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             Information from the European Southern Observatory

ESO Press Release 13/03

10 June 2003                                               [ESO Logo]

For immediate release
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One Thousand "Wonderful" Stars Discovered in Centaurus A

Part 1 of 2

First-Ever Census of Variable Mira-Type Stars in Galaxy Outside the
Local Group

An international team led by ESO astronomer Marina Rejkuba [1] has
discovered more than 1000 luminous red variable stars in the nearby
elliptical galaxy Centaurus A (NGC 5128).

Brightness changes and periods of these stars were measured
accurately and reveal that they are mostly cool long-period variable
stars of the so-called "Mira-type". The observed variability is
caused by stellar pulsation.

This is the first time a detailed census of variable stars has been
accomplished for a galaxy outside the Local Group of Galaxies (of
which the Milky Way galaxy in which we live is a member).

It also opens an entirely new window towards the detailed study of
stellar content and evolution of giant elliptical galaxies. These
massive objects are presumed to play a major role in the
gravitational assembly of galaxy clusters in the Universe (especially
during the early phases).

This unprecedented research project is based on near-infrared
observations obtained over more than three years with the ISAAC
multi-mode instrument at the 8.2-m VLT ANTU telescope at the ESO
Paranal Observatory.

The full text of this Press Release, with nine photos (ESO PR Photos
14a-i/03) and all related links, is available at:

http://www.eso.org/outreach/press-rel/pr-2003/pr-13-03.html

Mira-type variable stars

Centaurus A (NGC 5128) is the nearest giant galaxy, at a distance of
about 13 million light-years. It is located outside the Local Group
of Galaxies to which our own galaxy, the Milky Way, and its satellite
galaxies, the Magellanic Clouds, belong.

Centaurus A is seen in the direction of the southern constellation
Centaurus. It is of elliptical shape and is currently merging with a
companion galaxy, making it one of the most spectacular objects in
the sky, cf. PR Photo 14a/03. It possesses a very heavy black hole at
its centre (see ESO PR 04/01) and is a source of strong radio and
X-ray emission.

During the present research programme, two regions in Centaurus A
were searched for stars of variable brightness; they are located in
the periphery of this peculiar galaxy, cf. PR Photos 14b-d/03. An
outer field ("Field 1") coincides with a stellar shell with many blue
and luminous stars produced by the on-going galaxy merger; it lies at
a distance of 57,000 light-years from the centre. The inner field
("Field 2") is more crowded and is situated at a projected distance
of about 30,000 light-years from the centre.

Three years of VLT observations

Under normal circumstances, any team of professional astronomers will
have access to the largest telescopes in the world for only a very
limited number of consecutive nights each year. However, extensive
searches for variable stars like the present require repeated
observations lasting minutes-to-hours over periods of
months-to-years. It is thus not feasible to perform such observations
in the classical way in which the astronomers travel to the telescope
each time.

Fortunately, the operational system of the VLT at the ESO Paranal
Observatory (Chile) is also geared to encompass this kind of
long-term programme. Between April 1999 and July 2002, the 8.2-m VLT
ANTU telescope on Cerro Paranal in Chile) was operated in service
mode on many occasions to obtain K-band images of the two fields in
Centaurus A by means of the near-infrared ISAAC multi-mode
instrument. Each field was observed over 20 times in the course of
this three-year period; some of the images were obtained during
exceptional seeing conditions of 0.30 arcsec. One set of
complementary optical images was obtained with the FORS1 multi-mode
instrument (also on VLT ANTU) in July 1999.

Each image from the ISAAC instrument covers a sky field measuring 2.5
x 2.5 arcmin^2. The combined images, encompassing a total exposure of
20 hours are indeed the deepest infrared images ever made of the halo
of any galaxy as distant as Centaurus A, about 13 million
light-years.

Discovering one thousand Mira variables

Once the lengthy observations were completed, two further steps were
needed to identify the variable stars in Centaurus A.

First, each ISAAC frame was individually processed to identify the
thousands and thousands of faint point-like images (stars) visible in
these fields.  Next, all images were compared using a special
software package ("DAOPHOT") to measure the brightness of all these
stars in the different frames, i.e., as a function of time.

While most stars in these fields as expected were found to have
constant brightness, more than 1000 stars displayed variations in
brightness with time; this is by far the largest number of variable
stars ever discovered in a galaxy outside the Local Group of
Galaxies.

The detailed analysis of this enormous dataset took more than a
year. Most of the variable stars were found to be of the Mira-type
and their light curves (brightness over the pulsation period) were
measured, cf. PR Photo 14i/03. For each of them, values of the
characterising parameters, the period (days) and brightness amplitude
(magnitudes) were determined. A catalogue of the newly discovered
variable stars in Centaurus A has now been made available to the
astronomical community via the European research journal Astronomy &
Astrophysics.

Marina Rejkuba is pleased and thankful: "We are really very fortunate
to have carried out this ambitious project so successfully. It all
depended critically on different factors: the repeated granting of
crucial observing time by the ESO Observing Programmes Committee over
different observing periods in the face of rigorous international
competition, the stability and reliability of the telescope and the
ISAAC instrument over a period of more than three years and, not
least, the excellent quality of the service mode observations, so
efficiently performed by the staff at the Paranal Observatory."

What have we learned about Centaurus A?

The present study of variable stars in this giant elliptical galaxy
is the first-ever of its kind. Although the evaluation of the very
large observational data material is still not finished, it has
already led to a number of very useful scientific results.

 - Continued -

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