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In this illustration, light from a smaller black hole (left) curves around a larger supermassive black hole and forms an almost-mirror image on the other side. Credit: Caltech-IPAC.

Astronomers in Italy have devised a new method for establishing which of the countless specks of light in the night sky might correspond to pairs of supermassive black holes spiraling in towards one another within a common galaxy. Relying on data from a vast sky survey carried out by the European Space Agency’s Gaia spacecraft, the researchers say their technique could help theorists to refine models of merging galaxies.

Many cosmological models tell us that as the universe evolves smaller galaxies merge to form larger ones. With many galaxies thought to have a supermassive black hole at their core, these mergers should result in pairs of the exceptionally massive objects spiraling in around a common centre-of-mass until they combine and give off an intense burst of gravitational waves.

These pairs would be seen as dual active galactic nuclei (AGN), which are extremely bright regions at the centre of galaxies whose radiation is not generated by stars. Spotting such dual AGNs, however, has proved extremely tricky. Scientists have tried many approaches, including looking for sources within the Gaia catalogue whose position tends to vary over time. But so far, they have netted only four close-knit dual AGNs within the relatively early universe.

To increase their success, Elena Pancino, at the Arcetri Observatory in Florence, suggested to some her colleagues working on AGNs that it might be possible to use a different parameter from the Gaia catalogue – one which characterizes the shape of an object’s emission, with a single peak pointing to just one source and a double peak to two sources.

Using this method, Pancino, Filippo Mannucci and co-workers in Italy, the US and Norway identified 260 objects that could potentially be either dual AGNs, single AGNs that appear to be dual as a result of gravitational lensing, or single AGNs whose light has become mixed up with that from intervening stars1. They then trawled through the Hubble Space Telescope catalogue to try and obtain images of these objects, and found 26 matches — in each case confirming that the objects in question do indeed contain multiple sources. As a further step, they analysed the spectra of five objects that do not have matches in the Hubble catalogue, establishing that two of them were dual AGNs.

David Sanders, an astronomer at the University of Hawaii at Manoa, who was not involved in the research, says that the new method will “get a lot of attention”. But he says a couple of things remain unclear: why it was necessary to confirm the multiple sources using Hubble's images instead of Gaia images alone, and how many additional dual or lensed AGNs might turn up in more sensitive or higher-redshift survey data.

By going on to measure the properties of many dual AGNs, the researchers in Italy argue it should be possible to pin down the physical processes acting on supermassive black holes as they spiral in. In particular, they say that the statistical distribution of distances separating the two black holes might reveal whether spiraling requires the gravitational pull of dark matter, as well as that of ordinary matter.

However, more work is needed before such analyses can be done. Pancino says that her team will need fresh observations to classify the rest of the 260 candidates and understand how many are dual AGNs. If all goes well, she says, they will need “a couple of years” to complete the job.