DR EMILY BALDWIN
Posted: 17 March 2010
Two of the earliest and most primitive supermassive black holes have been uncovered by NASA's Spitzer Space Telescope, providing vital information about the nature of the early Universe.
Black holes lurk at the centres of most galaxies and are usually given away by a torus of dust and gas that swirls around them, and from which the black hole feeds. These disc-powered supermassive black holes, known as quasars, emit vast amounts of radiation as matter is consumed.Artist's impression of a primitive supermassive black hole at the core of a young galaxy. Usually black holes are surrounded by an accretion disc of dust and gas, but the primitive black holes do not host any dust. Image: NASA/JPL-Caltech.
“We have found what are likely first-generation quasars, born in a dust-free medium and at the earliest stages of evolution,” says Linhua Jiang of the University of Arizona’s Steward Observatory and lead author of the paper announcing the results that appear in this week's issue of the journal Nature.
Astronomers believe that the early Universe was dust free, which means that the first quasars must also be devoid of dust, however no one had observed such pristine examples until now. The two quasars, known as J0005-0006 and J0303-0019, are located 13 billion light years away from Earth and are the smallest quasars ever found. Each quasar is anchored by a supermassive black hole weighing more than 100 million Suns.
"The typical mass of other quasars at this distance is a few billion solar masses," says Jiang. "Small black holes are very difficult to find simply because they are too far away."
The duo were first identified in visible light data in the Sloan Digital Sky Survey and also in Chandra X-ray Observatory data, but it was the absence of hot dust in Spitzer's infrared images that got the astronomers' attention. “The most exciting discovery for us is what we don’t see,” says Xiaohui Fan who led the discovery team that first unveiled the quasars, “that is, the dust that typically surrounds all other quasars that have been found so far.”These two data plots show a primitive supermassive black hole (top) compared with a typical supermassive black hole (bottom), which displays evidence of a dusty ring. Image: NASA/JPL-Caltech/L.Jiang (University of Arizona).
The team think that these pristine black holes were born around the time when dust was first forming in the Universe, less than one billion years after the big bang. Prior to this time, the Universe did not contain any molecules that could coagulate to form dust; these arrived later when the first stars pumped the necessary elements into the Universe.
In the study group of twenty quasars the team also noticed that the amount of hot dust in a quasar increases with the mass of its black hole and hence its age, since as these behemoths grow they gather more and more dust around their midriffs. Given that J0005-0006 and J0303-0019 have the smallest masses they represent the youngest examples, born in an era devoid of dust.
Jiang adds that since Spitzer is no longer working at the infrared wavelengths needed to seek out dust-free quasars they will have to wait until the launch of the next-generation James Webb Space Telescope, planned for 2016, before resuming their quest. Spitzer ran out of coolant in May 2009 and has since been operating as a 'warm mission', with just the two shortest wavelengths – 3.6 and 4.5 µm – of the Infrared Array Camera in operation.