High-Spatial Resolution SED of NGC 1068 from Near-IR to Radio Disentangling the thermal and non-thermal contributions

We investigate the ideas that a sizable fraction of the interferometrically unresolved infrared emission of the nucleus of NGC 1068 might originate from other processes than thermal dust emission from the torus. We examine the contribution of free-free or synchrotron emissions to the central midand near-IR parsec-scale emitting region of NGC 1068. Each mechanism is constrained with parsec scale radio data available for NGC 1068 in the 109 − 1011 Hz regime, and compared to 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 recent studies suggest, the interferometric observations prefer 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 α = 0.29 ± 0.07 which we interprete in terms of either free-free emission or synchrotron radiation from quasi-monochromatic electrons. We apply emission models for both mechanisms in order to obtain physical parameters. Furthermore, we aimed for quantifying 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 from the accretion disk. By using an accretion disk spectrum which was adjusted to the luminosity and black hole mass of NGC 1068, we found out 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, favouring a clumpy torus.