Astronomers using ESA’s XMM-Newton X-ray observatory have discovered, for the first time, a pair of supermassive black holes in orbit around one another in an ordinary galaxy.

Most massive galaxies in the Universe are thought to harbor at least one supermassive black hole at their center.

Two supermassive black holes are the smoking gun that the galaxy has merged with another. Thus, finding binary supermassive black holes can tell astronomers about how galaxies evolved into their present-day shapes and sizes.

To date, only a few candidates for close binary supermassive black holes have been found. All are in active galaxies where they are constantly ripping gas clouds apart, in the prelude to crushing them out of existence. In the process of destruction, the gas is heated so much that it shines at many wavelengths, including X-rays. This gives the galaxy an unusually bright centre, and leads to it being called active.

On 10 June 2010, Dr Fukun Liu from Peking University in China with colleagues spotted a tidal disruption event in the galaxy SDSS J120136.02+300305.5 (J120136 for short). They were scanning the data for such events and scheduled follow-up observations just days later with XMM-Newton and NASA’s Swift satellite.

The galaxy was still spilling X-rays into space. It looked exactly like a tidal disruption event caused by a supermassive black hole but as they tracked the slowly fading emission day after day something strange happened. The X-rays fell below detectable levels between days 27 and 48 after the discovery. Then they re-appeared and continued to follow a more expected fading rate, as if nothing had happened.

“This is exactly what you would expect from a pair of supermassive black holes orbiting one another,” said Dr Liu, who is the lead author of the study published in the Astrophysical Journal (arXiv.org version).

Dr Liu found that two possible configurations were possible to reproduce the observations of J120136.

In the first, the primary black hole contained 10 million solar masses and was orbited by a black hole of about a million solar masses in an elliptical orbit.

In the second solution, the primary black hole was about a million solar masses and in a circular orbit.

In both cases, the separation between the black holes was relatively small – about 2 thousandths of a light year. This is about the width of our Solar System.

Being this close, the fate of this newly discovered black hole pair is sealed. They will radiate their orbital energy away, gradually spiraling together, until in about two million years time they will merge into a single black hole.

Now that astronomers have found this first candidate for a binary black hole in a quiescent galaxy, the search is inevitably on for more. This detection will also spur interest in a network of telescopes that search the whole sky for tidal disruption events.

“Once we have detected thousands of tidal disruption events, we can begin to extract reliable statistics about the rate at which galaxies merge,” said study co-author Dr Stefanie Komossa of the Max-Planck-Institut für Radioastronomie in Bonn, Germany.

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F.K. Liu et al. 2014. A Milliparsec Supermassive Black Hole Binary Candidate in the Galaxy SDSS J120136.02+300305.5. ApJ 786, 103; doi: 10.1088/0004-637X/786/2/103