Revealing the nuclear structure of Active Galactic Nuclei
The very luminous Active Galactic Nuclei found in distant galaxies mark
distinct periods of intense activity in the lifetime of these systems. During such
periods, the power source for the nuclear engine is generally assumed to be
the accretion of matter onto a massive compact object. However, the great
variety of nuclear signatures can only be explained with a non-isotropic spatial
distribution of strongly obscuring material. The nature of this obscuring
structure, its spatial distribution, and in particular, its relation to the nuclear
engine are topics of great interest.
Very Long Baseline Interferometry radio measurements using the European
VLBI Network (EVN) have been used to image molecular emission of a
prominent AGN called Markarian 231. In an article published in Nature, HansRainer Klöckner (Univ of Groningen, NL), Willem Baan (ASTRON, NL) and
Michael Garrett (JIVE, NL) have been able to infer the spatial distribution and
dynamics of an obscuring torus or "donut" like structure, that surrounds and
rotates about the nucleus of Markarian 231.
While the physical processes governing the nuclear engine of active galaxies
have been generally confirmed observationally, the direct imaging of the
structural components of the nuclear region has not yet been possible, except
in some very special cases. The new results uniquely probe the structure and
dynamics of the obscuring material on scales of several tens to hundreds of
light-years.
Markarian 231 is located 530 million light years away, and is the most
luminous infrared galaxy in the local universe. The prominent tidal tails seen
in optical images, together with its high dust content, conclusively
demonstrate that it is undergoing a major merger (i.e. it is the product of the
collision and interaction of two previously isolated galaxies). At the core of
Markarian 231 lies an active nucleus with powerful jets of radio emission
emanating from its centre.
The molecular emissions observed by Klöckner et al. in Markarian 231 are
due to the OH molecule that through self-amplification (maser action) can
become a million times more luminous than their Galactic counterparts. Such
"megamaser" emissions have been observed in around 100 galaxies, which
are also characterized by intense infrared emission. According to Klöckner et
al. the OH emission revealed by the EVN observations suggests a spatial
structure, which is best described as an inclined rotating torus (donut-like)
structure, surrounding the central engine with an (estimated) inner radius of
90 light years and a thickness of 200 light years. The total extent of this
structure is modeled to be about 600 light years across, and it forms the inner
part of a larger scale galactic disk also seen in the galaxy. Figure 1 shows the
main details of the model.
Figure 1: Inferred model of the nuclear torus in Markarian 231 is displayed as
a wire diagram with the symmetric cones indicating the nuclear outflow (the
ionization cones of the nuclear region). Superimposed is the velocity field of
the OH 1667 MHz emission in pseudo colour, where the bluish pattern
indicates the emission coming towards the reader (as seen by the Doppler
effect) and the redder part is moving from the reader into the paper. The
symmetry axis of the torus has been inclined by 56 degrees, and rotated anticlockwise by 35 degrees.
Although imaging of galactic nuclei has been performed in various other
experiments, these data provide a first clear view of the dynamics of a nuclear
component on such scales, and the interpretation is consistent with the
general understanding of Active Galactic Nuclei and Unified Schemes. that
explain the variety of observed characteristics.
About the European VLBI Network and JIVE:
The European Very Long Baseline Interferometry Network (EVN) is a network
of 14 nationally-funded radio telescopes in different countries that carry out
VLBI observations for the astronomical community in Europe. The data from
the telescopes is combined interferometrically to create an extremely
sensitive radio telescope whose diameter is as large as the largest separation
of the telescopes - many thousands of km across. This technique produces
the sharpest images mankind can now make of stars and galaxies in distant
parts of the Universe. The combination of EVN data takes place at the Joint
Institute for VLBI in Europe, in Dwingeloo, The Netherlands.
Web pages: www.evlbi.org and www.jive.nl
Further information and high quality images are available at:
www.astro.rug.nl/~hrkloeck/np/pubmrk231.html
Contact via secretariat of ASTRON:
phone : 0031 (0)521 595 223
fax
: 0031 (0)521 597 332
Hans-Rainer Klöckner
Kapteyn Institute, P.O. Box 800, 9700 AV Groningen,
The Netherlands
& ASTRON
e-mail : hrkloeck@astro.rug.nl
Willem A. Baan
ASTRON, Westerbork Observatory, P.O. Box 2, 7990 AA Dwingeloo,
The Netherlands
e-mail: baan@astron.nl
Michael A. Garrett
JIVE, Joint Institute for VLBI in Europe, P.O. Box 2, 7990 AA Dwingeloo,
The Netherlands
e-mail: garrett@jive.nl