Active black holes were at least 100 times more common 10 billion years ago. So what happened to these once ravenous monsters?
Only a fraction of super-massive black holes in the universe are actively feeding.
Galaxies living in clusters aren't any more likely than loners to have active super-massive black holes.
The fuel supply or the fueling mechanism for super-massive black holes is changing with time, scientists suspect.
The black hole at the center of the Milky Way, like about 99 percent of its super-massive siblings living in other galaxies, is relatively inactive and likely to stay quiescent for the foreseeable future, a new study shows.
Scientists aren't sure if there is less material around to feed black holes or if something else has changed. But they recently completed a key step in solving the puzzling by putting some hard numbers on what percentage of the universe's biggest black holes are actively sucking in material and what fraction is dead.
"We know that active black holes were at least 100 times more common 10 billion years ago and that they've been on the wane ever since," astrophysicist Paul Green, with the Harvard-Smithsonian Center for Astrophysics, told Discovery News.
Black holes are regions of space so dense with matter that not even photons can overcome the grip of gravity. Black holes are found by studying the trail of wreckage and other phenomena spawned as they consume nearby galaxies, stars, gases and other material in their paths.
The most active super-massive black holes -- those with a mass one million to 10 billion times the mass of the sun -- produce what are called active galactic nuclei, which are caused by the heating of gas on the brink of falling into the abyss. The gas glows brightly in X-ray light.
Researchers used data collected by NASA's Chandra X-ray Observatory to sample 100,000 galaxies and compared them with optical images from the Sloan Digital Sky Survey to infer what percent of super-massive black holes are active.
They restricted the sample to galaxies within 1.6 billion light-years to avoid skewing the results by missing dormant, distant galaxies.
"We only wanted to go out as far as we could fairly sample," Green said.
The scientists found that the fraction of galaxies which are active X-ray emitters supports previous less accurate results using different methods. What was more surprising is that the fraction of active black holes was about the same regardless of whether the host galaxy was part of a galaxy cluster or if it occupied a more isolated corner of the universe.
"We once thought that galaxies in clusters should have more active black holes because there may be more banging together and disruption of material which could then fall into the black hole," Green said.
Instead, the clusters seem to be very efficient at organizing the galaxies in mutually attractive bonds of gravity so that they have less of a chance to tango with one another -- at least that's the way it appears today.
"It may be a cosmic coincidence," Green said.
Perhaps the number of active black holes in galaxy clusters was once higher and now has decreased to the same level found in the so-called field galaxies, he added.
Scientists hope the information will lead to a better understanding of how the galaxies and their black holes formed.
"We need to understand the population holistically," said astrophysicist Daryl Haggard with Northwestern University.
Though only a small percentage of super-massive black holes are active today, they all had to be busy at some point in the past to grow as big as they are, Haggard added.
As activity amongst the universe's biggest black holes winds down, a trend Green and Haggard expect will continue, the smaller black holes may be beginning to step up. "It'd be very interesting to probe those galaxies," Haggard said.
The study appeared last month in the Astrophysical Journal.