The giant elliptical galaxy Centaurus A (NGC 5128) and its strange globular clusters
The strange galaxy Centaurus A in the constellation of Centaurus
Wide-field view of the giant galaxy Centaurus A
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Panning across the giant elliptical galaxy Centaurus A (NGC 5128) and its strange globular clusters
VLT discovers new kind of globular star cluster
Observations with ESO’s Very Large Telescope in Chile have discovered a new class of “dark” globular star clusters around the giant galaxy Centaurus A. These mysterious objects look similar to normal clusters, but contain much more mass and may either harbour unexpected amounts of dark matter, or contain massive black holes — neither of which was expected nor is understood.
Globular star clusters
are huge balls of thousands of stars that orbit most galaxies. They are
among the oldest known stellar systems in the Universe and have
survived through almost the entire span of galaxy growth and evolution.
Matt Taylor, a PhD student at the Pontificia Universidad Catolica de
Chile, Santiago, Chile, and holder of an ESO Studentship, is lead author
of the new study. He sets the scene: “Globular clusters and their
constituent stars are keys to understanding the formation and evolution
of galaxies. For decades, astronomers thought that the stars that made
up a given globular cluster all shared the same ages and chemical
compositions — but we now know that they are stranger and more
complicated creatures.”
The elliptical galaxy Centaurus A
(also known as NGC 5128) is the closest giant galaxy to the Milky Way
and is suspected to harbour as many as 2000 globular clusters. Many of
these globulars are brighter and more massive than the 150 or so
orbiting the Milky Way.
Matt Taylor and his team have now made the most detailed studies so
far of a sample of 125 globular star clusters around Centaurus A using
the FLAMES instrument on ESO’s Very Large Telescope at the Paranal Observatory in northern Chile [1].
They used these observations to deduce the mass of the clusters [2] and compare this result with how brightly each of the clusters shines.
For most of the clusters in the new survey, the brighter ones had
more mass in the way that was expected — if a cluster contains more
stars it has greater total brightness and more total mass. But for some
of the globulars something strange showed up: they were many times more
massive than they looked. And even more strangely, the more massive
these unusual clusters were, the greater the fraction of their material
was dark. Something in these clusters was dark, hidden and massive. But
what?
There were several possibilities. Perhaps the dark clusters contain
black holes, or other dark stellar remnants in their cores? This may be a
factor that explains some of the hidden mass, but the team concludes
that it cannot be the whole story. What about dark matter?
Globular clusters are normally considered to be almost devoid of this
mysterious substance, but perhaps, for some unknown reason, some
clusters have retained significant dark matter clumps in their cores.
This would explain the observations but does not fit into conventional
theory.
Co-author Thomas Puzia adds: “Our discovery of star clusters with
unexpectedly high masses for the amount of stars they contain hints
that there might be multiple families of globular clusters, with
differing formation histories. Apparently some star clusters look like,
walk like, and smell like run-of-the-mill globulars, but there may quite
literally be more to them than meets the eye.”
These objects remain a mystery. The team is also engaged in a wider
survey of other globular clusters in other galaxies and there are some
intriguing hints that such dark clusters may also be found elsewhere.
Matt Taylor sums up the situation: “We have stumbled on a new and
mysterious class of star cluster! This shows that we still have much to
learn about all aspects of globular cluster formation. It’s an
important result and we now need to find further examples of dark
clusters around other galaxies.”
Notes
[1] Up to now astronomers have studied star clusters to this detail
only in the Local Group. The relatively small distances make direct
measurements of their masses possible. Looking at NGC 5128, which is an
isolated, massive elliptical galaxy just outside the Local Group about
12 million light-years away, they were able to estimate masses of
globular clusters in a completely different environment by pushing
VLT/FLAMES to its limits.
[2] The FLAMES observations provide
information about the motions of the stars in the clusters. This orbital
information depends on the strength of the gravitational field and can
hence be used to deduce the mass of the cluster — astronomers call such
estimates dynamical masses. The light gathering power of a 8.2-metre VLT
Unit Telescope mirror and FLAMES’s ability to observe more than 100
clusters simultaneously was the key to collecting the data necessary for
the study.
More Information
This research was presented in a paper entitled
“Observational evidence for a dark side to NGC 5128’s globular cluster
system”, by M. Taylor et al., to appear in the Astrophysical Journal.
The team is composed of Matthew A. Taylor (Pontificia
Universidad Catolica de Chile, Santiago, Chile; ESO, Santiago, Chile),
Thomas H. Puzia (Pontificia Universidad Catolica de Chile), Matias Gomez
(Universidad Andres Bello, Santiago, Chile) and Kristin A. Woodley
(University of California, Santa Cruz, California, USA).
ESO is the foremost intergovernmental astronomy organisation in
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promoting and organising cooperation in astronomical research. ESO
operates three unique world-class observing sites in Chile: La Silla,
Paranal and Chajnantor. At Paranal, ESO operates the Very Large
Telescope, the world’s most advanced visible-light astronomical
observatory and two survey telescopes. VISTA works in the infrared and
is the world’s largest survey telescope and the VLT Survey Telescope is
the largest telescope designed to exclusively survey the skies in
visible light. ESO is a major partner in ALMA, the largest astronomical
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building the 39-metre European Extremely Large Telescope, the E-ELT,
which will become “the world’s biggest eye on the sky”.
Links
Contact
Matthew A. Taylor
Pontificia Universidad Catolica de Chile
Santiago, Chile
Tel: +56-9-91912386
Email: mataylor5128@gmail.com
Thomas H. Puzia
Pontificia Universidad Catolica de Chile
Santiago, Chile
Tel: +56-9-89010007
Email: tpuzia@gmail.com
Richard Hook
ESO, Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
Source: ESO
