Astronomers discover why supermassive black holes consume less material than expected

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Posted August 30, 2013
Why super massive black holes consume less material than expected

Why super massive black holes consume less material than expected
A composite image of the region around Sagittarius A* (Sgr A*), the supermassive black hole in the center of the Milky Way. X-ray emission from NASA’s Chandra X-ray Observatory is shown in blue, and infrared emission from the Hubble Space Telescope is shown in purple and yellow. The inset shows a close-up view of Sgr A* in X-rays only, covering a region half a light year wide. The diffuse emission is from hot gas captured by the black hole and being pulled inwards. Less than 1% of this material reaches the black hole’s event horizon, or point of no return, because much of it is ejected. Credit: X-ray: NASA/UMass/Q.D. Wang et al.; IR: NASA/STScI

Using NASA’s super-sensitive Chandra X-ray space telescope, a team of astronomers led by Q. Daniel Wang at the University of Massachusetts Amherst has solved a long-standing mystery about why most super massive black holes (SMBH) at the centers of galaxies have such a low accretion rate—that is, they swallow very little of the cosmic gases available and instead act as if they are on a severe diet.

“In principle, super massive black holes suck in everything,” Wang says, “but we found this is not correct.” Astronomers once thought SMBHs with their intense gravitational pull indiscriminately devoured all sorts of stars, dust and other matter in epic amounts. But in recent years, using X-ray emissions as a measure of heat given off by powerful gravitational forces, they unexpectedly found that most SMBH accrete matter at very low levels.

In fact, SMBHs’ signature X-ray emissions, which come from an area much larger than the black holes themselves, are often so surprisingly faint that the objects are difficult to distinguish from their galaxy centers. “There has been a big mystery about why most of these black hole signals are so faint,” says Wang, an expert in deep space X-ray analysis.

Now, taking advantage of very long observation times with the Chandra instrument and their detailed knowledge of the nearest SMBH, Sagittarius A* (Sgt A*), about 26,000 light years away at the center of our own Milky Way galaxy, he and an international team of astronomers tested the leading accretion models. For the first time, they were able to pinpoint and discriminate among X-ray sources near Sgt A* and identify exactly what the SMBH is feeding on. The advance is described in the current issue of Science.

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