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The Dusty Torus in the Circinus Galaxy Covery of True Type 2 Sources (Without Broad Emission Lines in Table 1

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The Dusty Torus in the Circinus Galaxy Covery of True Type 2 Sources (Without Broad Emission Lines in Table 1 Astronomy & Astrophysics manuscript no. 22698cir © ESO 2018 Friday 12th October, 2018 The dusty torus in the Circinus galaxy: a dense disk and the torus funnel⋆ Konrad R. W. Tristram1, Leonard Burtscher2, Walter Jaffe3, Klaus Meisenheimer4, Sebastian F. Hönig5, Makoto Kishimoto1, Marc Schartmann6,2, Gerd Weigelt1 1 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany email: [email protected] 2 Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, Gießenbachstraße, 85741 Garching, Germany 3 Leiden Observatory, Leiden University, Niels-Bohr-Weg 2, 2300 CA Leiden, The Netherlands 4 Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany 5 Dark Cosmology Center, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark 6 Universitätssternwarte München, Scheinerstr. 1, 81679 München, Germany Received 18 September 2013 / Accepted 26 November 2013 Abstract Context. With infrared interferometry it is possible to resolve the nuclear dust distributions that are commonly associated with the dusty torus in active galactic nuclei (AGN). The Circinus galaxy hosts the closest Seyfert 2 nucleus and previous interferometric observations have shown that its nuclear dust emission is particularly well resolved. Aims. The aim of the present interferometric investigation is to better constrain the dust morphology in this active nucleus. Methods. To this end, extensive new observations were carried out with the MID-infrared Interferometric instrument (MIDI) at the Very Large Telescope Interferometer, leading to a total of 152 correlated flux spectra and differential phases between 8 and 13 µm. To interpret this data, we used a model consisting of black-body emitters with a Gaussian brightness distribution and with dust extinction. Results. The direct analysis of the data and the modelling confirm that the emission is distributed in two distinct components: a disk-like emission component with a size (FWHM) of ∼ 0.2 × 1.1 pc and an extended component with a size of ∼ 0.8 × 1.9pc. The disk-like component is elongated along PA ∼ 46° and oriented perpendicular to the ionisation cone and outflow. The extended component is responsible for 80% of the mid-infrared emission. It is elongated along PA ∼ 107°, which is roughly perpendicular to the disk component and thus in polar direction. It is interpreted as emission from the inner funnel of an extended dust distribution and shows a strong increase in the extinction towards the south-east. We find both emission components to be consistent with dust at T ∼ 300 K, that is we find no evidence of an increase in the temperature of the dust towards the centre. From this we infer that most of the near-infrared emission probably comes from parsec scales as well. We further argue that the disk component alone is not sufficient to provide the necessary obscuration and collimation of the ionising radiation and outflow. The material responsible for this must instead be located on scales of ∼ 1 pc, surrounding the disk. We associate this material with the dusty torus. Conclusions. The clear separation of the dust emission into a disk-like emitter and a polar elongated source will require an adaptation of our current understanding of the dust emission in AGN. The lack of any evidence of an increase in the dust temperature towards the centre poses a challenge for the picture of a centrally heated dust distribution. Key words. galaxies: active, galaxies: nuclei, galaxies: Seyfert, galaxies: structure, galaxies: individual: Circinus, techniques: inter- ferometric 1. Introduction componentof AGN. First of all, the torus plays an importantrole in fuelling the AGN activity: it either forms the passive reservoir Active galactic nuclei (AGN) are thought to play a major of material for the accretion onto the supermassive black hole or, role in the formation and the evolution of galaxies. The AGN arXiv:1312.4534v1 [astro-ph.GA] 16 Dec 2013 more intriguingly, it is itself the active driver of the accretion to- phase provides mechanisms for feedback from the supermassive wards the black hole (Hopkins et al. 2012). Secondly, the dusty black hole to its hosting galaxy and the intergalactic medium. torus is held responsible for the orientation-dependent obscura- Therefore, a thorough knowledge of the accretion process in tion of the central engine (e.g. Antonucci 1993; Urry & Padovani AGN is required to understand their influence on