The black hole that lives at the heart of this galaxy, M87, could be two or three times more massive than previously thought.
Galaxy's giant black hole just got bigger
12 Jun 2009
Astronomers have used new computer modelling techniques to determine that the black hole at the heart of M87, one the largest nearby giant galaxies, is two to three times more massive than previously thought.
Weighing in at 6.4 billion times the Sun's mass, it is the most massive black hole yet measured with a robust technique, and suggests that the accepted black hole masses in other nearby large galaxies may be off by similar amounts.
This has consequences for theories of how galaxies form and grow, and might even solve a long-standing astronomical paradox.
To try to understand how galaxies evolve, astronomers need start with basic information about today's galaxies. What are they made of? How big are they? How much do they weigh? Astronomers measure this last category, galaxy mass, by clocking the speed of stars orbiting within the galaxy.
Studies of the total mass are important, but the crucial point is to determine whether the mass is in the black hole, the visible stars, or the "dark halo", a region surrounding a galaxy that extends beyond its main visible structure and containing the mysterious "dark matter."
< The Lonestar supercomputer can perform 62 trillion mathematical operations per second.
To model M87, Karl Gebhardt (The University of Texas at Austin) and Jens Thomas (Max Planck Institute for Extraterrestrial Physics) used one of the world's most powerful supercomputers, the Lonestar system at The University of Texas at Austin's Texas Advanced Computing Centre.
Gebhardt and Thomas' model of M87 was more complicated than previous models of the galaxy, because it took into account the galaxy's "dark halo".
The result was a mass for M87's black hole several times what previous models have found. "We did not expect it at all," Gebhardt said.
These new results for M87, together with hints from other recent studies and his own telescope observations, lead him to suspect that all black hole masses for the most massive galaxies are underestimated.
That conclusion "is important for how black holes relate to galaxies," Thomas said. "If you change the mass of the black hole, you change how the black hole relates to the galaxy."
The quasar paradox
Higher masses for black holes could also solve a paradox concerning the masses of quasars—active black holes at the centres of extremely distant galaxies, seen when the universe was very young.
Quasars shine brightly as the material spirals in towards the black hole, giving off copious radiation before crossing the event horizon (the region beyond which nothing—not even light—can escape).
"There is a long-standing problem in that quasar black hole masses were very large—ten billion solar masses," Gebhardt said. "But in local galaxies, we never saw black holes that massive, not nearly."
"The suspicion was [therefore] that the quasar masses were wrong," he said. But "if we increase the mass of M87 two or three times, the problem almost goes away."
Adapted from information issued by UT-Austin.
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