The Giant that Turned Out to be a Dwarf (Forwarded)

ESO Education and Public Relations Dept.

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Contact:

Ivo Saviane
ESO
Phone: +56 2 463 3000

Helmut Jerjen
Mount Stromlo Observatory, Australia
Phone: +61 2 6125 80 38

ESO Press Photo 09/07

For Immediate Release: 7 March 2007

The Giant that Turned Out to be a Dwarf

True Identity of a Dwarf Galaxy Revealed

New data obtained on the apparent celestial couple, NGC 5011 B and C, taken
with the 3.6-m ESO telescope, reveal that the two galaxies are not at the
same distance, as was believed for the past 23 years. The observations show
that NGC 5011C is not a giant but a dwarf galaxy, an overlooked member of a
group of galaxies in the vicinity of the Milky Way.

The galaxy NGC 5011C is located towards the Centaurus constellation, in the
direction of the Centaurus A group of galaxies and the Centaurus cluster of
galaxies. The former is about 13 million light-years from our Milky Way,
while the latter is about 12 times farther away.

The appearance of NGC 5011C, with its low density of stars and absence of
distinctive features, would normally lead astronomers to classify it as a
nearby dwarf elliptical galaxy. On the other hand, the galaxy's distance --
as reported in the scientific literature -- makes it a member of the more
distant Centaurus cluster. As such, it would belong to the same cluster as
its brighter, redder companion on the sky, the lenticular galaxy NGC 5011B,
which is seen almost edge-on.

"This is however a problem," says Ivo Saviane, from ESO, who together with
colleague Helmut Jerjen (Mt Stromlo Observatory, Australia) studied this
peculiar system, "as despite the small distance between the two galaxies
this would imply from their projection on the sky if they were indeed at the
same distance -- only 45,000 light-years, half the size of our Milky Way --
there is no obvious sign of interaction between the two."

Moreover, if the two galaxies were at the same distance, then NGC 5011C
would be larger than NGC 5011B in real size, making it a kind of galaxy
never seen before.

Saviane and Jerjen therefore used the ESO 3.6-m telescope at La Silla to
take images and spectra of the galaxies. The astronomers then found that
contrary to what is published, the two galaxies have very different
redshifts, with NGC 5011C moving away from us five times slower than its
companion on the sky. "This indicates they are at different distances and
not at all associated", says Jerjen. "Clearly, NGC 5011C belongs to the
close group of galaxies centred around Centaurus A, while NGC 5011B is part
of the much farther Centaurus cluster."

The astronomers also established that the two galaxies have very different
intrinsic properties. NGC 5011B contains for example more heavy chemical
elements than NGC 5011C, and the latter seems to contain only about 10
million times the mass of the Sun in stars and is therefore a true dwarf
galaxy. For comparison, our Milky Way contains thousands of times more
stars.

"Our new observations with the 3.6-m ESO telescope thus confirm a new member
of the nearby Centaurus A group whose true identity remained hidden because
of coordinate confusion and wrong distance estimates in the literature for
the last 23 years," says Saviane.

With this new distance determination, the astronomers also established that
NGC 5011C lies 500,000 light-years away from the dominant galaxy in its
group, Centaurus A. Centaurus A (NGC 5128) is the nearest giant elliptical
galaxy, at a distance of about 13 million light-years. It is currently
merging with a spiral companion galaxy. It possesses a very massive black
hole at its centre and is a source of strong radio and X-ray emission. (see
ESO 05/00, ESO 04/01, and ESO 13/03)

More Information

The research discussed here is presented in an article in press in the
Astronomical Journal, vol. 133, p. 1892: "NGC 5011C: An overlooked dwarf
galaxy in the Centaurus A group", by Ivo Saviane and Helmut Jerjen. It is
also available at
http://www.arxiv.org/abs/astro-ph?papernum=0701280

National contacts for the media:

Belgium: Dr. Rodrigo Alvarez, +32-2-474 70 50
Finland: Ms. Tiina Raivo, +358 9 7748 8369
Denmark: Dr. Michael Linden-Vnle, +45-33-18 19 97
France: Dr. Daniel Kunth, +33-1-44 32 80 85
Germany: Dr. Jakob Staude, +49-6221-528229
Italy: Dr. Leopoldo Benacchio, +39-347-230 26 51
The Netherlands: Ms. Marieke Baan, +31-20-525 74 80
Portugal: Prof. Teresa Lago, +351-22-089 833
Sweden: Dr. Jesper Sollerman, +46-8-55 37 85 54
Switzerland: Dr. Martin Steinacher, +41-31-324 23 82
United Kingdom: Mr. Peter Barratt, +44-1793-44 20 25

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