and importance of high-angular resolution (HR) MIR observations in order to isolate the AGN from surrounding starbursts, e.g., [6,4,12,1]. However, owing to the comparably low sensitivity and higher complexity of ground-based MIR observations (contrary to low-angular resolution space-based with, e.g., Spitzer), only relatively small samples have been observed and analyzed so far.
While HR enables us to resolve non-AGN emission regions in the nuclear regions of nearby galaxies, the AGN itself remains mostly unresolved even with 8 meter class telescopes (apart from the outer narrow line region). Many components of the AGN can possibly emit significant MIR emission, including the accretion disk surrounding the central supermassive black hole (SMBH), the perpendicularly emitted highly beamed jet outflow, the emission line clouds and the dusty obscuring structure ('torus') .
Finally, MIR interferometric observations enable us to resolve the AGN components but due to the sensitivity limits, only a relatively small number of AGN can be studied.
For these reasons, this project aims top provide an understanding of the MIR emitting structure in AGN by assembling HR MIR spectral energy distributions for a large representative sample of local AGN. This can then be used for multiwavelength studies as presented here, in particular the MIR-X-ray relation. The full analysis of this dataset will be published in Asmus et al. (in prep, a,b).

Sample selection and observations
High angular resolution is of utmost importance for the study of the MIR properties of local AGN. Therefore, we have considered imaging observations of only the largest single dish facilities in order to amass an AGN atlas of all ground-based MIR observations ever taken. In particular, we concentrate on facilities with public archives: Gemini/Michelle [5], Gemini/T-ReCS [13], Subaru/COMICS [8], and VLT/VISIR [11].
The base sample of this study is the uniform BAT 9-month AGN sample consisting of 104 objects [15]. Of those, we have observed a subsample of 80 sources with at least one of the above instruments during the last few years. Despite its selection method at hardest X-rays (14-195 keV), the BAT AGN sample underrepresents the highest absorbed part of the AGN population, in particular Compton-thick objects. Furthermore, the rather high flux limit leads to a cutoff of most low-luminosity AGN, which represent the majority of the nearby AGN. For these reasons, we have complemented the BAT AGN sample with all local AGN that have imaging observations available in at least one of the four instruments (COMICS, Michelle, T-ReCS and VISIR). In this work, we define 'local' as redshift z ≤ 0.4. This selection leads to a total sample of 249 AGN with a median redshift of 0.016, the 'AGN MIR atlas'. Note that this atlas is not complete in terms of volume or flux thresholds but simply contains everything that has been observed to date. However, with respect to [14], the AGN MIR atlas contains more than one-third of all optically identified AGN with a redshift < 0.01. Furthermore, it is sufficiently large to allow construction of well-matched samples based upon various selection strategies in future investigations. In the following of this work, we only distinguish between type 1 (Sy 1.0-1.5, and 1n), and type 2 (Sy 1.8-2.0 and 1h, 1i) AGN and LINERs (low-ionization nuclear emission line regions). With this definition, the majority of the objects are type 2. In total, about 1000 N-and Q-band images have been analyzed, of which more than 600 are yet unpublished. The majority of images have been obtained with VISIR, followed by T-ReCS, Michelle, and the least with COMICS. All observations have been carried out in standard chopping and nodding mode. Usually a flux standard star was observed within the two hours prior to or following the AGN and is used for the flux calibration. The flux measurements are carried out in the same way as described in [1] using a custom Gaussian fitting photometry method.

