Astronomical Science The Diversity of Dusty AGN Tori: Results from the VLTI/MIDI AGN Large Programme Leonard Burtscher1 BNLONMDMSLNCDKNESGDCTRSDLHRRHNMHMSGD"HQBHMTRF@K@WX Konrad R. W. Tristram2 and the MIDI AGN LP team* 1 1 Max-Planck-Institut für extra- terrestrische Physik, Garching, ( Germany 2 Max-Planck-Institut für Radio- astronomie, Bonn, Germany R L@ The molecular gas and dust surround- " ing the central engine of active galactic #$ b nuclei (AGN) plays a fundamental role in understanding their physics. Due to DS the small angular size of this torus, ER direct studies of its properties require .E very high angular resolution. The VLTI/ MIDI AGN Large Programme aims at l characterising a statistical sample of AGN tori for the first time by resolving their dust emission in the mid-infrared interferometrically. Measurements of 23 sources show that most tori are very l compact with sizes of 0.1 to 10 pc SF and emission from a two-component structure. The morphology of the torus ll differs significantly between individual .EERDSb1 L@R objects and does not show the expected dependency on viewing angle (edge- Figure 1. Colour image of the three-component perpendicular to the ionisation cone (cyan lines) and on vs. face-on). The torus is thus signifi- model, describing over 150 individual MIDI measure- which is coincident in orientation with water maser ments of both the visibility amplitude and differential emission (red–green–blue line); and (3) an extended cantly more complex than typically phase from the Circinus galaxy (Tristram et al., A&A, emission region roughly elongated in the polar direc- pictured, requiring new models and 2013). The model components are: (1) an unresolved tion with an increase of the silicate absorption imaging interferometry on milliarcsec- component; (2) a highly inclined disc, offset with towards the south-east (purple–green, with purple ond scales to decipher the physics. respect to component (1), whose major axis is representing deeper silicate feature). in resolving what is commonly referred to Although these two prominent sources Motivation as the torus in active galactic nuclei have much in common, they also show (AGNs; see Meisenheimer et al., 2008). marked differences. Most notably, hot The Mid-Infrared Interferometric Instru- This torus is considered to be responsi- dust (T ~ 800 K) on parsec scales has ment (MIDI) at the Very Large Telescope ble for the type 1 (broad-line) / type 2 only been detected in NGC 1068, while Interferometer (VLTI) is unique worldwide (narrow-line) dichotomy in AGNs and per- the Circinus galaxy, where a similar haps plays a crucial role in feeding of spatial resolution is reached, seems to *The MIDI AGN LP team: the central supermassive black hole (e.g., contain only warm (T ~ 300 K) dust. K. Meisenheimer (P.I.)1, L. Burtscher2, K. R. W. Tristram3, W. Jaffe4, S. F. Hönig5, R. I. Davies2, Schartmann et al., 2009). Until a couple of years ago, only a few M. Kishimoto3, J.-U. Pott3, H. Röttgering4, other extragalactic objects had been M. Schartmann3,6, G. Weigelt3, S. Wolf7 By interferometrically combining light from studied with MIDI, but it was clear that two of the large Unit Telescopes (UTs), a much larger sample was needed to 1 Max-Planck-Institut für Astronomie, Heidelberg, Germany AGNs with mid-infrared (MIR) fluxes as build a more coherent view of the nuclear 2 Max-Planck-Institut für extraterrestrische Physik, faint as ~ 100 mJy can be studied, and dust structures of AGNs. Garching, Germany structures as small as ~ 5 milliarcsec- 3 Max-Planck-Institut für Radioastronomie, Bonn, onds detected. The two brightest extra- This statistical sample has now been Germany 4 Sterrewacht Leiden, Universiteit Leiden, galactic southern sources in the MIR assembled by the first VLTI Large Pro- the Netherlands wavelength range (8–13μm) — NGC 1068 gramme (PI: K. Meisenheimer) together 5 Dark Cosmology Center, University of Copenhagen, and the Circinus galaxy — have now with two accompanying programmes Denmark been observed in great detail and reveal and data from the archive. The questions 6 Universitäts-Sternwarte München, Germany 7 Institut für Theoretische Physik und Astrophysik, complex morphologies on (sub-)parsec that this programme set out to answer Universität zu Kiel, Germany scales (see Figure 1). include: 62 The Messenger 154 – December 2013 – What are the elongations of the dust structures and how do they relate to the other axes in these systems? Is the dust chemistry of the nuclear structures different to that on larger scales? – Is there a connection between nuclear star clusters and the genesis of the torus? This more far-reaching question will be addressed using a combination of observational data (with an important constraint from the MIDI observations reported here) together with hydrody- -TLADQNE,(#(RNTQBDR namical simulations (Schartmann et al., 2010). Observations %KTWCDMRHSX@Sƅ)X Due to the specific distribution of MIR Figure 2. Histogram of mid-infrared fluxes of AGNs – What are the morphology and the size target brightnesses (Figure 2), enlarging that have been interferometrically detected with MIDI of the nuclear structures? Are type the sample was only possible by observ- (open bars); filled bars are for sources studied in Burtscher et al. (2013), and discussed in this article. 