Most models of galaxy formation and growth predict the existence of pairs of massive black holes or quasars that are separated by less than the diameter of a galaxy, but only a few examples have been found. Now, an active black-hole pair has been detected in a Chandra X-ray image, separated by only 490 light years, in the nearby spiral galaxy NGC 3393. The regular spiral morphology and predominantly old circumnuclear stellar population of this galaxy, and the closeness of the black holes embedded in the bulge, suggest that they are the result of a minor merger event.
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