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Summary: The nearby galaxy Centaurus A harbours a supermassive black hole at its
centre. Using the ISAAC instrument at the ESO Very Large Telescope (VLT), an
international team of astronomers [1] has peered right through the
spectacular dust lane of the peculiar galaxy Centaurus A, located
approximately 11 million light-years away.

They were able to probe the thin disk of gas that surrounds the very
center of this galaxy. The new measurements show that the compact
nucleus in the middle weighs more than 200 million solar masses!

This is too much just to be due to normal stars. The astronomers thus
conclude the existence of a supermassive black hole lurking at the
centre of Centaurus A.

A well studied galaxy with a hidden center

ESO PR Photo 08a/01

Caption: PR Photo 08a/01 shows a small area in the direction of the
heavily obscured centre of the peculiar radio galaxy Centaurus A, as
seen in visual light. It measures about 80 x 80 arcsec2, or 4400 x 4400
light-year2 at the distance of this galaxy, and has been reproduced
from exposures made with the FORS2 multi-mode instrument at the 8.2-m
VLT KUEYEN telescope at Paranal. The full field may be seen in PR Photo
05b/00. Technical information about this photo is available below.

The galaxy Centaurus A (NGC 5128) is one of the most studied objects in the
southern sky. The unique appearance of this galaxy was already noticed by
the famous British astronomer John Herschel in 1847 who catalogued the
southern skies and made a comprehensive list of “nebulae”. A fine photo
of Centaurus A from the VLT was published last year as PR Photo 05b/00.

Herschel could not know, however, that this beautiful and spectacular
appearance is due to an opaque dust lane that covers the central part of
the galaxy. This dust is likely the remain of a cosmic merger between a
giant elliptical galaxy, and a smaller spiral galaxy full of dust.

Centaurus A is even more spectacular when observed with radio telescopes.
It is in fact one of the brightest radio sources in the sky (its name
indicates that it is the strongest radio source in the southern
constellation Centaurus). At a distance of merely 11 million light-years,
it is also the nearest radio galaxy. The radio emission from the very
compact centre exhibits strong activity. It has for some time been
suspected that this powerful energy release is due to accretion of
material onto a massive black hole.

The details of the centre have remained largely unknown, due to the dense
dust lane that completely obscures the central part of the galaxy in optical
light, cf. PR Photo 08a/01. Observations of the dust emission in the
mid-infrared spectral region were carried out with the ISOCAM camera onboard
the ESA Infrared Space Observatory. They revealed a structure extending over
5 arcmin (16,500 light-years or 5 kpc), centred on the compact radio source,
and very similar to that of a small barred galaxy. This bar may serve to
funnel gas towards the active nucleus of the galaxy.

Peering through the dust

To look into the very centre of the galaxy, the observations must be carried
out at wavelengths longer than those of visual light, e.g., in the infrared
spectral region. This is because the dust absorbs much less the infrared
radiation. Infrared observations of the innermost regions (of Centaurus A
(on an arcsec scale) were recently done by a team of astronomers from Italy,
UK and USA [1], by means of the multi-mode ISAAC instrument on the ESO Very
Large Telescope (VLT) at Paranal Observatory.

In fact, the team started their infrared studies of this galaxy already in
1997, using the NICMOS camera on board the Hubble Space Telescope (HST).
That close view of the galaxy nucleus revealed a thin gaseous disk of
material close to the center, which looked very much like an accretion disk
that was feeding material into a central black hole. The HST image prompted
further spectroscopic observations to probe the rotation of the disk, and
thus to measure the mass of the central object.

The ISAAC spectra

ESO PR Photo 08b/01

Caption: PR Photo 08b/01 shows two wavelength regions of one of the
infrared ISAAC spectra of the center of Centaurus A. The direction of
the long spectrograph slit is vertical and the dispersion (wavelength)
direction is horizontal; longer wavelengths are towards the right. The
two emission lines shown originate in singly ionized Iron ([FeII]; rest
wavelength 1256.68 nm) and in Hydrogen (Paschen-Beta; 1281.81 nm) and
both are clearly tilted. This is due to the rapid rotation of the
accretion disk surrounding the supermassive black hole in the center
of the galaxy. The light from the receding edge of the disk is
Doppler-shifted towards the red (to the right) and the light from the
part of the disk approaching us is shifted to the left. This may be
better seen in the inserted enlargments. Therefore the inclined disk
shows a tilted spectra. These motions may be represented in a rotation
curve, cf. PR Photo 08c/01. There are other emitting areas above and
below the nucleus, especially in the Paschen-Beta line. Technical
information about these photos is available below.