1/2 sources intrinsically different on ing targets that were at least ten times In order to increase the sample beyond the two parsec scales? These phenomenologi- fainter (and much more distant) than the brightest sources, the sensitivity had to be increased cal questions are directly addressable two initially studied AGNs. NGC 1068 and by more than a factor of ten. from the interferometric data and are the Circinus galaxy are among the clos- the focus of this article (see Burtscher est AGNs with distances of 14 and 4 Mpc et al., 2013); respectively. The most distant object that (L@FDRO@BD(MSDMRHSXCHRSQHATSHNM L@R Figure 3. Nuclear mor- phologies of AGNs in the MIR revealed by MIDI. Left: resolved source with an addi- tional unresolved com- ponent (visibility de­­ creases with baseline L@R length, but is always > 0); middle: over-resolved 1DRNKUDC@[email protected] QDRNKUDC@MCTMQDRNKUDCB@RD $RRDMSH@KKXTMQDRNKUDCB@RD source with an addi- tional unresolved com- %NTQHDQRO@BD5HRHAHKHSX ponent (visibility is ~ constant with baseline ,"& -&" ,QJ length and < 1); right: essentially unresolved source (visibility is ~ 1 on all baselines with only a shallow decrease). Top row: intensity dis- tribution in image space; 5HRHAHKHSX bottom row: radial visi- bility profiles of repre- sentatives of the three different nuclear MIR morphologies observed in the MIDI AGN Large Programme: MCG-05- 23-16, NGC 4151 and /QNIDBSDCA@RDKHMDKDMFSGL /QNIDBSDCA@RDKHMDKDMFSGL /QNIDBSDCA@RDKHMDKDMFSGL Mrk 1239. The Messenger 154 – December 2013 63 Astronomical Science Burtscher L. & Tristram K. R. W., The Diversity of Dusty AGN Tori Figure 4. Point source 2. Over-resolved sources, where the fraction (~ minimum visibility was less than 100%, but no observed visibility) as a function of resolution. change of visibility with baseline or Type 1 AGNs are plotted position angle was observed; and in blue, type 2 AGNs in 3. Essentially unresolved sources. red; triangles show lim- its. To overcome resolu- tion / distance effects in Most of the resolved (case 1) and all of our flux-limited sample, the over-resolved (case 2) sources show we normalise the resolu- clear signs of a two-component struc- tion to the inner radius Q@BSHNM ture, where the larger component is (at of dust rin (using the DE empirically derived rela- least) several tens of milliarcseconds in tion between bolometric size (about 1 pc in the nearest galax- RNTQB luminosity and r ). If all S in ies — 30 pc in the most distant ones) and HM tori were equal, the the compact, unresolved point source is /N point source fraction would only depend on less than about 5 milliarcseconds in size. this "intrinsic resolution" — which is clearly not The low-resolution spectra obtained with the case. Type 1 sources MIDI generally show an almost feature- show higher levels of -&" unresolved flux than less continuum with only the broad sili- "HQBHMTR SXOD &- type 2 sources (but are cate feature imprinted in absorption or nonetheless statistically SXOD &- emission. Here we show only the visibili- of the same size). ties at 12 μm (Figure 3) since the sources are best resolved at this wavelength. 1DRNKTSHNMHMMDQQ@CHTRQHM is observed with MIDI is the quasar 3C MIR-bright AGNs, therefore, a sub - A large sample of (partly) resolved AGN 273 at a redshift z = 0.158. It is marginally stantial fraction of the nuclear 12 µm tori resolved. emission must originate from very com- pact (0.1–10 pc) scales, that are only Apart from the generally high level of visi- To scientifically exploit these observa- partially resolved even on the longest bility in the weak sources, another sur- tions, a substantial effort was started to baselines with MIDI. prising result was the large diversity in better understand the data reduction morphology among the sources. This is of the weak objects; correlation losses This finding was somewhat unexpected, most readily seen when looking at the (that appear when the signal-to-noise since scaling relations, derived from lowest visibility measured for each source ratio is very low) and calibration uncer- near-infrared (NIR) dust reverberation (we call this the “point source fraction”). tainties were quantified (Kishimoto et mapping and interferometry, predicted If the tori in the sources were all of equal al., 2011; Burtscher et al., 2012). This more extended dust structures if based size, we would expect that the point allowed us to consistently analyse the on the bright sources that had been ini- source fraction depends only on the res- newly acquired data as well as all previ- tially studied with MIDI.