A First Look at the Doughnut Around a Giant Black Hole

First detection by infrared interferometry of an extragalactic object

Active galactic nuclei (AGN) are one of the most energetic and
mysterious phenomena in the universe. In some galaxies indeed, the core
generates amounts of energy which surpass those of normal galaxies, such
as the Milky Way, by many orders of magnitude.

The central engine of these power stations is thought to be a
supermassive black hole. Indirect lines of evidence have suggested that
these massive black holes are enshrouded in a thick doughnut-shaped
structure of gas and dust, which astronomers call a “torus”. However,
due to the limited sharpness of images that can be obtained with present
telescopes in the 10-m range, such a torus has never been imaged to date.

Using the new and powerful VLT Interferometer [1] – a mode of
the ESO Very Large Telescope that combines light
from at least two telescopes to obtain information on very fine scales –
a team of European astronomers [2] has succeeded for the first
time in resolving structures in the dusty torus of the prototype AGN,
the famous galaxy NGC 1068. The structures have a size of roughly 0.03
arcsec, corresponding to about 10 light-years at the distance of the
galaxy.

This important achievement shows that the VLT Interferometer, using the
recently inaugurated MIDI instrument [3], proves an invaluable
tool in the study of objects outside our own Galaxy.

The full text of this Press Release, with three photos (ESO PR Photos
18a-c/03) and all related links, is available at:

http://www.eso.org/outreach/press-rel/pr-2003/pr-17-03.html

Cosmic power station

Active galaxies are among the most spectacular objects in the sky. Their
compact nuclei (AGN) are so luminous that they can outshine an entire
galaxy. These objects show many interesting observational
characteristics over the whole electromagnetic spectrum, ranging from
radio to X-ray emission.

Active galaxies take many forms: some have bright nuclei emitting
high-energy (i.e. ultraviolet and X-rays) photons, some have high-energy
nuclei but appear to be surrounded by a more-or-less “normal” galaxy,
while some have long narrow jets or beams of matter streaming out from
the centre.

There is now much evidence that the ultimate power station of these
activities originate in supermassive black holes with masses up to
thousands of millions times the mass of our Sun (see e.g. ESO PR 04/01).
The black hole is fed from a tightly wound
accretion disc encircling it. Material that falls towards such black
holes will be compressed and heated up to tremendous temperatures. This
hot gas radiates an enormous amount of light, causing the active galaxy
nucleus to shine so brightly.

Enshrouded in the mystery torus

The central region of an active nucleus is currently believed to be
surrounded by a doughnut of dense and opaque gas and dust. It was first
thought that the different types of active galaxies were fundamentally
different objects. Astronomers now prefer the so-called “unified” model
of AGN, meaning that most or all AGN are actually just different
versions of the same object. What the object looks like depends on the
orientation of the doughnut on our line of sight : can we see through
the doughnut hole deep into the bright centre or can we only see the
opaque walls ? Some AGN appear indeed very luminous because we see
straight down to the emission site, while others would be very dim,
since the torus hides the central power station from our view. These
doughnuts or tori are, however, very difficult to resolve because of
their very small size, typically a few tens of light-years. For the
nearest active galaxy, this corresponds to an estimated angular diameter
less than 0.05 arcsec, much smaller than what can be observed with
present single large telescopes in the 10-m range.

Since, so far, evidence for the tori is only indirect, a large variety
of models has been proposed as to how these tori could be, varying from
very dense and compact tori, to very extended and fluffy tori. What the
astronomers really need, in order to differentiate among the models, is
a direct image of a torus. But until now, no telescope could see sharp
enough to spot one.

Finding a needle in a haystack

This is where interferometry with large telescopes makes a difference.
Interferometry is the technique which combines two or more telescopes to
achieve an angular resolution equal to that of a telescope as large as
the separation of the individual ones (cf ESO PR 06/01 and 23/01). The
recently inaugurated ESO Very Large Telescope Interferometer on top of
the Paranal mountain has the ambitious goal of making interferometry a
tool available to every astronomer. Just a few months ago, the first of
the powerful instruments for the VLTI was installed, the 10 micron beam
combiner mid-infrared interferometric instrument (MIDI, cf. ESO PR
25/02). This will be followed in early 2004 by the AMBER instrument [4].

MIDI is sensitive to light of a wavelength near 10 microns, i.e. in the
mid-infrared spectral region (the so-called “thermal infrared”). Located
at the heart of the VLT Interferometer with its multiple baselines of up
to 200 m, MIDI can reach an angular resolution of about 0.01 arcsec.
Combined with two powerful 8.2-m VLT Unit Telescopes, MIDI has for the
first time in infrared interferometry enough sensitivity to study
objects far away from our galaxy, the Milky Way.

