NGC 6240

Like many merging galaxies, NGC 6240 offers not one supermassive black hole but two. Eventually, though, they will merge to form a single object.

The two galaxies of NGC 6240 began their merger process about 30 million years ago (as seen from Earth). Gravitational interactions between the two disk-shaped galaxies pulled out great streamers of millions of stars that spread across hundreds of thousands of light-years. Vast clouds of gas and dust in the two galaxies rammed together, triggering a vigorous bout of starbirth. The galaxies are packed with millions of young stars, and many millions more should be born before the merger is completed.

A supermassive black hole inhabited the core of each galaxy. Today, the black holes are only about 3,000 light-years apart. They are difficult to see because they are veiled by dense clouds of dust. The dust is warmed by the newborn stars, superhot gas around the black holes, or both, so it emits copious amounts of infrared radiation.

The dusty cloaks make it more difficult to measure the masses of the black holes. A 2011 study, which measured the orbital speeds of stars in the core of one of the two galaxies, estimated the black hole's mass at roughly 870 million to 2 billion times the mass of the Sun. The other black hole has not been measured.

As the two galaxies merge, the black holes will loop around each other, drawing tighter with each loop. Each will grow larger as it sucks in vast amounts of gas and dust. In millions of years the black holes likely will merge, spawning an outburst of gravitational waves that will ripple across the universe.

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This document was last modified: May 1, 2013.