Waltzing behemoths

A supermassive black hole is rather like a gravitational version of the Incredible Hulk. Massive, mean, able to take on anything thrown at it, and virtually unstoppable. While the smaller black holes may form when a massive but otherwise normal star has a temper tantrum and explodes, the supermassive ones are altogether different. No one’s quite sure exactly how they formed, but chances are good that they’re very old indeed. Ancient and invincible, anything a supermassive black hole can’t pull into its gluttonous maw is shredded by gravitational forces capable of carrying entire galaxies in tow. So you probably wouldn’t like to have a close encounter with one of them. But how about two of them?

At the heart of the vast spiral galaxy NGC 3393, two giants are dancing a mighty gravitational waltz. Separated by only around 490 light years (closer than Earth is to Betelgeuse), these two are hurtling around each other, seemingly dragging clouds of interstellar dust in their wake. This is actually the first time a pair of black holes like this has been spotted inside a spiral galaxy – and here you can see an optical image in gold, and an X-ray image in blue. The gold shows clouds of gas and dust (and probably quite a few shredded stars) swirling around with the black holes, while the blue shows superheated gas. The X-ray emission in that gas includes emission from iron atoms, which is usually a good way of spotting feeding black holes.

As for why there are two, the most likely reason is that NGC 3393 used to be two galaxies. Probably at least a billion years ago, two galaxies collided with each other, splashing together and flinging stars out into intergalactic space. The stars which were held tightly enough by gravity coalesced into the NGC 3393 we see today, and the black holes at the cores of both galaxies have been spiralling inwards towards each other ever since.

Hats off to Chandra for the most gorgeous image I’ve seen this year. Go and have a look for a closeup of those dancing singularities!


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