With its high sensitivity to thermal radiation, MIDI is ideally suited
to study cosmic material near a central object and heated by its
radiation. The ultraviolet and optical radiation from the hot material
surrounding the black hole indeed heats the dust torus to several
hundred degrees. The absorbed energy is then re-radiated in the thermal
infrared between 5 and 100 microns.

The MIDI instrument on the VLTI is thus the most appropriate instrument
to peer at the enigmatic dust and gas tori believed to be located around
giant black holes at the centres of quasars and Active Galactic Nuclei.

And since nobody has ever been able to use interferometry to study faint
objects in the thermal infrared, MIDI enters into a whole unexplored
territory.

On the nights of June 14 to 16, a team of European astronomers [2]
conducted a first series of observations to verify the
scientific potential of MIDI on the VLTI. Among them, they studied the
active galaxy NGC 1068.

NGC 1068: a prototype AGN

NGC 1068 is among the brightest and most nearby active galaxies. Located
in the constellation Cetus at a distance of about 60 million light
years, it is also known as Messier 77. It is in fact one of the biggest
galaxies in Messier’s catalogue and one of the first recognised spiral
galaxies. On optical images, NGC 1068 looks indeed like a rather normal
barred spiral galaxy. The core of the galaxy, however, is very luminous
not only in the optical, but also in ultraviolet and X-ray light. A
black hole with a mass equivalent to approximately 100 million stars
like our Sun is required to account for the nuclear activity in NGC 1068.

Fringes in the distant dust: resolving the torus in NGC 1068

The MIDI observations used two of the 8.2 m VLT Unit Telescopes (Antu
and Melipal), separated by a baseline of 102 m. Due to projection
effects, the actual baseline for the NGC 1068 observations amounted to
79 m. While observing NGC 1068, the astronomers detected interferometric
fringes (ESO PR Photo 18c/03). Fringes are produced
when beams of light from two telescopes are brought together exactly in
phase. For a point-like source, such fringes have the maximum possible
theoretical contrast (i.e. 100%): the source is unresolved. However,
sources of increasing angular size produce fringes with decreasing
contrast. In the case of NGC 1068, the measured contrast was only about
10% of the maximum one. An exact interpretation of this result will
follow in the context of additional measurements along different
baselines, which are planned for this coming Autumn [5].
Already this initial result is nevertheless very convincing: the fringes
were obtained with consistent values on several measurements over 2
consecutive nights, thanks also to the excellent observing conditions at
the Paranal site (“seeing” values were between 0.3 and 0.6 arcsec). It
is already possible to state that a structure on a spatial scale of
approximately 0.03 arcsec (corresponding to about 10 light-years) has
been detected in the dust torus in NGC 1068. The relative size of this
structure is shown in ESO PR Photo 18b/03.

A breakthrough in interferometry

This measurement represents the first observation ever by the technique
of long-baseline interferometry of an extragalactic object in the
thermal infrared. This new success of the VLTI opens the door to a
completely new field in astronomy: the study of gas and dust structures
surrounding and feeding the biggest monsters in the universe. MIDI and
the VLTI will offer for years to come the best combination for
astronomers from all over the world to carry out these studies.

Notes
[1] More information about the VLTI and photos of many of the components
of the facility are available at the VLTI website, as well as in
ESO PR 06/01
(“First Light” in March 2001 and explanation of the interferometric
measurements), ESO PR 23/01 (observations with two 8.2-m telescopes in
October 2001) and ESO PR 16/02 (observations with four 8.2-m telescopes
in September 2002), ESO PR 22/02 (measurements of the diameters of
small stars in November 2002) and ESO PR 11/03
(installation of the first MACAO adaptive optics unit in May 2003).

[2]: The observations were planned and carried out by a team led by
Andrea Richichi (ESO) and including C. Leinert, R. Koehler, K.
Meisenheimer (MPIA), R. Waters (Amsterdam), F. Malbet (Grenoble), M.
Schoeller, S. Morel , F. Paresce, A. Glindemann, M.Tarenghi (ESO), H.
Roettgering and W. Jaffe (Leiden).

[3]: The MIDI instrument (http://www.mpia-hd.mpg.de/MIDI/) is the result
of a collaboration between German, Dutch and French institutes. See ESO
PR 25/02.

[4]: The AMBER instrument will equip the VLTI starting from 2004. It
will cover the 1-2.5 micron range, combining up to three different
telescopes. http://www.obs-nice.fr/amber/

[5]: NGC 1068 is well visible in September/October. The present
measurement was obtained under demanding pointing in June, in the very
last hour of the night.

Contacts

A. Richichi

ESO

Garching, Germany

Phone: +49 89 3200-6803

email: arichich@eso.org

C. Leinert [MIDI]

Max-Planck-Institut fur Astronomie

Heidelberg, Germany

Phone: +49 6221 528 264

email: leinert@mpia.de

H. Roettgering [NEVEC]

Leiden Observatory, The Netherlands

Phone: +31 71-5275851
email: rottgeri@strw.leidenuniv.nl