Science Matters: Light on black holes
Astronomers are preparing to point telescopes at a peculiar black hole, known as HLX-1, perched on the outskirts of an ancient galaxy, 300 million light years away.PT5M19S http://www.canberratimes.com.au/action/externalEmbeddedPlayer?id=d-2sdsl 620 349 August 22, 2013
Astronomers are getting ready to point their telescopes, radio dishes and X-ray satellites towards a fuzzy patch of southern sky in the remote constellation of Phoenix. Their quest: a peculiar black hole, known prosaically as HLX-1, perched on the outskirts of an ancient galaxy, 300 million light years away.
The unusually positioned black hole, scientists believe, could provide vital clues as to how galaxies form and evolve.
Stars burn for billions of years by converting hydrogen into helium. When they eventually run out of fuel, they end their days in a variety of unseemly ways. Stars much bigger than our sun sometimes explode as supernovae. When this happens, their cores may collapse under the force of gravity to become extremely dense and fast-spinning neutron stars – or sometimes black holes.
The galaxy in which the HLX-1 black hole was found. Photo: NASA
The black holes themselves cannot be observed but some of the matter spiralling into them may be detected as light, radio waves or X-rays.
Black holes discovered so far come in two varieties: relatively small ones formed from the collapse of massive stars and supermassive ones at the centre of many galaxies, including our own Milky Way.
"HLX-1 does not fit into either class," says Rob Soria, an astrophysicist and black hole expert at the International Centre for Radio Astronomy Research at Curtin University in Perth.
"It's not located at the centre of a galaxy but is too luminous and too massive to be a black hole of the stellar variety. It is probably the first black hole belonging to an elusive intermediate class with a mass of a few-thousand times that of our sun."
Discovered by Sydney University astronomer Sean Farrell, along with British and French collaborators, HLX-1 is odd, to say the least: it flares up every 370 days, releasing vast quantities of X-rays and radio-emitting jets.
"HLX-1 had the characteristic X-ray emissions of a black hole feeding from a disc of gas," Dr Farrell says. "But it was not at the centre of its host galaxy – and was about 1000 times brighter in X-rays than expected from low-mass black holes formed through the collapse of massive stars."
All the evidence, he explains, points to the presence of a black hole with about 10,000 times the sun's mass. "We think the hole is embedded in a dense cluster of stars drifting around in the halo of a large galaxy," Dr Farrell says.
HLX-1 stays very bright for a few weeks, then declines and almost vanishes until the following year. At the peak of its annual flare, the black hole is 300 million times more luminous than the sun.
The most likely reason for HLX-1 flaring up so regularly is that it feeds from an orbiting star, scientists believe. "The star loses some gas each time it passes close to the black hole," Dr Soria notes. "As the gas sinks towards the hole, it gets very hot and produces a big flare. We don't know how many more years it will take before the star is fully eaten."
Astrophysicists are also at a loss to explain how and when HLX-1 formed. "It is way too big to have come from a dead star," says Dr Soria. "But it is not in the centre of a galaxy, either. We discovered a faint blue spot at the position of the black hole, suggesting there is some gas and a small cluster of stars around it." Theoretical models suggest the small group of stars may be all that remains of a small galaxy that was once the satellite of a larger host galaxy. "It seems to have been completely absorbed into the larger one," Dr Soria adds. "HLX-1 may have been the central black hole of that satellite galaxy, and is now the last sign of that act of cosmic cannibalism."
Large galaxies generally grow by munching smaller ones. In the course of its lifetime, our Milky Way, for example, has absorbed smaller satellites and, in a few hundred million years, will shred and gobble up the Magellanic Cloud familiar to southern sky viewers.
"If small galaxies contain their own central black holes, these may remain inside the larger galaxy long after all trace is lost of the little satellite," Dr Soria explains.
Finding more examples of black holes such as HLX-1 will help scientists understand why the size of a galaxy seems to be roughly proportional to the size of its central black hole.
"For now, we're hoping that the annual feeding frenzy of HLX-1 occurs on schedule at the end of this month," Dr Soria says. "If so, we'll get the chance to discover more about its intriguing properties; but if not, and HLX-1 stays dark and quiet, we'll have to discard our current models and go back to the drawing board."
