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

Abstract

Context. Detailed high resolution studies of AGN with mid-infrared (MIR) interferometry have revealed parsec-sized dust emission 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 mid-infrared 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 investigate the significance of the detection of an elongated shape in the MIR emission by fitting elongated Gaussian models to the interferometric data at 12 um. We pay 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; NGC5506 is a new detection. The observed axis ratios are typically around 2 and the position angle of the 12 um emission for all the elongated sources seems to be always closer to the polar axis of the system than to the equatorial axis. Two other objects, NGC4507 and MCG-5-23-16 with a reasonably well mapped (u,v) coverage and good signal-to-noise ratios, appear to have a less elongated 12 um emission. Conclusions. Our finding that sources showing elongated mid-infrared emission are preferentially extended in polar direction sets strong constraints on torus models or implies that both the torus and the NLR/outflow region have to be modeled together. Especially also models used for SED fitting will have to be revised to include emission from polar dust.