NASA's Chandra Observatory Images Gas Flowing Toward Black Hole
The galaxy NGC 3115 contains a supermassive black hole at its center. New Chandra data has been used to clearly image hot gas as it falls toward this giant black hole. NASA/CXC/Univ. of Alabama/K.Wo

A black hole at the center of a large galaxy has been seen attracting the motion of gas with its intense gravity and dominating the flow of gas towards it. As gas flows toward a black hole, it becomes squeezed, making it hotter and brighter, a phenomenon now confirmed by NASA's Chandra X-ray Observatory.

The Chandra data also show the gas close to the black hole in the center of the galaxy is denser than gas further out, as predicted.

The researchers, using the observed properties of the gas and theoretical assumptions, estimated that each year gas weighing about 2 percent the mass of the sun is being pulled across the Bondi radius toward the black hole.

The researchers have observed a critical threshold, by imaging the hot gas at different distances from a super-massive black hole, where the motion of gas first becomes dominated by the black hole's gravity and falls inward. This distance from the black hole is known as the "Bondi radius".

The black hole is at the center of a large galaxy known as NGC 3115, which is located about 32 million light years from Earth. A large amount of previous data has shown material falling toward and onto black holes, but none with this clear a signature of hot gas.

The astronomers found the increase in gas temperature begins about 700 light years from the black hole, giving the location of the Bondi radius. This suggests the black hole in the center of NGC 3115 has a mass about two billion times that of the sun, making it the closest black hole of that size to Earth.

"It's exciting to find such clear evidence for gas in the grip of a massive black hole. Chandra's resolving power provides a unique opportunity to understand more about how black holes capture material by studying this nearby object," said Ka-Wah Wong of the University of Alabama, who led the study that appears in July 20 issue of The Astrophysical Journal Letters.

The astronomers would expect to find a source that is more than a million times brighter in X-rays than what is seen in NGC 3115, making certain assumptions about how much of the gas's energy changes into radiation.

"A leading mystery in astrophysics is how the area around massive black holes can stay so dim, when there's so much fuel available to light up," said co-author Jimmy Irwin, also of the UA in Tuscaloosa. "This black hole is a poster child for this problem."

For this discrepancy, there are at least two possible explanations. The first is that much less material actually falls onto the black hole than flows inside the Bondi radius. Another possibility is that the conversion of energy into radiation is much less efficient than is assumed.

Different models describing the flow of material onto the black hole make different predictions for how quickly the density of the gas is seen to rise as it approaches the black hole. A more precise determination of the rise in density from future observations should help astronomers rule out some of these models.

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