Black Holes in Star Clusters stir up Time and Space

An artist's representation of the burst of gravitational waves resulting from the collision of a colliding pair of black holes. Credit: LIGO Scientific Collaboration (LSC) / NASA.

Within a decade scientists could be able to detect the merger of tens of pairs of black holes every year, according to a team of astronomers at the University of Bonn’s Argelander-Institut fuer Astronomie, who publish their findings in a paper in Monthly Notices of the Royal Astronomical Society. By modelling the behaviour of stars in clusters, the Bonn team find that they are ideal environments for black holes to coalesce. These merger events produce ripples in time and space (gravitational waves) that could be detected by instruments from as early as 2015.

Clusters of stars are found throughout our own and other galaxies and most stars are thought to have formed in them. The smallest looser ‘open clusters’ have only a few stellar members, whilst the largest tightly bound ‘globular clusters’ have as many as several million stars. The highest mass stars in clusters use up their hydrogen fuel relatively quickly (in just a few million years). The cores of these stars collapse, leading to a violent supernova explosion where the outer layers of the star are expelled into space. The explosion leaves behind a stellar remnant with gravitational field so strong that not even light can escape – a black hole.

When stars are as close together as they are in clusters, then although still rare events, the likelihood of collisions and mergers between stars of all types, including black holes, is much higher. The black holes sink to the centre of the cluster, where a core that is completely made of up of black holes forms. In the core, the black holes experience a range of interactions, sometimes forming binary pairs and sometimes being ejected from the cluster completely.

Now Dr Sambaran Banerjee, Alexander von Humboldt postdoctoral fellow, has worked with his University of Bonn colleagues Dr Holger Baumgardt and Professor Pavel Kroupa to develop the first self-consistent simulation of the movement of black holes in star clusters.

The scientists assembled their own star clusters on a high-performance supercomputer, and then calculated how they would evolve by tracing the motion of each and every star and black hole within them.

According to a key prediction of Einstein’s General Theory of Relativity, black hole binaries stir the space-time around them, generating waves that propagate away like ripples on the surface of a lake. These waves of curvature in space-time are known as gravitational waves and will temporarily distort any object they pass through. But to date no-one has succeeded in detecting them.

In the cores of stars clusters, black hole binaries are sufficiently tightly bound to be significant sources of gravitational waves. If the black holes in a binary system merge, then an even stronger pulse of gravitational waves radiates away from the system.

Based on the new results, the next generation of gravitational wave observatories like the Advanced Laser Interferometer Gravitational-wave Observatory (Advanced LIGO) could detect tens of these events each year, out to a distance of almost 5000 million light years (for comparison the well known Andromeda Galaxy is just 2.5 million light years away).

Advanced LIGO will be up and running by 2015 and if the Bonn team are right, from then on we can look forward to a new era of gravitational wave astronomy.

Sambaran comments, “Physicists have looked for gravitational waves for more than half a century. But up to now they have proved elusive. If we are right then not only will gravitational waves be found so that General Relativity passes a key test but astronomers will soon have a completely new way to study the Universe. It seems fitting that almost exactly 100 years after Einstein published his theory, scientists should be able to use this exotic phenomenon to watch some of the most exotic events in the cosmos.”

CONTACTS

Dr Sambaran Banerjee
Alexander von Humboldt postdoctoral fellow
Argelander Institut fuer Astronomie
University of Bonn
Tel: +49 (0) 228 73 3649
Mob: +49 (0) 1577 8030745
E-mail: sambaran@astro.uni-bonn.de

Dr Holger Baumgardt
Argelander Institut fuer Astronomie
University of Bonn
Tel: +49 (0) 228 73 6790
E-mail: holger@astro.uni-bonn.de

Professor Pavel Kroupa
University of Bonn
Argelander Institut fuer Astronomie
University of Bonn
Tel: +49 (0) 228 73 6140
Mob: +49 (0)177 9566127
E-mail: pavel@astro.uni-bonn.de

IMAGES

An aerial view of the Laser Interferometer Gravitational wave Observatory (LIGO at Hanford in the United States. The two interferometer arms are at right angles and each arm is about 4 km long. By 2015, the present LIGO will be upgraded to Advanced LIGO (also known as LIGO 2) which will be about 10 times more sensitive than the present instrument. See http://www.ligo.caltech.edu/ for details. Credit: LIGO Scientific Collaboration (LSC)

http://www.ligo.org/multimedia/gallery/lho-images/Aerial5.jpg

An artist's representation of the burst of gravitational waves resulting from the collision of a colliding pair of black holes. Credit: LIGO Scientific Collaboration (LSC) / NASA

http://www.ligo.org/science/GW-Overview/images/binary-wave.jpg

MOVIE

A movie depicting the movement of black holes (black dots) and stars (represented by green asterisks) in a star cluster. The black holes, formed when the most massive stars exhaust their hydrogen fuel, are initially spread over a wide region of the cluster. Then, as they are more massive than the rest of the stars, they begin to sink and concentrate within a small region at the cluster centre. When this central black hole cluster becomes dense enough, gravitational waves are emitted due to the mergers of black holes in binary systems.

The propagation of gravitational waves from these mergers is depicted by outgoing circles and the wobbling of the whole cluster, representing space-time distortion, while the cluster stars are unmoving (since the gravitational waves, travelling at the speed of light, cross the cluster before the stars can move significantly). Note that the implied propagation speed and the wobbling are not to scale.

The time line in millions of years is given on the top axis and the passage of time is denoted by the movement of the white marker. In this model the first binary black hole forms in the cluster and begins radiating gravitational waves after about 600 million years. Credit: University of Bonn

Quicktime:
http://www.astro.uni-bonn.de/~sambaran/clustergw_en.mov

Quicktime (for a slower Internet connection):
http://www.astro.uni-bonn.de/~sambaran/clustergw_small_en.mov

Flash:
http://www.astro.uni-bonn.de/~sambaran/clustergw_en/clustergw_en.html

PREPRINT OF MNRAS PAPER

A preprint of the paper, which will appear in Monthly Notices of the Royal Astronomical Society, is available at http://arxiv.org/abs/0910.3954

NOTES FOR EDITORS

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Royal Astronomical Society Press Release
Ref: RAS PN 09/65
15th December 2009
EMBARGOED UNTIL 0001 GMT, 16TH DECEMBER 2009

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