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Galactic black holes may be more massive than thought

Predictions and observations could resolve seeming mismatch between close and distant giants

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6:08pm, June 8, 2009
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PASADENA, Calif.—And now for something truly monstrous.

Astronomers report that some of the biggest supermassive black holes in nearby galaxies are at least twice and possibly four times as heavy as previously estimated. The findings come from new simulations by two independent teams of researchers, as well as new observations of stars whipping around a handful of supermassive black holes at the centers of massive galaxies no more than a few hundred million light-years from Earth.

The results, some of which were reported June 8 at a meeting of the American Astronomical Society, may resolve a long-standing puzzle about the mismatch between the masses of giant black holes in distant versus nearby galaxies. The findings may also suggest that supermassive black holes, already known to grow in lockstep with a galaxy’s central bulge of stars, may play an even bigger role in governing the growth and maximum size of galaxies than had been suspected.

In simulations presented at the meeting, Karl Gebhardt of the University of Texas at Austin and Jens Thomas of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, used a supercomputer to recalculate  the mass of the biggest black hole in the nearby universe, which lies at the center of the galaxy M87, 50 million light-years from Earth.  The team’s study is the first to include the presence of dark matter in assessing the mass of a giant black hole. Dark matter, the invisible material believed to make up about 85 percent of the mass in the universe, envelops each galaxy in a vast halo. 

By clocking how rapidly stars orbit the galaxy’s center, researchers measure the total mass of stars plus black hole in the galaxy’s central region. To figure out how much the black hole at the core of the galaxy contributes to that total mass, astronomers have to figure out the amount of mass in stars and subtract it from the total. However, it turns out to be trickier than thought to determine the stellar mass.

At first glance, dark matter wouldn’t seem to be important in calculating stellar mass in the galaxy’s center because the invisible stuff is negligible at the core of a galaxy. But it comes into play because of the indirect method astronomers use to measure the stellar mass, the team said.

Astronomers calculate that mass by recording the amount of visible starlight and using a relationship, called the mass-to-light ratio, to translate the intensity of starlight into stellar mass. In the past, calculations of that ratio assumed that all the mass astronomers measured was in stars. But many stars reside in the outer regions of the galaxy, where they are outweighed by dark matter. Subtracting the dark matter from the total mass lowers the amount of mass attributed to stars, reducing the stellar mass-to-light ratio, said Gebhardt.

He and Thomas found that the mass-to-light ratio for stars is about half the old estimate.

Using the revised ratio, and assuming that stars’ mass-to-light ratio is the same in the inner part of the galaxy as in the outer part, the team found a much lower stellar mass near the core and therefore a much higher mass for the black hole. The team reports that the supermassive black hole in M87 weighs the equivalent of 6.4 billion suns, about twice as much as the currently accepted estimate.

Accounting for dark matter “is an effect that in retrospect is obvious,” said Gebhardt, and “in some galaxies like M87, it can be very important.” Unpublished simulations of three other galaxies show signs of a similar increase, he notes. And high-resolution observations of M87 by Gebhardt and colleagues using the Gemini North telescope atop Hawaii’s Mauna Kea agree with the revised theoretical estimate, he said.

“It’s high time that someone included the effect of dark matter,” said John Kormendy of the University of Texas, not a member of Gebhardt’s team. The revised mass estimates, he said, “will have a welcome audience.” That’s because for more than 25 years it has been a puzzle why the most luminous distant quasars are powered by black holes weighing the equivalent of 10 billion solar masses, yet no nearby black holes appear to be this hefty.

With the new mass estimate, the black hole in M87 is now a much closer match to the mass of those that power quasars in the distant universe, said Avi Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. 

Another team, including Remco van den Bosch of the University of Texas, used a different type of analysis that doesn’t include dark matter and also found that supermassive black holes at the cores of massive galaxies may have double their estimated heft.

Van den Bosch and his collaborators realized that massive galaxies tend to be more out of round, like squashed footballs, with stars on highly elongated orbits, than smaller galaxies. If the true orbits of these stars are ignored, astronomers will calculate slower stellar speeds and dramatically underestimate the mass of the central black hole. In the one massive galaxy, called NGC 3379, for which the team did its analysis, the estimated mass of the black hole doubled, the team reported online  http://dl.getdropbox.com/u/360230/RvdB_triaxbh.pdf.

“The fact that in NGC 3379 the black hole mass goes up does not prove that it will go up in other [out-of-round] galaxies too, but it is very likely,” says van den Bosch.

Kormendy suggests that when both effects — the influence of dark matter and the out-of-round shape of massive galaxies — are combined, the estimated mass of giant black holes in nearby galaxies may be quadrupled.

“Stay tuned,” said Kormendy. “The story isn’t over yet.”

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