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Escaped: supermassive black hole and heavyweight star
Posted: 11 May 2010

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In two separate reports released today, scientists announce a supermassive black hole being ejected from its host galaxy, and a heavyweight star racing away from a stellar nursery.

As part of a final year project, undergraduate student Marianne Heida of the University of Utrecht has discovered what appears to be a supermassive black hole being ejected from its host galaxy. Based at the SRON Netherlands Institute for Space Research, Heida used the Chandra Source Catalogue, a compilation of Chandra X-ray Observatory data, to study the correlation of hundreds of thousands of X-ray sources with the locations of galaxies.

A Hubble Space Telescope image of a galaxy (white circle marks the centre) and its suspected offset black hole (red circle). Image: STScI / NASA.

Supermassive black holes lie at the centres of most galaxies, revealed by the strong X-ray emission thrown out as material falls into their gaping jaws. X-ray observations can penetrate the thick dust and gaseous disc that obscures the centre of a galaxy, providing a clear view of the black hole environment to give it away as a bright point of light.

In the case of one particular galaxy, Heida noticed that the tell-tale light source was offset from the centre of the galaxy, and with supervisor SRON researcher Peter Jonker, concluded that the observation can be explained if the supermassive black hole is in the process of being kicked out of its galaxy at high speed. Such an event is the likely culmination of a merger event of two smaller black holes.

There may be many more examples of so-called 'recoiling' black holes lurking in the data set, too. "We have found many more objects in this strange class of X-ray sources," says Heida. "With Chandra we should be able to make the accurate measurements we need to pinpoint them more precisely and identify their nature."

A wide field view of 30 Doradus taken by ESO's Wide Field Imager at the 2.2 metre telescope on La Silla and a close-up of the stellar runaway captured by Hubble's Wide Field and Planetary Camera 2. The young star likely originated from within the cluster, marked by a circle in the main image. Image: ESO/NASA, ESA, J. Walsh (ST-ECF), and ESO.

Meanwhile, the Hubble Space Telescope has spied a heavyweight star racing away from a stellar nursery at a speed of over 250,000 miles per hour. The homeless star was found on the outskirts of the 30 Doradus nebula, and may have originated from the star cluster R136, 375 light years from its present location, and a cluster known to host stars that top 100 solar masses.

Dynamical processes like stellar ejection have been predicted by models but this is the first direct observation of the process. “Less massive runaway stars from the much smaller Orion Nebula Cluster were first found over half a century ago, but this is the first potential confirmation of more recent predictions applying to the most massive young clusters,” says Nolan Walborn of the Space Telescope Science Institute.

Close-up of the runaway star. Image: NASA, ESA, J. Walsh (ST-ECF), and ESO.

Stars are thought to be expelled in one of two ways, either having encountered heavier siblings in a dense cluster, or by getting a kick from a supernova explosion in a binary system. “It is generally accepted, however, that R136 is sufficiently young, 1 million to 2 million years old, that the cluster’s most massive stars have not yet exploded as supernovae,” adds team member Danny Lennon. “This implies that the star must have been ejected through dynamical interaction.”

Ian Howarth of University College London first observed the misfit star in 2006 using the Anglo-Australian Telescope at Siding Spring Observatory, finding it to be an unusually hot, massive blue-white star that is usually associated with a cluster. The newly installed Hubble Cosmic Origins Spectrograph instrument confirmed that the star is extremely massive, perhaps some 90 times heavier than the Sun. Delving into archival data, an optical image of the star taken with Hubble's Wide Field Planetary Camera 2 in 1995 revealed that it is located at one end of an egg-shaped cavity, the glowing edges of the cavity stretching out behind the star and pointing back in the direction of 30 Doradus. Observations with the Very Large Telescope in Chile show that the star's velocity is constant and that it displays an unusual motion relative to its surroundings, providing further evidence that this is a runaway star.

Two other hot, massive stars have also been spotted beyond the edges of 30 Doradus, suggesting that these too have been expelled from their home, and astronomers will further study the region to determine if 30 Doradus is indeed mass-evicting its inhabitants.