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Farthest Black Hole Weighs In
24 March 2003 (All day)
Using a novel technique, astronomers have weighed the most distant black hole in the universe. It resides in a luminous galaxy, or quasar, so far away that its light took 13 billion years to reach Earth. The monstrous black hole in the quasar's core turns out to be a whopping 3 billion times more hefty than the sun. If the finding holds up, astronomers may need to rework their theories to explain how such a massive object could have formed so soon after the big bang.
In quasars, supermassive black holes are surrounded by whirling disks of hot gas that give off enormous amounts of radiation as they gradually spiral into oblivion. The traditional technique for weighing them takes advantage of the fact that glowing hydrogen atoms in the disk emit visible light at a particular wavelength. The corresponding narrow line in the quasar's spectrum is broadened by the disk's rapid rotation--the broader the line, the faster the rotation and the more massive the black hole. The technique doesn't work for very remote galaxies, however, because the expansion of the universe shifts the blue hydrogen light into a hard-to-observe infrared wavelength.
Last year, two astronomers in the United Kingdom, Ross McLure at the University of Edinburgh and Matt Jarvis of the University of Oxford, showed that for distant galaxies, the emission line of ionized magnesium--which shows up in the ultraviolet part of the spectrum--can be used instead. Working with Chris Willott at the Herzberg Institute of Astrophysics in Victoria, Canada, and Douglas Pierce-Price of the Joint Astronomy Centre in Hawaii, McLure and Jarvis have now applied the new technique to the most distant supermassive black hole known, in the quasar SDSS J1148+5251. According to Willott, the mass estimate--3 billion solar masses--is accurate to within a factor of two-and-a-half.
The result comes as a surprise to Heino Falcke of the University of Nijmegen, the Netherlands. Supermassive black holes in the cores of galaxies are thought to fatten slowly over eons, so finding such a heavyweight so early in the history of the universe is "really pushing it," he says. "You would need a very, very fast growth instead." Jarvis says his group is now using the new technique to weigh supermassive black holes at a wide range of distances and other epochs in the history of the universe. "It will be really exciting to compare the results with the evolution of galaxies, given the close links between black hole and galaxy formation," says Willott.