Not Too Big, Not Too Small
New Type of Mid-Sized Black Holes Discovered
The white circle shows the general location of a suspected mediumsized black hole within the galaxy M82. The yellow dots indicate recent supernovas, which can produce small black holes. Several of these may have merged to form the medium-size black hole. (Carnegie Mellon Astrophysics)
By Kenneth Chang
April 13 — This is a Goldilocks story of astronomy …
Once upon a time, astronomers looked through their telescopes and detected Baby Bear black holes, dead stars collapsed to a few miles wide — so dense that not even light can break the gravitational binds. Then the astronomers looked at the center of galaxies and found some gargantuan Papa Bear black holes, with masses up to a billion suns.
Now they’ve discovered objects that appear … well, if not “just right,” at least in-between: several Mama Bear-sized black holes with masses between 100 and 10,000 suns.
Of course, no one actually sees black holes, which by definition emit no light. But as particles swirl into a black hole, they emit X-rays, which are detectable.
For a couple of decades, astronomers have been perplexed by unusual X-ray glows within many galaxies. The glows are brighter than the X-rays from collapsed-star black holes, but dimmer than those from supermassive, galactic black holes.
There appeared two possible explanations: they could be bigger versions of the small black holes or quiet versions of the galactic type.
Analyzing X-ray data from 39 galaxies, Edward Colbert and Richard Mushotzky, astronomers at NASA’s Goddard Space Flight Center in Greenbelt, Md., found that the “colors” of X-rays coming out of at least three medium-bright beacons resemble those from small black holes.
“You don’t see that kind of behavior from a supermassive black hole,” Colbert says. “Also, the sources are not located at the center of the galaxies,” as would be expected for supermassive black holes.
More Mass, More Light
But according to models of stellar-type black holes, the black holes would have to be between 100 and 10,000 times the mass of the sun to produce the observed brighter X-ray intensities. Such a black hole would be be smaller than the moon.
Colbert presented a talk about the medium-sized black holes today at the meeting of the American Astronomical Society’s High Energy Astrophysics Division in Charleston, S.C. The findings will also appear in an upcoming issue of The Astrophysical Journal.
In a separate talk, Andrew Ptak and Richard Griffiths from Carnegie Mellon University in Pittsburgh revealed they have also found a mid-sized black hole, more than 460 times the mass of the sun, in the galaxy M82 located 12 million light-years from Earth. Ptak and Griffiths calculated that if the black hole were any lighter — and the gravitational pull any weaker — the outflowing X-ray radiation would disperse the cloud of inflowing gas, and the black hole would fall quiet.
“It’d be basically blowing itself apart,” Ptak says.
How Did They Get There?
The discoveries create a new puzzle. How can the universe create a collapsed-star black hole that’s bigger than any uncollapsed star?
“There’s not really a good foundation for how to explain this intermediate black hole,” Colbert says.
Possibilities include several small black holes merging into an medium-sized one, a small black hole growing by sucking down a surrounding cloud of gas or a super-large star producing a bigger- expected black hole.
“These are that kind of ideas that haven’t been checked before,” Colbert says. “We’re just starting to do that work.”
For astronomers, a puzzle to solve is a happy ending and a happy beginning.
“There’s not really a good foundation for how to explain this
intermediate black hole.”
Gina "Nanogirl" Miller
"The science of nanotechnology, solutions for the future."