Resolving the mid-infrared cores of local Seyferts^*

¹ RIKEN Cosmic Radiation Lab, 2-1 Hirosawa, Wakoshi, Saitama 351-0198, Japan e-mail: pg@crab.riken.jp
² Institut für Theoretische Physik und Astrophysik, Christian-Albrechts-Universität zu Kiel, Leibnizstr. 15, 24098 Kiel, Germany
³ Zentrum für Astronomie, ITA, Universität Heidelberg, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ European Southern Observatory, Alonso de Cordova 3107, Casilla 19001, Santiago, Chile
⁶ Istituto Nazionale di Astrofisica (INAF) – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁷ Dipartimento di Astronomia, Università degli Studi di Bologna, via Ranzani 1, 40127 Bologna, Italy

Received: 17 November 2008
Accepted: 6 February 2009

Abstract

We present new photometry of 16 local Seyferts including 6 Compton-thick sources in N-band filters around 12-μm, obtained with the VISIR instrument on the 8-m Very Large Telescope. The near-diffraction-limited imaging provides the least-contaminated core fluxes for these sources to date. Augmenting these with our previous observations and with published intrinsic X-ray fluxes, we form a total sample of 42 sources for which we find a strong mid-infrared:X-ray (12.3 μm:2–10 keV) luminosity correlation. Performing a physically-motivated subselection of sources in which the Seyfert torus is likely to be best-resolved results in the correlation L_MIR  L_X, with a reduction of the scatter in luminosities as compared to the full sample. Consideration of systematics suggests a range of 1.02–1.21 for the correlation slope. The mean 2-keV:12.3-μm spectral index () is found to be -1.100.01, largely independent of luminosity. Indirectly-computed 12-μm bolometric corrections range over ≈10–30 if a known luminosity trend of X-ray bolometric corrections is assumed. Comparison with ISO data spanning a wider luminosity range suggests that our correlation can be extended into the quasar regime. That unobscured, obscured, and Compton-thick sources all closely follow the same luminosity correlation has important implications for the structures of Seyfert cores. The typical resolution-limit of our imaging corresponds to ~70 pc at a median , and we use the tightness of the correlation to place constraints on the dominance of any residual emission sources within these physical scales. An upper-limit for any contaminating star formation of ≈40% of the unresolved flux is inferred, on average. We suggest that uncontaminated mid-IR continuum imaging of AGN is an accurate proxy for their intrinsic power.