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Most, if not all, galaxies are believed to have a huge black hole residing at their centre. When matter falls into one of these galactic black holes, large amounts of radiation are released, producing a quasar — an object that looks like a star but is 10 billion times brighter. Todd Boroson of the National Optical Astronomy Observatory in Tucson, Arizona, has been studying quasars for more than 25 years. During an analysis of data from the Sloan Digital Sky Survey, he and his colleague Tod Lauer uncovered evidence of a binary black-hole system: that is, the presence of two black holes at a galaxy's centre (see page 53). Many scientists agree that such systems, produced by the merging of two large galaxies that each harbour a black hole, ought to be common throughout the Universe. But few have been identified, possibly because they are very distant from us. Out of the hundreds of billions of galaxies in the Universe, only the four dozen or so nearest to our own can be studied closely enough for black holes to be directly detected. Boroson tells Nature more.

What is the Sloan Digital Sky Survey?

It's a massive astronomical survey managed by the Astrophysical Research Consortium and scheduled for completion in 2014. It covers more than a quarter of the sky and has been used to create three-dimensional maps containing more than 930,000 galaxies and 120,000 quasars. We looked at an analysis of spectra from 17,500 of these.

Were you looking for a binary black-hole system?

No. We were studying quasar characteristics when this popped up. We found something that I had never seen before — two very distinct sets of emission lines in one spectrum. This indicated the presence of two different objects, each travelling at a different velocity. In a normal quasar spectrum you would see only one. We deduced that there are two velocities because two black holes are orbiting each other.

Can you be sure?

The object has two sets of broad emission lines, which indicates that material is falling into the black holes. We can't rule out the possibility that these black holes aren't orbiting each other. But finding out should be fairly straightforward because if they are then we should see changes in the emission lines in as little as one year. We're also planning to get good images from the Hubble Space Telescope. Using the two velocities and the assumption that the black holes are orbiting each other, we can estimate the distance between them. That distance is so small that, even with Hubble, this object should look like a single point of light, not two.

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Abstractions. Nature 458, 6 (2009).

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