A second kind, small black holes that are known to form out of the collapse of an old star, might have a mass no more than our Sun, but packed into a tiny sphere.
Middleweight black holes, as the name suggests, are in-between. They can be tens or thousands of times more massive than a stellar black hole and generally are spread throughout galaxies outside their cores. Until now, researchers have been puzzled as to how they are created.
One newly proposed scenario suggests that middleweight black holes may not be middleweight at all, but rather stellar black holes creating puffed-up appearances.
Building blocks
In three separate studies presented here at the annual meeting of the American Astronomical Society, researchers showed that these middleweight black holes, if they are in fact what they seem, may well be common. The Chandra X-ray Telescope alone is "nearly every day" finding more of them, said Kimberly Weaver of NASA's Goddard Space Flight Center.
But in one galaxy, Weaver and her colleagues, David Strickland and Timothy Heckman of Johns Hopkins University, uncovered some strange goings-on that illustrate how smaller black holes evolve into larger ones and migrate toward the galactic core.
The galaxy, called NGC 253, is known as a starburst galaxy. Large numbers of young, massive stars and supernova explosions generate the bulk of the galaxy's brightness, most of it coming from outside the galactic core.
The gas expelled from all this activity may collide and collapse into middleweight black holes, Weaver said, perhaps providing an explanation for how some of them are created. But out of 10 of these objects spotted, three were within 3,000 light-years of the galaxy's core, closer than is typically seen.
Weaver suggests that these three black holes are gravitating to the center of the galaxy and might eventually form a single, supermassive black hole that feeds off other nearby matter to grow to gargantuan proportions.
In an interview, Weaver said the core of NGC 253, like many galaxies in which a lot of star formation is going on, is loaded with stars, gas and dust. "They're just really messy," she said, and all of this mess means objects and clouds of material are more apt to collide.
"As these black holes orbit around the center of the galaxy, they're going to interact with all this stuff," she said. "And the friction they encounter will cause them to lose angular momentum, and they will slowly sink in."
Weaver stressed that this idea is speculative and has not yet been tested. It's possible that in many galaxies such a process would take too long to be viable, she said. But within a certain distance of the center of some galaxies, it is not unexpected for objects to gravitate toward the center.
Turning a galaxy inside out
If middleweight black holes did swirl inward, Weaver said, it would effectively turn the galaxy inside out. While the bright X-ray sources are now scattered around the center of NGC 253, the galaxy would one day shine intensely from its core as the newly created supermassive black hole pulls nearby matter into a frenetic swirl, speeds it up and heats it up, then generates high-energy X-rays before swallowing it forever.
And some evidence shows this is already beginning to happen. Weaver's team found another type of X-ray emission that they say looks like the signature of a budding active galactic nucleus, or AGN -- essentially a supermassive and well-fed black hole.
"We have known for several years that starburst activity can be associated with AGN activity," Weaver said. "In NGC 253, Chandra may have found a causal connection."
Survey says...lots of middleweights
A separate study found that middleweight black holes seem to exist in at least a fifth of all galaxies.
Andrew Ptak of Carnegie Mellon University and Edward Colbert of Johns Hopkins looked for middleweight black holes in a study of 750 galaxies within 200 million light-years of Earth, the largest such survey done so far.
Using the German-led ROSAT satellite, the researchers found that about one in 35 of these galaxies had middleweight black holes as massive as 100 Suns, and about one in five harbored fainter black holes, each roughly 10 times as massive as the Sun.
The researchers say there may be many more middleweight black holes that were not spotted because they are dormant, not currently feeding due to the fact they have already ingested the bulk of the gas and dust that was in their vicinity.
"Never before has there been a complete accounting of the numbers of these peculiar objects," Colbert said. "Now we have a large database to use for follow-up observations and we can begin to search for clues as to what [these objects] really are."
Colbert and Ptak, working on separate teams back in 1999, made the first discoveries of middleweight black holes.
They might not be giants
Finally, Andreas Zezas of the Harvard-Smithsonian Center for Astrophysics led a study of the Antennae Galaxies, finding several apparent middleweight black holes.
But Zezas and his colleagues speculate that these middleweight black holes might be stellar black holes in disguise. Zezas explained that black holes are detected because they are accompanied by a companion, a normal star from which they siphon matter. This process in a so-called binary system creates a thin disk of incoming material around the black hole, called an accretion disk.
Incoming matter strikes the black hole's surface and generates X-rays that travel in all directions -- the signature that researchers use to identify a black hole.
"When you have a lot of mass falling on the black hole, then this disk becomes very thick," Zezas told SPACE.com.
The X-rays might then be directed into relatively narrow regions above and below the thickened accretion disk, Zezas said. This funneling effect could concentrate the energy emitted by even a relatively small, stellar-mass black hole, making its output appear similar to that produced by a middleweight black hole.
The researchers agreed that more study is needed, and that Chandra would provide significant data.
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