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

TL;DR

This study analyzes the infrared emission of NGC 1068's nucleus, demonstrating that dust emission dominates over synchrotron or free-free processes at parsec scales, with less than 20% contribution from non-thermal mechanisms.

Contribution

It provides the first detailed constraints on the non-thermal contributions to the IR emission of NGC 1068 using high-resolution radio and interferometric data.

Findings

Unresolved IR emission (<1pc) is dominated by dust, not non-thermal processes.

Non-thermal contributions are estimated to be less than 20%.

Radio data slope suggests free-free or synchrotron emission from quasi-monochromatic electrons.

Abstract

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 mid- and near-IR parsec-scale emitting region of NGC 1068. Each mechanism is constrained with parsec scale radio data available for NGC 1068 in the 10^9 - 10^11 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 (<~1pc) is still dominatedd 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 alpha = 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. (abridged)