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Mid-Infrared Interferometry of Active Galactic Nuclei: an Outstanding Scientific Success of the VLTI

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Mid-Infrared Interferometry of Active Galactic Nuclei: an Outstanding Scientific Success of the VLTI Astronomical Science Mid-infrared Interferometry of Active Galactic Nuclei: an Outstanding Scientific Success of the VLTI Klaus Meisenheimer1 and below the torus are visible. The ob- time (Leinert et al., 2003). The sensitivity David Raban2 ject then appears as a Seyfert 2 galaxy. required for most AGN observations 1, 3 Konrad Tristram Spectropolarimetric observations of (correlated flux Fcorr ≤ 1 Jy in the N-band) Marc Schartmann1,4,5 Seyfert 2 galaxies, showing broad lines in can only be reached by the combination Walter Jaffe 2 scattered light, support this idea (see of two Unit Telescopes (UTs) of the VLTI. Huub Röttgering 2 review by Antonucci, 1993). The UV-opti- The highest sensitivity for detecting Leonard Burtscher1 cal light which is trapped by dust in the and tracking the interferometric fringes is torus should heat the dust to a few hun- obtained by inserting a prism into the dred Kelvin, and the dust should re-radi- interferometric beams, that spectrally dis- 1 Max-Planck-Institut für Astronomie, ate in the mid-infrared. Indeed, the Spec- perses the N-band light with a spectral Heidelberg, Germany tral Energy Distributions (SEDs) of both resolution ,25. For brighter objects a 2 Sterrewacht Leiden, the Netherlands Seyfert 1 and Seyfert 2 galaxies display grism with a higher resolution of 250 can 3 Max-Planck-Institut für Radioastro- signatures of AGN heated dust between be used. In both cases MIDI delivers nomie, Bonn, Germany ,3L and 30 µm. It is an open issue two spectra (phase-shifted by 180 de- 4 Max-Planck-Institut für Extraterres- whether dust obscuration plays a similar grees) onto its detector, containing spec- trische Physik, Garching, Germany role in radio galaxies. tral and interferometric information at 5 Universitäts-Sternwarte, München, the same time. A special analysis pipeline Germany Before the advent of the VLT Interferom- is needed to extract this information. We eter (VLTI), the size, shape and internal use the Expert Work Station (EWS) pipe- structure of the torus remained unknown, line developed in Leiden by Walter Jaffe. Active Galactic Nuclei (AGN) are pow- although the mid-infrared spectra located ered by accretion onto a supermassive the dust within a few parsec of the core. Observations of the scientific target have black hole. The unified scheme for Single 8-m-class telescopes cannot re- to be complemented by standard star strongly accreting AGN postulates that solve mid-infrared structures of this size. observations obtained with an identical the central engine is enshrouded by a Even in the L-band (3.6 µm) a diffrac tion- instrumental set-up. The essential result of doughnut-shaped structure of gas and limited 8-m telescope is limited to 93 mas the pipeline analysis is a spectrum of the dust – the so-called torus. We report resolution (Full Width at Half Maximum, (calibrated) correlated flux Fcorr(L ) in the observations with the MID-Infrared FWHM). At the distance of nearby range 7.8 to 13.2 µm (see Figure 1b, c, d). Interferometric Instrument (MIDI) at the Seyfert galaxies, such as NGC 1068 and Fcorr(L ) corresponds to the Fourier Trans- VLT Interferometer, which resolve the NGC 4151 (14 Mpc), this corresponds to form of the source emission evaluated tori in the nearest Seyfert 2 galaxies, 6.5 parsec. at a coordinate (called ‘uv-point’ or ‘base- and suggest a complex structure, con- line’) given by the projected separation sisting of a compact inner disc embed- The situation changed dramatically between the telescopes as viewed from ded in a patchy or filamentary outer in December 2002, when MIDI, the MID- the source. Spatial information about the torus. The prominent nearby radio gal- Infrared Interferometric Instrument, source structure can be obtained com- axy Centaurus A, however, shows little became operational at the VLTI. MIDI paring Fcorr(L ) at different uv-points. To sign of a torus. Instead, its mid-infrared observes in the N-band (wavelengths 8 to the actually measured uv-points can be emission is dominated by non-thermal 13 µm). Using the widest telescope sepa- added the total flux Ftot(L ) registered by radiation from the base of the radio jet. ration (UT1–UT4) of 125 m, the width of a single telescope, essentially equivalent Thus, not all classes of AGN contain a the point-spread function at 8 µm is only to an observation with zero baseline (see thick torus. 