Researchers Confirm New Kind of Black Hole
Astronomers peering into an explosive nearby galaxy with the eagle-eyed Chandra X-ray Observatory announced Tuesday that they have confirmed the discovery of an entirely new class of black hole--a “mid-sized” version.
The finding could explain the formation of certain types of black holes, some of the strangest and most compelling objects that reside at the cores of most galaxies. That explanation, in turn, could eventually explain how all galaxies, including our own, came to be.
Black holes are regions of space stuffed with so much mass that nothing, not even light, can escape their gravitational force. For years, scientists have classified black holes in just two categories: “super-massive” heavyweights that pack in the mass of as many as a billion suns; and featherweights just a few times more massive than our own diminutive sun.
“Little black holes are the compact economy cars of the black hole world, jumbo black holes are the luxury SUVs and there was nothing in between,” said Douglas Richstone, an astronomer at the University of Michigan who commented on the new finding. A “mid-sized black hole,” he said, “fills the gap.”
The newly confirmed black hole was detected in a galaxy called M82, a fiery cosmic neighbor just 11 million light-years away. M82 is a starburst galaxy--so full of new, exploding and dying stars that astronomers liken it to a fireworks factory.
Last year, Andrew Ptak of Carnegie Mellon University and colleague Richard Griffiths discovered an extremely bright X-ray glow from M82 that they suspected was associated with a mid-sized black hole.
At the same time, Ed Colbert and Richard Mushotzky of NASA’s Goddard Space Flight Center reported finding peculiar X-ray glows suggesting medium-sized black holes in three other galaxies.
While the findings stirred much interest, they were not completely accepted. Because of the poorer resolution of earlier telescopes, ordinary light sources, such as binary star systems with hot jets of gas, couldn’t be ruled out.
The new observation, taken with the more powerful Chandra telescope, confirms and extends the case for mid-sized holes. “It’s satisfying,” said Ptak. “I went out on a limb with an ambitious explanation and it turned out to be right.”
The new evidence is threefold. First, Chandra confirms the existence of an “astonishingly bright” object in M82, said Andrea Preswich, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., who was part of the team that made the discovery.
Astronomers cannot see black holes directly, because they absorb all light. But X-ray telescopes can measure energy given off by matter falling into the black hole. Bigger black holes attract more matter and are therefore considered brighter.
The brightness of the new object indicates a black hole with at least as much mass as 500 of our suns, Preswich said. It could be as massive as 80,000 suns, but is squeezed into a region the size of our moon, she said. Small black holes are just a few miles across.
It is also different from previously known black holes because it is in a strange place, offset from the center of the galaxy by 600 light years. Super-massive black holes are found only at the centers of galaxies.
“It’s not the first evidence [mid-sized black holes] exist, but it is the best evidence,” said Roger Blandford, a theoretical astrophysicist at Caltech who studies active black holes and other astronomical objects.
Astronomers are certain they have found a black hole and not a bright supernova because the object is flickering. Black holes flicker at steady rates, like lighthouse beams, that can be used to estimate their size.
The flickering is caused by gas swirling around the black hole.
Blandford said he was most interested in the flickering because it might provide the strongest clues to what was happening around the edges of the black hole.
Astronomers know roughly how small black holes form. Called “stellar” holes, they are the end points of stars’ life cycle.
Super-massive black holes are more of a mystery, said Martin Ward, an X-ray astronomer from the University of Leicester in England who led the research team. They may form all at once during a galaxy’s birth, or they may start as small black holes and grow into monsters.
The finding of a mid-sized hole deepens these mysteries. Astronomers said they didn’t think they could come from the collapse of a single star, because that would require a “hyperstar” far bigger than any yet known, astronomers said.
“There’s no way this object can form by one star collapsing down,” said Philip Kaaret, a Harvard-Smithsonian astrophysicist.”There has to be a [different] process, like merging.”
Ptak noted that a black hole that starts as a large collapsed star can grow by swallowing dust and gas that surrounds it. The starburst M82 is loaded with dust and gas that could help a black hole pack on mass. Medium-sized black holes might form only in such volatile galaxies, he said.
Richstone suspects the holes form as a “runaway merger in a dense stellar neighborhood.” And Preswich theorizes that the black hole is probably continuing to grow by swallowing gas. The merging of two black holes could also occur, she said.
The mid-sized objects “may be first-generation black holes” on their way to becoming the super-massive black holes at the cores of galaxies, said Kaaret. There is no known black hole in the center of M82, adding to the tumultuous galaxy’s mystery.
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A New Kind of Black Hole
An X-ray image of the central region of the galaxy M82 shows a medium-sized black hole (center bright object) that bridges the gap between small and large black holes. The black hole, with a mass of at least 500 suns, is the first confirmed case of such a large black hole outside the center of a galaxy (white cross). The hole is 600 million light-years from the center.
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M82, which is 11 million light-years from Earth, is the nearest starburst galaxy. The galaxy is a cauldron of activity, filled with exploding stars, black holes and remnants of supernovae, seen here as a multitude of bright spots. M82 is much more active than our comparatively quiet Milky Way.
