U.S., European Observations Link Black Hole Rotations to Galactic Evolution


U.S. and European X-ray space observatories have measured the spin rate of a super massive black hole in a bright nearby galaxy, demonstrating a feature of Einstein's General Theory of Relativity that offers a promising new technique for use across the universe to study galactic evolution.

Artist's conception of NGC 1365 and supermassive black hole. Image Credit: NASA

The focus of a 36 hour joint observation using NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) and the European Space Agency's XMM-Newton X-ray telescope was a two million-solar mass black hole in the galaxy NGC 1365, which lurks 56 million light years distant.

The enormous spin rate, 84 percent of Einstein's theoretical limit, generates a rotational energy equivalent to the light coming from a billion stars shining for a billion years.

"We know that spinning black holes actually twist space-time and distort it," Fiona Harrison, NuStar's principal investigator from the California Institute of Technology, told a NASA hosted news briefing on the discovery which coincided with publication of the findings in the journal Nature. "If you were standing near the event horizon of this particular black hole, you would have to turn around. Because your space-time is twisting, you would be turning around once every four minutes just to stand still."

XMM-Newton, launched in 1999, and NuStar, launched in mid-2012, permit observations across low- to high-energy X-ray bands to measure the rotations. The X-rays, which come from a high velocity jet emerging from the black hole, are reflected off an accretion disk of dust and gas spiraling into the massively dense object.

Astronomers believe most galaxies are anchored by a central supermassive black hole. They started relatively small, 20-30 solar masses when the universe was about 10 percent of its 
current age, and grew in two basic ways. When galaxies merge, their supermassive black holes merge as well. And matter pulled into the accretion disk by intense gravity continuously falls into the black hole.

The spin rate increases as the black hole mass grows.

"The result is very interesting and important as a proof of general relativity, but it is also quite important for its consequences regarding galaxy formation and evolution," said Guido Risaliti, an astronomer with the Harvard-Smithsonian Center for Astrophysics who led the joint study.

"What we would really like to do is to extend these studies to the more distant universe and see how the average back hole spin changes with cosmic time," explained Arvind Parmar, who 
heads ESA's Astrophysics and Fundamental Physics Missions Division.  "This would allow us to probe the importance of accretion and of mergers in creating the universe we see today."

While Risaliti and his co-investigators are extending their joint observations to three other bright “nearby" supermassive black holes, equivalent measurements on the far cosmic frontier must await the development of vastly more powerful X-ray space telescopes.

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