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Black holes grow faster than expected

Artist's impression of a black hole about to devour a star

Artist’s impression of a black hole about to devour a star. Supermassive black holes are thought to be at the heart of all major galaxies. Australian researchers have determined that as a galaxy grows, its black hole grows even faster.

  • Supermassive black holes have up to billions of times more mass than the Sun
  • How they became this big has been a long-standing mystery
  • Australia research shows big galaxies breed even bigger black holes

ASTRONOMERS FROM SWINBURNE UNIVERSITY of Technology in Australia have discovered how supermassive black holes grow – and it’s not what was expected.

For years, scientists had believed that supermassive black holes – millions or billions of times the mass of our Sun – located at the centres of galaxies, increased their mass in step with the growth of their host galaxy.  However, new observations have revealed a dramatically different behaviour.

“Black holes have been growing much faster than we thought,” Professor Alister Graham from Swinburne’s Centre for Astrophysics and Supercomputing said.

Within galaxies, there is a competition of sorts for the available gas; for either the formation of new stars or feeding the central black hole.

For more than a decade the leading models and theories have assigned a fixed fraction of the gas to each process, effectively preserving the ratio of black hole mass to galaxy mass. New research to be published in The Astrophysical Journal reveals that this approach needs to be changed.

“We now know that each ten-fold increase of a galaxy’s stellar mass is associated with a much larger 100-fold increase in its black hole mass,” Professor Graham said. “This has widespread implications for our understanding of galaxy and black hole co-evolution.”

The following animation depicts a star being devoured by a black hole.

Unexpected behaviour

The researchers have also found the opposite behaviour to exist among the tightly packed clusters of stars that are observed at the centres of smaller galaxies and in disc galaxies like our Milky Way.

“The smaller the galaxy, the greater the fraction of stars in these dense, compact clusters,” Swinburne researcher Dr Nicholas Scott said. “In the lower mass galaxies the star clusters, which can contain up to millions of stars, really dominate over the black holes.”

Previously it was thought that the star clusters contained a constant 0.2 per cent of the galaxy mass.

Black holes = gravitational prisons

The research also appears to have solved a long-standing mystery in astronomy. ‘Intermediate mass’ black holes with masses between that of a single star and one million stars have been remarkably elusive.

The new research predicts that numerous galaxies already known to harbour a black hole – albeit of a currently unknown mass – should contain these missing `intermediate mass’ black holes.

Artist's impression of a black hole in a star field

Intermediate or middle-sized black holes have proved elusive (artist’s impression).

“These may be big enough to be seen by the new generation of extremely large telescopes,” Dr Scott said.

Professor Graham said these black holes were still capable of readily devouring any stars and their potential planets if they ventured too close.

“Black holes are effectively gravitational prisons and compactors, and this may have been the fate of many past solar systems,” Professor Graham said. “Indeed, such a cosmic dance will contribute at some level to the transformation of nuclear star clusters into massive black holes.”

The researchers combined observations from the Hubble Space Telescope, the European Very Large Telescope in Chile and the Keck Telescope in Hawaii to create the largest sample to date of galaxies with reliable star cluster and supermassive black hole mass measurements.

Adapted from information issued by Swinburne University of Technology. Images by Gabriel Perez Diaz.

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Black hole to destroy cloud

  • Giant gas cloud about to enter black hole
  • The black hole is at the centre of our galaxy
  • Astronomers to watch it happen in 2013

THE BLACK HOLE AT THE CENTRE of the our galaxy, formally known as Sagittarius A* – pronounced Sagittarius A star – is about to unleash its destructive power. By mid-2013, a gas cloud is expected to pass in its vicinity at a distance of only 36 light-hours (equivalent to 40,000,000,000 km), which is extremely close in astronomical terms. The cloud will be ripped apart.

For the past 20 years, Stefan Gillessen, an astrophysicist at the Max Planck Institute for Extraterrestrial Physics in Munich, Germany, has been studying the black hole. “So far there have been only two stars [we have seen] that came that close to Sagittarius A*”, he says. “They passed unharmed, but this time will be different: the gas cloud will be completely ripped apart by the tidal forces of the black hole.”

A black hole is what remains after a supermassive star dies. When the “fuel” of a star runs low, it will first swell and then collapse to a dense core. If this remnant core has more than three times the mass of our Sun, it will transform to a black hole.

Direct observations of such black holes are impossible because they are coal-black and do not emit light or matter. But astronomers can identify a black hole indirectly due to the affect it has on objects in its vicinity.

So-called supermassive black holes are the largest type. Their mass equals hundreds of thousands to a billion times the mass of our Sun. The centres of all galaxies are thought to contain supermassive black holes. But their origin is not fully understood and astrophysicists can only speculate as to what happens inside them. Hence the imminent collision is of great interest, as it should provide some new insights.

Reinhard Genzel (European Southern Observatory) leads the team of astronomers that discovered the cloud and studied its trajectory. According to their observations, its speed has nearly doubled in the last seven years, reaching more than 8 million km/h.

The cloud’s edges have already started to shred and it is expected to break up completely over the coming months. As it nears the collision, the cloud is expected to get much hotter and probably emit X-rays.

Adapted from information issued by the Max Planck Institute for Extraterrestrial Physics.