the formation 1995): when oriented face-on, a direct view of the central engine and evolution of galaxies. Especially little is known about the is possible through the cavity in the torus (Type 1 AGN); when accretion process on parsec scales. oriented edge-on, the view towards the centre is blocked by the A toroidal distribution of warm molecular gas and dust sur- gas and dust of the torus (Type 2 AGN). This scenario is suppor- rounding the central engine, the so-called dusty torus, is a key ted by multiple observational evidence, most importantly by the detection of broad emission lines in the polarised light of several ⋆ Based on observations collected at the European Southern Type 2 nuclei (e.g. Antonucci & Miller 1985; Lumsden et al. Observatory, Chile, programme numbers 073.A-9002(A), 060.A- 2004), indicating that Type 2 sources host the same central en- / 9224(A), 074.B-0213(A B), 075.B-0215(A), 077.B-0026(A), 081.B- gine as Type 1 AGN. However, there is also a growing number 0893(A), 081.B-0908(A/B), 383.B-0159(A), 383.B-0993(A), 087.B- 0746(C), 087.B-0971(A-C), and 087.B-0266(H). of observations that challenge this simple picture, e.g. the dis- 1 K. R. W. Tristram et al.: The dusty torus in the Circinus galaxy covery of true Type 2 sources (without broad emission lines in Table 1. List of the MIDI observation epochs for the Circinus polarised light) and X-ray column densities discrepant with the galaxy including observing date, baseline parameters (telescope optical classification (for a recent discussion of AGN obscura- combination, range of baseline lengths and position angles) and tion, see e.g. Bianchi et al. 2012). number of (un)successful measurements. The dust in the torus is heated by the emission from the ac- cretion disk. It re-emits this energy in the infrared (Rees et al. date baseline BL [m] PA [°] good bad 1969): the innermost dust is close to its sublimation temperature 2004-02-12 U3-U2 43 to 43 19 to 21 2 0 and mainly emits in the near-infrared, while the dust at larger 2004-06-03 U3-U2 20 to 29 92 to 129 2 0 distances is at lower temperatures and emits in the mid-infrared 2005-02-21 U2-U4 87 35 0 1 (Barvainis 1987). Direct observations of the torus are thus best 2005-03-01 U3-U4 49 to 62 44 to 130 6 0 carried out at infrared wavelengths. However, the dust distribu- 2005-04-18 U2-U4 89 60 1 0 2005-05-26 U2-U3 35 to 43 12 to 69 6 0 tions are very compact: they are essentially unresolvedby single- 2006-05-18 U2-U3 23 to 31 84 to 114 4 0 dish observations even with the largest currently available tele- 2008-04-17 U1-U3 75 to 84 42 to 60 0 5 scopes (e.g. Horst et al. 2009; Ramos Almeida et al. 2009). Only 2008-04-18 U2-U4 76 to 89 33 to 156 26 2 by employing interferometric methods in the infrared is it pos- 2008-04-26 E0-G0 12 to 15 14 to 155 23 2 sible to resolve the nuclear dust distributions in AGN. 2009-04-15 U1-U3 54 to 92 8 to 90 17 1 To date, several interferometric studies of the nuclear dust 2009-04-27 E0-G0 14 to 16 16 to 113 16 1 distributions of individual galaxies have been carried out. In the H0-G0 24 to 25 138 to 164 3 1 Seyfert 2 galaxy NGC 1068, the interferometric observations re- 2011-04-14 U1-U2 35 65 1 0 veal a hot, parsec-sized disk that is surrounded by warm dust 2011-04-17 U3-U4 47 to 62 33 to 124 10 0 ff 2011-04-18 U2-U4 87 to 87 32 to 41 7 1 extended in polar direction (Wittkowski et al. 2004; Ja e et al. 2011-04-19 U1-U3 86 to 86 35 to 37 2 0 2004; Weigelt et al. 2004; Poncelet et al. 2006; Raban et al. 2011-04-20 U2-U4 77 to 81 127 to 150 16 0 2009). A two-component structure was also found in the nuc- 2011-05-06 C1-A1 14 to 15 26 to 104 10 1 leus of the Circinus galaxy (Tristram et al. 2007, see below). D0-B2 26to27 34to 38 0 2 In NGC 424 (Sy 2) and NGC 3783 (Sy 1) the thermal dust totals: 152 17 emission appears extended along the polar axis of the system (Beckert et al. 2008; Hönig et al. 2012, 2013). In NGC 4151, Notes. U in the baseline stands for Unit Telescope (UTs); A1 to H0 a Seyfert 1.5 galaxy, interferometric measurements in the near- denote the stations of the Auxiliary Telescopes (ATs). Measurements and mid-infrared have provided evidence of both hot dust at the were considered successful (“good”) if a fringe signal was tracked and sufficient signal was detected (see also Appendix A). inner rim of the torus as well as warm dust farther out (Swain et al. 2003; Burtscher et al. 2009; Kishimoto et al. 2009b; Pott et al. 2010). In the radio galaxy NGC 5128 (Centaurus A) on the other hand, only half of the mid-infrared emission appears diverse models using different assumptions and parameters can to be of thermal origin (Meisenheimer et al.
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