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First observation of an active galactic nucleus with the VLTI : resolution of the torus at the heart of NGC 1068

1er mai 2004 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 Vue de la galaxie spirale NGC 1068 dans le domaine optique. À la distance de NGC 1068, soit 17 Mpc, 1 kpc correspond à un champ de 12 secondes d’angle. Cliquer sur l’image pour l’agrandir

The 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 La taille moyenne des étoiles Mira et de leur enveloppe moléculaire est comparée à la taille du système solaire interne. Les orbites de la Terre, Mars et Jupiter sont représentées, ainsi que la position du Soleil. Quand le Soleil deviendra une étoile de type Mira dans environ 5 milliards d’années, il atteindra presque l’orbite de Mars, et son enveloppe moléculaire ira bien au delà de Mars. Cliquer sur l’image pour l’agrandir [en]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 it

The 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 Le graphique du haut est le spectre de l’objet mesuré par MIDI entre 8 et 13 µm. Les deux graphiques suivants sont les flux corrélés mesurés avec les bases de 46m et 98m dont les projections dans la direction de NGC 1068 sont respectivement de 42 et 78 m. Cliquer sur l’image pour l’agrandir

The 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.

Contact

  • Guy Perrin (Observatoire de Paris, LESIA)
  • Hélène Sol (Observatoire de Paris, LUTH)
  • Julien Woillez (Observatoire de Paris, LESIA et W.M. Keck Observatory, Hawaii)
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