ESO PR Photo 08c/01

Caption: PR Photo 08c/01 shows the rotation curve (velocity vrs. Distance
from the centre) of the disk surrounding the black hole at the centre of
Centaurus A. From the ISAAC spectra displayed in PR Photo 08b/01, the
‘average’ gas velocities along the slit direction can be derived.
Position ‘0’ on the horizontal axis indicates the exact position of the
galaxy nucleus; at the distance of Centaurus A, 1 arcsec corresponds to
55.5 light-years (17 pc). The blue triangles and the red square
correspond to emission lines from singly ionized Iron atoms ([Fe II])
and Hydrogen (Paschen-Beta), respectively. The high velocities are the
hallmark of a central black hole. The thick solid line represents the
expected velocities, assuming the presence of a 200 million solar-mass
black hole at the centre. Technical information about these photos is
available below.

The spectroscopic observations required both a high sensitivity in the
infrared and excellent seeing conditions. This combination was achieved
using ISAAC at VLT. Peering through the thick walls of dust enshrouding
the nuclear region of Centaurus A, the astronomers succeeded in acquiring
several high-quality spectra of the thin central disk; the exposure time
for each spectrum was (about) 35 min.

The spectra did show the characteristic shape of a rotating disk, cf. PR
Photo 08b/01. High-speed motions of the gas in this disk were detected (PR
Photo 08c/01), which are the hallmark of a black hole. An analysis of the
rotational speed of the disk leads to determination of the total mass of
the material inside the disk. This showed that about 200 million solar
masses of material resides inside the nuclear disk.

A massive black hole

The astronomers quickly realized that this enormous mass within the central
region cannot be caused by normal stars, as it would then be much more
luminous. Instead they conclude that the most conservative explanation for
the dark, central mass concentration observed in Centaurus A is indeed a
supermassive black hole. The most likely mass of this “central beast” is
then about 200 million times the mass of our Sun.

This discovery confirms the previous suspicion that the active nucleus of
Centaurus A is powered by a supermassive black hole. It is the first time
infrared spectroscopy has been used to weigh a black hole. Many other
galaxies have dust-enshrouded nuclei, and the excellent capabilities of
ISAAC now hold a great potential to discover and weigh many more black

More Information

The research described in this Press Release is reported in a research
article (“Peering through the dust: Evidence for a supermassive Black Hole
at the Nucleus of Centaurus A from VLT IR spectroscopy”), that will appear
in the international research journal the Astrophysical Journal on March 10,
2001. The full article is also available on the web as astro-ph/0011059.


[1]: The team is composed by Ethan Schreier (Principal Investigator; Space
Telescope Science Institute – STScI, Baltimore, USA), Alessandro Marconi
(Arcetri Observatory, Italy), Alessandro Capetti (Turin Observatory, Italy),
David Axon (University of Hertfordshire, United Kingdom), Anton Koekemoer
(STScI, USA) and Duccio Macchetto (ESA/STScI, USA).

Technical information about the photos

PR Photo 08a/01 is reproduced from three exposures, obtained during the
night of January 31 – February 1, 2000. It is a composite of three exposures
in B (300 sec exposure, image quality 0.60 arcsec; here rendered in blue
colour), V (240 sec, 0.60 arcsec; green) and R (240 sec, 0.55 arcsec; red).
The field covered corresponds to about 80 x 80 arcsec2 (395 x 395 pix2, 1
pix = 0.2 arcsec). North is up and East is left. PR Photo 08b+c/01: The
original ISAAC spectra were exposed for 35 min each with an average seeing
of 0.5 arcsec. Three spectrograph slits were used, but only one of these is
shown here. It was centered on the nucleus of Centaurus A and oriented at
33 deg, measured counter-clockwise from the North direction. The spectral
pixel size is 0.6 Angstrom x 0.15 arcsec (i.e., 14 km/sec x 8.3 light-year).
The large and small figures cover 2300 km/s x 1665 light-years and
1150 km/s x 330 light-years, respectively.