Astrophys. J. (in the press); preprint at http://arxiv.org/abs/1412.3120 (2015)

The collision of two galaxies is a dramatic event — all the more so when they host supermassive black holes in their centres. Such an event is taking place 134 million light years away, where two galaxies, collectively known as Arp 299, are moving towards each other, causing an overflow of gas that enhances star formation and fuels the black holes. The mechanism triggering a black hole to start feasting on the gas is still unknown, so Andrew Ptak and colleagues combined the data from NASA's NuSTAR high-energy X-ray observations with visible-light images from the Hubble Space Telescope to expose the gluttonous black holes.

The NuSTAR telescope can identify the origin of X-ray emissions, so the observations could be used to pinpoint which of the two black holes is releasing more high-energy X-rays following gas consumption. It turns out that one of the black holes is either inactive or obscured by dust and gas. The other one, however, is very actively feeding on the gas and releasing energetic X-rays.