Galaxy harbors many star-snacking black holes

July 2, 2012
In the centre of Arp 220, a galaxy 250 million light years away, scientists at Chalmers have discovered evidence for a large number of black holes. Credit: NASA / ESA / R. Thompson, M. Rieke, G. Schneider (U. of Arizona), N. Scoville (CalTech), A. Evans (U. of Virginia)

( -- Astronomers have found evidence of hundreds of black holes in a galaxy 250 million light years away. The discovery, made with a worldwide network of radio telescopes, gives scientists a new way to find out how black holes are created.

A team led by astronomers at Chalmers University of Technology and Onsala Space Observatory has been monitoring radio signals from the core of the galaxy Arp 220, which lies 250 million light years from Earth. Besides a number of supernovae, they also found some sources that were at first sight difficult to understand.

“We found three remarkable sources whose brightness was different each time we looked at them. In the beginning we had no idea what they could be” says Fabien Batejat, astronomer at Chalmers, who led the study.
The scientists followed the three peculiar radio sources over several years. Now they think they know what is behind the : jets created by black holes.
“We believe we are seeing radio emission from binary star systems in which one star has already exploded and left behind a black hole. The black hole “eats” gas which it draws from its companion, producing powerful jets that emit radio waves”, says Fabien Batejat.
The newly discovered black holes in galaxy Arp 220 are only three of many more, the scientists believe.

John Conway, professor of observational radio astronomy at Chalmers and deputy director of Onsala Space Observatory, explains.
“Jets from black holes are visible at this distance only if they are pointing right towards us. Probably there are many more systems like this in this galaxy, but their jets point in other directions”, he says.
The galaxy Arp 220 is already famous for creating new stars at a furious pace. Previous research by the same team has also demonstrated that there are many supernova explosions in the galaxy, up to 250 times more than in our galaxy. and black holes are related. Astronomers believe that black holes are created when stars with masses more than about 20 times the sun explode.

This discovery in Arp 220 gives astronomers hope to soon be able to put this idea to the test. Only a dozen black holes of this type are known in the Milky Way, and only a few are known in other .
“By studying large numbers of these small, star-snacking black holes, we have a new way to learn about how they are created. So far, black hole statistics has only been possible for distant, supermassive ”, says Anthony Rushton, member of the team in Onsala.
The discovery was made with a network of around the world, linked together to create very sharp images, using the technique VLBI (Very Long Baseline Interferometry). Radio telescopes can follow events in the dense centers of galaxies that lie behind thick layers of dust, invisible to other telescopes. In order to discover what the radio sources in Arp 220 are, the team made measurements at different radio wavelengths over a period of 17 years.
“This result has only emerged after many years of painstaking observations and improvements in VLBI techniques”, says Philip Diamond, member of the team and Chief of CSIRO Astronomy and Space Science in Australia.
These objects, known to astronomers as microblazars, were theoretically predicted over a decade ago. Astronomers believe microblazars are scaled-down versions of the cosmic beacons known as blazars. In a blazar, a supermassive black hole feasting on dense gas at the centre of a galaxy creates powerful jets which can be observed from if they are directed towards us.
“Our new results from Arp 220 are the best evidence yet for microblazars. It also seems that galaxies like this one can contain very large numbers of them“, says Fabien Batejat.

Explore further: Chandra Sheds Light on Galaxy Collision

More information: The results are published in a paper in the June issue of the journal Astronomy & Astrophysics (“Rapid variability of the compact radio sources in Arp220” by F. Batejat et al.).

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1 / 5 (2) Jul 03, 2012

A more fundamental question is why do black hole binaries exist at all?

1, Before the supernova, the original giant star would have been between 50% and 300% more massive than the black hole itself, since a star loses between 1/3 and 2/3 of it's original mass during a supernova. This means the gravity of a black hole is generally far weaker than it's parent star, at least at "stellar orbital distances".

2, The black hole then must migrate inward towards the other star, even though that makes no sense, because the gravitation of the system is actually weaker after the explosion than before. How or why this happens, if black hole binaries indeed formed from a supernova, has never been explained.

3, It seems more likely that the black hole and the second star capture one another after the fact, millions or billions of years later, and were not orbiting one another at all at the time the black hole formed.

4) Or a microscopic black hole consumes star A while it orbits star B...
1 / 5 (2) Jul 03, 2012
"4" above leaves a mostly undisturbed gravitational environment, except obviously at or below the original surface radius of hte consumed star, and also seems to make more sense than the "supernova theory".

Indeed, the microscopic black hole may actually CAUSE a supernova, by consuming the core of a star and crushing the hydrogen and helium gases on the outer layers under a deeper gravity well, blowing of the outer shells...

In that way, you could have a "supernova-like" event in some cases, while the black hole still manages to eat most of the original mass, and keeping the gravitational environment stable.

The "Parent star in an otherwise stable binary explodes and then it's black hole product eats the second star" scenario contradicts conservation of momentum.
1.5 / 5 (2) Jul 03, 2012
The Milky Way probably contains multiple of black holes too.

why do black hole binaries exist at all
They're probably remnants of older galaxies sucked with main galaxy. The stellar black holes are quite tiny with compare to these giant monsters at the center of galaxies.
not rated yet Jul 03, 2012
'wide' angle views of star fields have always struck me as showing arrangements of stars in a circular patten .. as if millions of BH's exist and are performing 'lensing' patterns in our sky ..
rather than suggesting a 'filimentous' patterns of star positioning ..

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