PDS456 was discovered in 1997 and lies at a redshift of only 0.184 (a mere 800 million light-years away - our backyard by quasar standards).Its energy output is equivalent to that of 25,000 billion Suns requiring a black hole of roughly a billion solar masses. Such objects are relatively common with high redshifts in the distant universe, but nearby ones are scarce. "Fortunately, PDS456 is sufficiently close that we can study it in great detail and thereby learn about the structure of these extraordinary objects," says Dr Paul O'Brien of the University of Leicester, who will present the findings.

Members of the research team have been studying PDS456 for several years. Their previous work has measured the large power output and other basic properties of PDS456, but now, using X-ray data from the XMM-Newton X-ray satellite and ultraviolet data from the Hubble Space Telescope, they have discovered the new and unexpected twist. The team had suspected that PDS456 is accumulating matter at almost the maximum rate allowed for feeding a black hole. Theory suggests that a black hole may "choke" under these circumstances, expelling matter outward.

Before being sucked into the black hole, accreting material generally forms a flattened, rotating disc which allows matter to move inwards while radiating energy away. However, the energy output of PDS456 is so large that even the enormous gravitational pull of the black hole cannot capture it all. Instead the radiation blows the top off the inner disc. The material travels at close to the speed of light because this process happens very close to the black hole - within a region about the same size as the solar system. "It appears that PDS 456 is expelling matter at the rate of many times the mass of our sun every year" comments Dr James Reeves. "This massive outflow may tip the balance of power in this quasar, implying that a large fraction of its total energy output is involved in driving the flow. Such high mass, high velocity flows pose a real problem for current models of quasars."

In one respect, PDS456 is similar to 3C273, the very first quasar discovered back in 1962. "Our observations show that PDS456 looks remarkably similar to 3C273, though brighter, right across the spectrum from the ultraviolet through to the infrared," comments Paul O'Brien. "But 3C273 is a harder object to study in X-rays because it has a powerful jet pointing almost directly at us. The jet beams radiation, including X-rays, at us, contaminating our view of the centre. It is like trying to see a light bulb next to the glare of a searchlight. In the case of PDS456, any jet is pointing away from our line of sight so we get a much clearer view of its disc."

The clear view to the centre of PDS456 means the researchers can use it as a template object to determine what all powerful quasars are like, particularly those accreting matter at a high rate. In the early universe, when galaxies were young, their central black holes were growing rapidly and may have been accreting matter at a high rate. Today only a few, like PDS 456, continue to do so.

CONTACTS

Dr. Paul O'Brien,
Department of Physics & Astronomy,
University of Leicester
(+44) (0)116 252 5203
pto@star.le.ac.uk
Will be present at the NAM 8-11 April; giving presentation

Dr. James Reeves
NASA Goddard Space Flight Center
(+1) 301 286 1592
jnr@milkyway.gsfc.nasa.gov
Will be present at the NAM 8-11 April

Professor Martin Ward
Department of Physics & Astronomy,
University of Leicester
(+44) (0)116 252 3540
mjw@star.le.ac.uk
Will be present at the NAM 8-11 April

Professor Ken Pounds
Department of Physics & Astronomy,
University of Leicester
(+44) (0)116 252 3509
kap@star.le.ac.uk

NOTE Artist's impressions of the centre of a galaxy containing a supermassive black hole may be found at

http://hubblesite.org/newscenter/archive/1990/29/image/a

and

http://chandra.harvard.edu/xray_sources/blackholes_sm.html