Mid-infrared interferometry of 23 AGN tori: On the significance of polar-elongated emission

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: nlopez@strw.leidenuniv.nl
² Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, Gießenbachstr., 85741 Garching, Germany
³ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, PO Box 218, Hawthorn, Victoria 3122, Australia

Received: 19 October 2015
Accepted: 11 February 2016

Abstract

Context. Detailed high-resolution studies of active galactic nuclei (AGN) with mid-infrared (MIR) interferometry have revealed parsec-sized dust emission that is elongated in the polar direction in four sources.

Aims. Using a larger, coherently analyzed sample of AGN observed with MIR interferometry, we aim to identify elongated MIR emission in a statistical sample of sources. More specifically, we wish to determine if there is indeed a preferred direction of the elongation and whether this direction is consistent with a torus-like structure or with a polar emission.

Methods. We investigated the significance of the detection of an elongated shape in the MIR emission by fitting elongated Gaussian models to the interferometric data at 12 μm. We paid special attention to (1) the uncertainties caused by an inhomogeneous (u,v) coverage; (2) the typical errors in the measurements; and (3) the spatial resolution achieved for each object.

Results. From our sample of 23 sources, we are able to find elongated parsec-scale, MIR emission in five sources: three type 2s, one type 1i, and one type 1. Elongated emission in four of these sources has been published before; NGC 5506 is a new detection. The observed axis ratios are typically around 2 and the position angle of the 12 μm emission for all the elongated sources always seems to be closer to the polar axis of the system than to the equatorial axis. Two other objects, NGC 4507 and MCG-5-23-16, with reasonably well-mapped (u,v) coverage and good signal-to-noise ratios, appear to have a less elongated 12 μm emission.

Conclusions. Our finding that sources showing elongated MIR emission are preferentially extended in polar direction sets strong constraints on torus models or implies that both the torus and NLR/outflow region have to be modeled together. In addition, models used for SED fitting will have to be revised to include emission from polar dust.