The Milky Way is thought to have a supermassive black hole at its centre, says Cambridge University astronautics engineer Graham Dorrington, now at RMIT University in Melbourne. Called Sagittarius A, it weighs about 4.2 million suns – relatively small when compared with holes of up to 6 billion solar masses.
Our galaxy also has many dwarf galaxies and clusters, such as Omega Centauri, which is easily observable from Australia. "It is thought to contain an intermediate black of hole of about 40,000 solar masses but there is no hard evidence of this," Dr Dorrington says.
If dwarf galaxies commonly collide with larger galaxies to produce objects such as HLX-1, then why have scientists not found more of them? "The answer is probably that the phase in which they emit X-rays is short-lived – in astronomical terms – making them hard to catch in the act," Dr Farrell replies.
In a separate discovery, scientists have come upon a highly energetic object in deep space that suggests black holes are more powerful than imagined.
It is a type of quasar, which is much smaller than a normal galaxy but shines far more brightly. The micro-quasar, weighing about as much as a typical star and powered by a stellar black hole, shoots powerful jets of radio-emitting particles hundreds of light years into space.
The active phase of the black hole started about 200,000 years ago, scientists say. Since then, the black hole has swallowed as much gas as the sun contains, producing about 300 times more energy than the sun will have generated during its lifetime.
This is what led to the discovery of the micro-quasar labelled S26, which sits inside a galaxy catalogued NGC 7793, about 13 million light years away in the southern constellation of Sculptor. The team was led by Manfred Pakull and Christian Motch of France's University of Strasbourg.
Quasars, which dominated the universe billions of years ago, are powered by black holes, some much bigger than the sun. But astronomers have been puzzled as to how much of the black holes' energy the quasar jets transmit to the gas through which they travel.
The gas is the raw material for forming stars, and the effects of jets on star formation have been hotly debated. As matter falls towards black holes, gravitational energy is extracted and released. That is why the hot gas immediately outside black holes shows up as the most energetic or luminous objects in the universe.
"Typically, the energy extracted from the falling matter comes out either as radiation – photons, especially in the X-ray band – or as narrow jets of particles moving close to the speed of light," Dr Soria says.
Quasar jets can be indirectly detected from the plumes of radio emissions created by their fast-moving electrons spiralling around a magnetic field.
Calculating the total jet power from radio emissions is not simple. The swirling cloud of radio-emitting electrons carries only a small and poorly known fraction of the jet power. "The rest of the energy is transferred from the jet to the surrounding gas, which is heated and swept away, forming a hot bubble around the black hole," Dr Soria notes. "It is more difficult to observe this effect directly."
This is where the micro-quasar helped. It has the longest jets ever detected from a stellar black hole, stretching for about 1000 light years.
Observations revealed the heating effect of the jets on a bubble of hot gas around the black hole, expanding at 250 kilometres per second. Having measured both the radio emissions from fast-moving electrons and the heating effect on surrounding gas, astronomers could estimate the black hole's jet power.
"The total power is a few hundred times the power carried by the radio-emitting electrons," Dr Soria says. "This is more than previously thought. It makes S26 one of the most powerful stellar black holes ever found."
This suggests the power of many other black holes, especially in the distant universe, has been underestimated.
"It was thought that radio jets were associated only with moderately weak black holes, while jets could not be launched by black holes that grow very fast," Dr Soria says. "The discovery of S26 has challenged this scenario. It seems now that at least some of the most powerful black holes can have strong jets."
This has implications for the universe's early history. For the first billion years or so, the cosmos was dominated by quasars. Thereafter, they switched off after their gas supplies became exhausted.
To understand what makes black holes turn their jets on or off, and what types of black holes have the most powerful jets, Dr Soria – along with collaborator James Miller-Jones and other colleagues – is scrutinising nearby stellar black holes using the Australia Telescope Compact Array and X-ray telescopes.
Over the past year or so, another two powerful micro-quasars have been unearthed in the nearby universe: one in the Andromeda galaxy, discovered by Dr Miller-Jones and his colleagues, and another in the M83 galaxy, which Dr Soria identified.
Watch Fay Dowker's lecture on the thermodynamics of black holes at: www.youtube.com/watch?v=VhHE86d-Th8
Read the article "Radio lobes and X-ray hot spots in the micro-quasar S26" at: arxiv.org/abs/1008.0394
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