First observation of an active galactic nucleus with the VLTI: resolution of the torus at the heart of NGC 1068 |
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First observation of an active galactic nucleus with the VLTI: resolution of the torus at the heart of NGC 1068
The paradigm to explain the large variety of active galaxy nuclei is the existence of a supermassive black hole, surrounded by a torus of gas and dust, hiding it for certain lines of sight. The Seyfert 2 galaxy, NGC 1068, is precisely in this case, where the active nucleus is hidden to us by a torus. For the first time, the emission of the dust torus is clearly resolved by the VLT interferometer, the VLTI. According to the European consortium, with researchers from Paris Observatory, at the origin of these observations, the torus could be decomposed into a compact and hot part of 1 pc and a wider and colder one of size 3.4 pc.
The instrument MIDI (Mid-Infrared Interferometer) of the large interferometer VLTI (Very Large Telescope Interferometer) of the European Southern Observatory in Chile is the first instrument observing between 8 and 13 microns to offer an angular resolution of about 10 to 20 mas (milli arcsecond) according to the configuration (resolution corresponding to a building of ten floors on the Moon seen from the Earth).
MIDI was carried out by a European consortium of German, Dutch and French institutes (Observatories of Paris and Côte d'Azur). The first scientifically exploitable data were obtained during the year 2003 and, after a delicate phase of treatment and analysis, the results now start to accumulate.
It is the first observation of an active galaxy nucleus by an interferometer in the thermal infra-red. It was made possible thanks to the use of the VLT sensitive telescopes, 8.2 m in diameter, and with the baselines of 98m (UT1-UT3) and 46m (UT2-UT3), thus offering resolutions corresponding to telescopes of 98 and 46 m in diameter, still unaccessible. The active galaxy nuclei (AGN) indicate the central heart of very active galaxies, thought to shelter a supermassive black hole, several hundreds of million times the mass of the Sun, surrounded by an accretion disc in which gas falls in spiraling towards the center, thus releasing a gigantic energy. Ionized matter jets are sometimes ejected at very high speed in two opposite directions, perpendicular to the disc. In periphery of this central engine which remains inaccesible with the current instruments at high angular resolution, the astronomers expect to find matter more disorganized, probably in the form of a torus containing gas and dust. This torus is the last border, in the case of NGC 1068 which is seen edge-on, before the energetic central engine. Its observation is thus crucial for the understanding of the nature of the nucleus and its energetic mechanisms.
A large field image of the galaxy is given in Figure 1. The spiral arms are clearly visible. The region observed by MIDI is the bright nucleus.
Figure 1 Image of the spiral galaxy NGC 1068 in the optical domain. At the distance of NGC 1068, about 17 Mpc, 1 kpc corresponds to a 12 second field of view.
Click on the image to enlarge itThe observations proceeded in June and November 2003. Only the central part of Figure 2, very bright in the infra-red compared to the galaxy host, was observed by MIDI.
Figure 2 Field of view of the instrument MIDI projected on the sky. The image in the background has been obtained by adaptive optics at 2,2 microns. The width of the slit of MIDI is 1 arc second.
Click on the image to enlarge itThe output of MIDI is the correlated flux of the source. It is the product of the source spectrum by the visibility which is also wavelength-dependent. The smaller the visibility of the object is and the more the object is resolved, a point source having a visibility of 1. It is this which informs about the size of the object. Combined with spectral information, it allows to know certain spatial and physical properties of the source.
Figure 3 The top graph is the spectrum of the object measured by MIDI between 8 and 13 microns. The two following graphs are the correlated fluxes measured with baselines 46m and 98m whose projections in the direction of NGC 1068 are respectively 42 and 78 m.
Click on the image to enlarge itThe ratio between the correlated flux and the spectrum gives the visibility. The visibility decreases with the baseline length, a sign that the source is resolved by the interferferometer.
The visibility curves from MIDI are given in Figure 3. Visibilities are smaller (down to 5%) at longer baselines thus showing that the object has been resolved. The spectrum of NGC 1068 has an absorption feature around 9.7 µm, a signature of silicate grains. A visibility gap is clearly visible at this wavelength, an evidence that the object size is larger. Silicate grains are therefore likely to be components of the larger structure in the object. This information shows that the object can be modelled by two components. A hot and compact component of characteristic size 1 pc (12 mas) and of temperature higher than 800 K, and a more extended component of size 3,4 pc (50 mas) and temperature of about 320 K. The previous adaptive optics observations had led to maximum sizes of 120 mas for this structure. The observations of MIDI show that the nature of the object is up to now in agreement with current AGN models. The impulse is now given for the exploration of the AGN at high angular resolution in the near infra-red. Still better angular resolutions will be necessary to resolve the most central regions. The next step will soon be made with the VLTI instrument AMBER operating between 1 and 2.4 microns. The Paris Observatory also works at the realization of an interferometer of 800 m, 'OHANA, coupling the large telescopes of Mauna Kea in Hawaii and allowing in the long term to reach resolutions below the mas between 1 and 2.4 microns, required to study remote AGN sources.
References
The central dusty torus in the active nucleus of NGC 1068: W. Jaffe, K. Meisenheimer, H. J. A. Rottgering, Ch. Leinert, A. Richichi, O. Chesneau, D. Fraix-Burnet, A. Glazenborg-Kluttig, G.-L. Granato, U. Graser, B. Heijligers, R. Kohler, F. Malbet, G. K. Miley, F. Paresce, J.-W. Pel, G. Perrin, F. Przygodda, M. Schöller, H. Sol, L. B. F. M. Waters , G. Weigelt1, J. Woillez & P. T. de Zeeuw, à paraître dans l'edition du 6 Mai 2004 de la revue Nature.Links
VLTI: http://eso.org/projects/vlti/
MIDI: http://www.eso.org/instrumets/midi/
Contacts
Hélène Sol (Observatoire de Paris, LUTH)
Julien Woillez (Observatoire de Paris, LESIA et W.M. Keck Observatory, Hawaii)