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Most distant quasar found

Artist’s impression of quasar ULAS J1120+0641

This artist’s impression shows how ULAS J1120+0641, a very distant quasar powered by a black hole with a mass two billion times that of the Sun, may have looked. This quasar is the most distant yet found and is seen as it was just 770 million years after the Big Bang.

ASTRONOMERS HAVE DISCOVERED the most distant quasar to date—a development that could help further our understanding of the universe when it was still in its infancy following the Big Bang.

Quasars are distant galaxies that have very bright cores, believed to be powered by supermassive black holes at their centres. Their brilliance makes them powerful beacons that may help to probe the era when the first stars and galaxies were forming.

“It is a very rare object that will help us to understand how supermassive black holes grew a few hundred million years after the Big Bang,” says Stephen Warren, the study’s team leader.

The quasar, named ULAS J1120+0641, is seen as it was only 770 million years after the Big Bang, giving it a redshift of 7.1. The light we see coming from it took 12.9 billion years to reach us.

Striking gold

Although more distant objects have been confirmed—such as a gamma-ray burst at redshift 8.2 and a galaxy at 8.6—the newly discovered quasar is hundreds of times brighter than these. In fact, amongst objects bright enough to be studied in detail, this is the most distant by a large margin.

Objects so away cannot be found in visible-light surveys because their light, stretched by the expansion of the Universe, falls mostly in the infrared part of the spectrum by the time it gets to Earth.

The European UKIRT Infrared Deep Sky Survey (UKIDSS) which uses the UK’s dedicated infrared telescope in Hawaii was designed to solve this problem. The team of astronomers hunted through millions of objects in the UKIDSS database to find those that could be the long-sought distant quasars, and eventually struck gold.

“It took us five years to find this object,” explains Bram Venemans, one of the authors of the study. “We were looking for a quasar with redshift higher than 6.5. Finding one that is this far away, at a redshift higher than 7, was an exciting surprise.”

A rare find

Because the object is comparatively bright it is possible to take a spectrum of it (which involves splitting the light from the object into its component colours). This technique enabled the astronomers to find out quite a lot about it.

These observations show that the mass of the black hole at the centre of ULAS J1120+0641 is about two billion times that of the Sun. This very high mass is hard to explain so early on after the Big Bang, as current theories for the growth of supermassive black holes predict a slow build-up in mass as the object pulls in matter from its surroundings.

“We think there are only about 100 bright quasars with redshift higher than 7 over the whole sky,” concludes Daniel Mortlock, the leading author of the paper. “Finding this object required a painstaking search, but it was worth the effort to be able to unravel some of the mysteries of the early Universe.”

Adapted from information issued by ESO / University of Nottingham / M. Kornmesser.

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Black hole in the ‘Eye of Sauron’

Eye of Sauron image of NGC 4151

This false-colour image (using X-ray, visible light and radio wave data) of the core of galaxy NGC 4151 resembles the Eye of Sauron from the Lord of the Rings movies. In reality, it shows the region surrounding a supermassive black hole.

  • Spiral galaxy NGC 4151 has a growing, giant black hole at its centre.
  • Dubbed “The Eye of Sauron” for its resemblance to the “The Lord of the Rings” character

AT THE HEART OF MANY (perhaps most) galaxies there lives a dark, malevolent force—a black hole.

And they aren’t just ordinary black holes. They are giants…what astronomers call ‘supermassive’ black holes, which can have masses hundreds of millions or billions of times the mass of our Sun.

One such galaxy is NGC 4151. Located about 43 million light-years from Earth, it is one of the nearest galaxies to contain an actively growing black hole.

A new false-colour image put together using different wavelength data makes the local region surrounding the black hole look like the ‘Eye of Sauron’ from the Lord of the Rings movies.

In the ‘pupil’ of the eye, X-rays (coloured blue) detected by the Chandra X-ray Observatory are combined with visible light wavelengths (yellow) showing positively charged hydrogen atoms (from observations with the Jacobus Kapteyn Telescope in the Canary Islands).

The red surrounding the pupil shows neutral hydrogen detected by radio observations with the Very Large Array radio telescope in the USA.

Because it is so close (in astronomical terms), NGC 4151 offers one of the best opportunities to study the interaction between an active supermassive black hole and the surrounding gas of its host galaxy.

Such interaction, or ‘feedback’, is recognised to play a key role in the growth of both black holes and their host galaxies.

Gas falls into the black hole, feeding it and making it grow larger.

But as the gas approaches the black hole, it heats up…to the point where some of it shoots back into the galaxy in a process known as an outflow. That hot gas emits X-rays.

If the X-ray emission seen in the core of NGC 4151 indeed originates from hot gas heated by the outflow from the black hole, it would be strong evidence for black hole feedback occurring within individual galaxies.

Such feedback has already been seen on larger scales—in clusters of galaxies such as the Perseus Cluster, where active black holes interact with surrounding gas.

Adapted from information issued by Chandra X-ray Centre. Image credit: X-ray, NASA / CXC / CfA / J.Wang et al.; optical, Isaac Newton Group of Telescopes, La Palma / Jacobus Kapteyn Telescope; radio, NSF / NRAO / VLA.

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