The small dispersion of the mid IR – hard X-ray correlation in active galactic nuclei

Free access article

Issue		A&A Volume 457, Number 2, October II 2006


Page(s)		L17 - L20
Section		Letters
DOI		http://dx.doi.org/10.1051/0004-6361:20065820

A&A 457, L17-L20 (2006)
DOI: 10.1051/0004-6361:20065820

Letter

The small dispersion of the mid IR - hard X-ray correlation in active galactic nuclei

H. Horst^{1, 2}, A. Smette¹, P. Gandhi^{3, 1} and W. J. Duschl^{2, 4}

¹ European Southern Observatory, Casilla 19001, Santiago 19, Chile
e-mail: hhorst@eso.org
² Zentrum für Astronomie, ITA, Universität Heidelberg, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
³ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
⁴ Steward Observatory, The University of Arizona, 933 N. Cherry Ave, Tucson, AZ 85721, USA

(Received 13 June 2006 / Accepted 27 July 2006 )

Abstract
Context.We investigate mid-infrared and X-ray properties of the dusty torus in unification scenarios for active galactic nuclei.
Aims.We use the relation between mid IR and hard X-ray luminosities to constrain AGN unification scenarios.
Methods.With VISIR at the VLT, we have obtained the currently highest angular resolution ( $0 \farcs 35$ FWHM) narrow-band mid infrared images of the nuclei of 8 nearby Seyfert galaxies. Combining these observations with X-ray data from the literature we study the correlation between their mid IR and hard X-ray luminosities.
Results.We find that the rest frame $12.3 \, \mu$ m ( L_MIR) and 2-10 keV ( L_X) luminosities are correlated at a highly significant level. The best fit power-law to our data is $\log{L_{\mathrm{MIR}}} \propto \left(1.60 \pm 0.22 \right) \log{L_{\mathrm{X}}}$ , showing a much smaller dispersion than earlier studies.
Conclusions.The similarity in the $\log{L_{\mathrm{MIR}}}$ / $\log{L_{\mathrm{X}}}$ ratio between Sy1s and Sy2s even using high angular resolution MIR data implies that the similarity is intrinsic to AGN and not caused by contamination from extra-nuclear emission. This supports clumpy torus models. The exponent of the correlation constrains the inner geometry of the torus.

Key words: galaxies: active -- infrared: galaxies -- X-rays: galaxies

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