High-spatial resolution SED of NGC 1068 from near-IR to radio

Disentangling the thermal and non-thermal contributions

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: shoenig@mpifr-bonn.mpg.de
² Instituto Astrofisica de Canarias, 38200 La Laguna, Tenerife, Spain

Received: 19 February 2008
Accepted: 27 March 2008

Abstract

We investigate the idea that a sizable fraction of the interferometrically unresolved infrared emission of the nucleus of NGC 1068 might originate from processes other than thermal dust emission from the torus. We examine the contribution of free-free or synchrotron emission to the central mid- and near-IR parsec-scale emitting region of NGC 1068. Each mechanism is constrained using parsec scale radio data available for NGC 1068 in the  Hz regime, and compared with the highest-resolution interferometric data available in the mid-infrared. It is shown that the unresolved emission in the interferometric observation (1 pc) is still dominated by dust emission and not by contributions from synchrotron or free-free emission. As previous studies suggest, the interferometric observations appear to infer a clumpy structure of the dust distribution. Extrapolation of the radio free-free or synchrotron emission to the IR indicates that their contribution is <20% even for the unresolved fraction of the interferometric flux.
The slope of the available radio data is consistent with a power law exponent that we interpret in terms of either free-free emission or synchrotron radiation from quasi-monochromatic electrons. We apply emission models for both mechanisms to obtain physical parameters.
Furthermore, we attempt to quantify the possible contribution of the accretion disk to the near-infrared emission. It has been suggested, that the unresolved K-band flux in VLTI/VINCI interferometric observation at 46 m baseline (40% of the total K-band flux) might originate in the accretion disk. By using an accretion disk spectrum that has been adjusted to the luminosity and black hole mass of NGC 1068, we find that the expected accretion-disk flux in the K-band is negligible. Moreover, the scenario of detecting the accretion disk through holes in a clumpy torus is extremely unlikely. We conclude that all current IR data of NGC 1068 trace the torus dust emission, favoring a clumpy torus.