Discovery of smallest supermassive black hole ever provides clues to growth

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In what NASA dubs as an oxymoronic discovery, researchers have found the smallest every supermassive black hole at the centre of a disk shaped galaxy some 340 million light years away.

Located at the centre of dwarf disk galaxy, called RGG 118, the latest supermassive black hole was discovered using NASA’s Chandra X-ray Observatory and the 6.5-meter Clay Telescope in Chile. Researchers say though the mass of the black holes if 50,000 times that of our Sun, it is still less than half the mass of the previous smallest black hole at the center of a galaxy.

Researchers discovered the supermassive black hole by studying the motion of cool gas near the center of the galaxy using visible light data from the Clay Telescope and the X-ray brightness of hot gas swirling toward the black hole using the Chandra data. Researchers found that the outward push of radiation pressure of the hot gas is about 1 per cent of the black hole’s inward pull of gravity, matching the properties of other supermassive black holes.

Previously, scientists had noted a relationship between the mass of supermassive black holes and the range of velocities of stars in the center of their host galaxy. This relationship also holds for RGG 118 and its black hole.

“We found this little supermassive black hole behaves very much like its bigger, and in some cases much bigger, cousins,” said co-author Amy Reines of the University of Michigan. “This tells us black holes grow in a similar way no matter what their size.”

According to the estimates put forward by researchers, the black hole in RGG 118 is nearly 100 times less massive than the supermassive black hole found in the center of the Milky Way and about 200,000 times less massive than the heaviest black holes found in the centers of other galaxies.

Astronomers say that the discovery will give them an opportunity to study a nearby small supermassive black hole and based on that garner understanding about how supermassive black holes may have formed in the early universe.

Astronomers think supermassive black holes may form when a large cloud of gas, with a mass of about 10,000 to 100,000 times that of the sun, collapses into a black hole. Many of these black hole seeds then merge to form much larger supermassive black holes. Alternately, a supermassive black hole seed could come from a giant star, about 100 times the sun’s mass, that ultimately forms into a black hole after it runs out of fuel and collapses.

“We have two main ideas for how these supermassive black holes are born,” said Elena Gallo of the University of Michigan. “This black hole in RGG 118 is serving as a proxy for those in the very early universe and ultimately may help us decide which of the two is right.”

Researchers will continue to look for other supermassive black holes that are comparable in size or even smaller than the one in RGG 118 to help decide which of the models is more accurate and refine their understanding of how these objects grow.

The study has been published in the Astrophysical Journal Letters.