7 mas, or 0.5 parsec at the distance of Figure 1a). Different baselines can either NGC 1068. But at the start of MIDI’s op- be realised by using different telescope erations two major questions remained: combinations or by observing the target The unified scheme for Active Galactic first, would MIDI be sensitive enough during its movement across the sky with Nuclei (AGN) explains various types of to reach extragalactic targets? Second, a fixed telescope combination. AGN by a line-of-sight effect: it postulates would observations with a handful of that the central engine – an accreting baselines allow us to reconstruct the dust As evident from Figure 1, the AGN spec- supermassive black hole – is embedded distribution in the torus and thus provide tra between 8.5 and 12.5 µm are often in a doughnut-shaped torus of gas and scientifically meaningful insights? This dominated by a broad absorption trough, dust. Thus, the hot accretion disc and the article demonstrates that today both caused by silicate dust grains. The exact surrounding Broad Line Region (BLR) is questions can be answered unequivo- profile of this ‘silicate feature’ depends only visible when looking along the torus cally: yes! on the chemical composition, size and axis. This is the case in Seyfert 1 galaxies, crystalline structure of the grains. Thus the optical spectra of which are charac- the N-band interferometry of an AGN not terised by a blue continuum and broad Mid-infrared interferometry with MIDI only resolves the spatial structure of the emission lines. In an edge-on case, how- nuclear dust but also can give insight ever, the direct view onto the core is MIDI operates as classical stellar interfer- into the dust properties within the inner blocked by the dusty torus and only nar- ometer of the Michelson type. It com- few parsecs. row emission lines from regions above bines the beams of two telescopes at a 36 The Messenger 133 – September 2008 25 of 14.4 Mpc, one parsec corresponds to is about 3.5 pc, but its exact shape re- (a) an angular scale of 14 mas, i.e. parsec- mains to be determined by shorter base- 1 20 scale structures can just be resolved with lines along the East-West direction. MIDI at the VLTI. ) y J The major axis of the hot component is ( 15 x u The earliest MIDI observations of perfectly aligned with a spur of water l F l NGC 1068 were obtained half a year after masers extending about 20 mas towards a t 10 o T MIDI became operational, during VLTI NW from the (radio-)core, although the Science Demonstration Time (SDT). Jaffe relative astrometric position cannot be 5 et al. (2004) demonstrated for the first determined. Surprisingly, the orientation time that a compact, geometrically thick of its minor axis (P. A. = 48˚), which might dust structure – as expected for the dust mark the symmetry axis of an inclined 8 9 10 11 12 13 Wavelength (µm) torus – indeed exists in Seyfert 2 galax- disc, does not fit well to the source axis ies. Essentially only two visibility points as determined from outflow phenome- 6 were observed at that time. The corre- na. The inner radio jet points almost ex- 5 Ozone lated fluxes were best modelled by two actly North (P. A. = 2˚), while the ionisa- components, a small, relatively hot one tion cone opens between P. A. , –5 – 4 (T > 800 K, diameter about 1 pc), embed- –30˚. For the standard torus scenario this 3 ded in a larger component of 320 K and is a puzzle: the open funnel which allows 2 about 3.5 pc diameter. the ionising UV-photons to escape should be caused by the angular momentum 1 (b) New observations with MIDI (Raban et al., barrier and thus be aligned with the rota- ) 2008b) cover the uv-plane much better: tion axis of the gas distribution. How y J ( 2.5 15 visibility points were obtained with the could a tilted disc form out of this gas? x u tele scope combinations UT1–UT3, Perhaps the hot inner component is l 2 F UT1–UT4, and UT2–UT3. An additional not a rotationally supported structure d 1.5 e t measurement with the orthogonal base- (disc) but rather a filament or hot channel. a l 1 e line UT3–UT4 proved essential for the r r o 0.5 (c) following results. To study the details of Further insight into the dust properties C the silicate absorption profile, the higher can be inferred from the depth of the sili- (R ,230) resolution grism was used. cate feature. In the total flux, which is 2.5 2 Even with this more complete uv-cover- 1.5 age, direct image reconstruction is not possible because MIDI observes only two 1 telescopes at a time and rapid atmos- 0.5 ((dd) pheric phase shifts cannot be recovered 0 by phase closure techniques. The meas- 8 10 12 ured Fcorr(L ) spectra for different baselines Wavelength (µm) still have to be interpreted by simple mod- els. Remarkably, a model of two compo- 40 –40 Figure 1. Results of MIDI observations of NGC 1068. nents with Gaussian brightness distribu- (mas) (a) Total flux F ( ): the contribution of the hot com- tot L tion and black-body spectrum describes ponent is shown in red, that of the extended compo- the correlated flux data reasonably well.
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