This Echo is READ ONLY !   NO Un-Authorized Messages Please!
 ~~~~~~~~~~~~~~~~~~~~~~~~   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

             Information from the European Southern Observatory

ESO Press Release 17/03

19 June 2003                                               [ESO Logo]

For immediate release
---------------------------------------------------------------------

A First Look at the Doughnut Around a Giant Black Hole

Part 1 of 2

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.

 - Continued -

@Message posted automagically by IMTHINGS POST 1.30
---