August 30, 2013
Researchers Explain Why Supermassive Black Holes Eat Less Material Than Expected
[ Watch the Video: Black Hole On A Diet ]
April Flowers for redOrbit.com - Your Universe OnlineA long-standing mystery concerning supermassive black holes (SMBH) at the center of galaxies has been solved by a team of astronomers using NASA’s super-sensitive Chandra X-ray Space Telescope.
The team, led by Q. Daniel Wang at the University of Massachusetts Amherst, investigated why these SMBHs have such a low accretion rate, or rather, why 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." SMBHs, with their intense gravitational pull, were once thought to indiscriminately devour all sorts of stars, dust and other matter in epic amounts.
Recently, however, astronomers have used X-ray emissions as a measure of heat given off by powerful gravitational forces to discover most SMBH accrete matter at very low levels. The results of this study were published in the journal Science.
SMBHs have signature X-ray emissions which come from an area much larger than the black holes themselves. These emissions 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.
The international team of researchers combined long observation times -- five weeks -- with the Chandra instrument and detailed knowledge of the nearest SMBH, Sagittarius A* (Sgr A*), which is about 26,000 light-years away at the center of the Milky Way galaxy, to test the leading accretion models. The researchers were able to pinpoint and discriminate among X-ray sources near Sgr A* for the first time, which allowed them to identify exactly what the SMBH is feeding on. The data indicated that less than one percent of the gas initially within Sgr A*'s gravitational grasp ever reaches the point of no return, also called the event horizon. Much of the gas is ejected before it gets near the event horizon and has a chance to brighten, instead. This leads to a feeble X-ray emission.
"We think most large galaxies have a supermassive black hole at their center, but they are too far away for us to study how matter flows near it," said Wang. "Sgr A* is one of very few black holes close enough for us to actually witness this process."
Previous studies, in an attempt to explain the faint X-ray signals, theorized emissions from areas around SMBH had nothing to do with the black hole itself but rather with concentrations of low-mass stars associated with SMBHs.
The new observations showed the gas near the black hole likely originates from winds produced by a disk-shaped distribution of young massive stars.
Wang adds, "There are also a huge number of young, massive stars as well as low-mass stars near these SMBHs, so it's very crowded in the downtown area of the galaxy. Hard to tell what was going on.
"The massive stars have extremely high winds associated with them and the winds are colliding and swirling at very high speeds, which make the gases in this environment very hot. We found that first, the SMBH has difficulty in accreting such gases. Second, the gases are too hot for the black hole to swallow. Instead it rejects about 99 percent of this super hot material, only letting a small amount in. This makes sense because the hotter the gases, the more difficult it is for the black hole to pull them in."
Wang explains a diet of cooler gases would accrete in a more orderly fashion, but with increasing gas temperatures, both the SMBH’s sphere of influence and its ability to accrete new material decrease.
"Most of the gas must be thrown out so that a small amount can reach the black hole", said Feng Yuan of Shanghai Astronomical Observatory in China. "Contrary to what some people think, black holes do not actually devour everything that's pulled towards them. Sgr A* is apparently finding much of its food hard to swallow."
The gas Sgr A* has available to feed on is very diffuse and super-hot, making it hard for the black hole to capture and swallow it. Black holes that devour huge amounts of gas to power quasars and produce massive amounts of radiation have gas reservoirs that are much cooler and denser than that of Sgr A*.
"Now we have physically resolved it and for the first time we've made the connection observationally between the massive stars moving around black holes and the X-ray emitting material. We can definitively rule out that these X-rays are coming from a concentration of low-mass stars. We don't see the expected energy signature predicted by that scenario,” explained Wang, who did this NASA-supported work while on a four-month sabbatical as a Raymond and Beverly Sackler Distinguished Visiting astronomer at the University of Cambridge, UK
The Chandra data not only allowed the astronomers to detect the X-ray source, but to describe its elongated shape for the first time. "Now we know what kind of material is getting into the black hole, though exactly how it happens is still another question."