Results
Out of the 249 AGN, 200 have been clearly detected. The majority of images show compact or point-like nuclear emission, and no host galaxy emission is detected (Fig. 1). In total, ∼ 18% of the objects show consistent evidence of at least partly resolved nuclear or circumnuclear MIR emission. The unresolved nuclear MIR emission measured in all images is used for further analysis in the following. In particular, we concentrate on the MIR emission at ∼ 12 µm measured either directly with suitable narrow-band filters (most objects) or inferred from adjacent filters. The spectral region around 12 µm is free of strong spectral features and thus serves as the best monochromatic estimate of the nuclear MIR continuum luminosity.
The method described in [1] allows for constraining the contribution of SF toward the nuclear 12 µm emission by using a tight empirical correlation between the strength of the polycyclic aromatic hydrocarbon (PAH) emission feature at 11.3 µm and the continuum MIR emission in starburst galaxies. PAHs are commonly used as a SF tracer [3]. Therefore, it is possible to scale a SF template spectrum from [2] to the PAH flux in the individual AGN. This PAH flux is in turn either constrained from the lower angular resolution Spitzer/IRS spectra of the AGN (if available), or suitable HR photometric covering the 11.3 µm feature. This method yields an upper limit for the relative nuclear SF contribution at 12 µm. In the majority of AGN, the maximum SF contribution at sub-arcsec scales is minor. This is particularly the case for type 1 AGN, while type 2 AGN tend to exhibit higher possible SF contribution.
In the following, we present preliminary results for the majority of detected AGN from the AGN MIR atlas. Similar to [4,7,1], we have compiled absorption-corrected 2-10 keV X-ray luminosities, L X , for most of the detected AGN from the most recent satellite missions and publications. Details and final results for all AGN will be published in Asmus et al., in prep. b.

Global MIR emission
Whenever the angular resolution is insufficient to separate AGN and host emission, as in particular in higher redshift studies, it is important to assess whether the AGN dominates the observed total MIR emission. For this reason, we compare the HR measurements with low angular resolution measurements from IRAS, the beam of which contains most of the host galaxy emission. Fig. 2 demonstrates that the AGN dominates the global MIR emission only at intrinsic X-ray luminosities of L X 10 43 erg/s although with a large scatter.

Relation to X-ray emission
The MIR-X-ray correlation has proven to be a powerful tool for AGN studies [10,4,12]. For 140 detected AGN of the atlas, we thus compare the X-ray luminosities to the observed monochromatic 12 µm luminosities, L MIR , in Fig. 3. As expected from previous investigations, a strong correlation best described by log L MIR = 0.33 ± 0.03 + (0.99 ± 0.03) log L X is found. The observed scatter in the MIR-to-X-ray ratio is ∼ 0.42 dex, and the intrinsic scatter is estimated to be ∼ 0.28 dex with linmix_err [9]. The same relation is also present in flux space and therefore is not caused by distance-related effects.  Figure 2: Percentage of the fraction of AGN-to-global MIR luminosity versus the absorption-corrected X-ray luminosities for 140 detected AGN (preliminary). Blue squares: type 1 Seyferts (type 1.5 or lower); red diamonds: type 2 Seyferts; green triangles: LINERs; "C"s mark Compton-thick objects.

Conclusions
with a typical starburst template, we have excluded SF as the main cause of the point-like nuclear emission. Instead, most of the observed MIR emission appears to originate from AGN-heated dust, which still needs further verification. A comparison between high and low angular resolution MIR photometry shows that the AGN dominates the global emission of the galaxies only above AGN luminosities of 10 43 erg/s. Finally, we find a strong correlation of the observed nuclear MIR emission with the absorption-corrected X-ray emission. Such a correlation can be approximately understood by a link between the MIR and X-ray emission via reprocessing of primary radiation from the accretion disk, which is strongest in the ultraviolet (UV). The latter is reprocessed on the one hand in the innermost regions around the SMBH, where the UV photons are up-scattered from hot corona electrons into the X-ray regime. And on the other hand, much further out in the torus region, the UV photons are absorbed by dust and reemitted as thermal radiation in the MIR. More detailed studies are necessary, however, to test the origin of the MIR emission and its independence from other AGN parameters.

PoS(Seyfert 2012)007
Mid-infrared properties of local active galactic nuclei at high angular resolution Daniel Asmus