Weighing a black hole at the edge of the Universe
Posted: March 20, 2003

Artist's impression of the heart of a quasar, where a black hole is hidden in a disk of gas and dust. Credit: NASA Education and Public Outreach at Sonoma State University - Aurore Simonnet
For the first time, astronomers have weighed a black hole at the furthest reaches of the Universe. A team of astronomers from Canada and the United Kingdom studied infrared light from the most distant quasar known, and found that the quasar contains a black hole one quadrillion (1,000,000,000,000,000) times as massive as the Earth. The observations were made with the United Kingdom Infrared Telescope (UKIRT) in Hawaii, using the new UKIRT Imager Spectrometer (UIST) and are scheduled to be published today (March 20th) in the "Astrophysical Journal Letters" electronic edition.

Quasars are exceptionally luminous galaxies which are far brighter than can be explained by normal starlight. A quasar is powered by the release of gravitational energy as matter is pulled toward a supermassive black hole at its centre, a process called accretion. Their extreme brightness makes quasars visible at very great distances.

Team leader Dr. Chris Willott, from the National Research Council's Herzberg Institute of Astrophysics in Victoria, Canada, said "We looked at the most distant known quasar, SDSS J1148+5251, with UKIRT. We're seeing this quasar as it looked when its light was emitted 13 billion years ago, back when the Universe was only 6% of its current age."

The astronomers used the UKIRT Imager Spectrometer, UIST, to measure the infrared spectrum of the light from the quasar. They looked for a characteristic feature in the spectrum - a line emitted by MgII ions. These are atoms of magnesium with single electrons stripped off. The magnesium ions are part of the gas around the black hole at the heart of the quasar.

Willott explained "We can determine the mass of the black holes in these distant quasars by looking at the MgII emission line and comparing it with the same emission line in closer quasars. The basic idea here is that the width of the line gives an indication of the speed of the gas close to the quasar. More massive black holes will have faster moving material."

The team measured the width of the MgII emission line, which allowed them to measure the mass of the black hole as 3 billion (3,000,000,000) times the mass of our own Sun, or one quadrillion (1,000,000,000,000,000) times the mass of the Earth. They also used the wavelength of the emission line to determine a precise redshift for the quasar of 6.41. The redshift measures the distance to the object, confirming it as the most distant quasar known, approximately 13 billion light years from Earth.

The extreme brightness of this quasar also shows that the black hole in its core is swallowing matter at the maximum rate possible. This maximum rate is called the "Eddington Limit". If the black hole were accreting matter any faster, it would shine even brighter, and the intense luminosity would actually exert enough pressure to stop any more material falling in.

Dr. Ross McLure from the Institute for Astronomy in Edinburgh added "This quasar pinpoints the first massive structures to have formed in the Universe. It confirms predictions that such huge black holes do exist so early in the Universe, but they are rare. They are also surrounded by a reservoir of fuel which allows them to accrete material right up to the Eddington Limit."

Dr. Matt Jarvis of Oxford University explained what the team plan to do next: "We'll apply our black hole mass measuring techniques to other quasars, over a wide range of redshifts. We hope to trace out the evolution of black holes and the galaxies they reside in from the early Universe to the present day."

The research is scheduled to be published on 20th March in the Astrophysical Journal Letters online edition, appearing in the 10th April paper edition, volume 587.

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