EMBARGOED for 2 pA popular perception has grown in recent years that supermassive black holes, those containing as much material as millions or even billions of stars, reside at the cores of most or possibly even all galaxies.
The perception is fed by a simple fact: Every galaxy that has been studied in detail has had one.
But a peek inside the nearby galaxy M33, along with preliminary studies of other galaxies, shows that most galaxies in the universe might not contain a black hole at all. In two separate efforts, scientists used the same data provided by the Hubble Space Telescope and determined that if M33 has a black hole, it is by no means supermassive, confirming a ground-based study that goes back nearly a decade.
The new research indicates that black hole formation is tied directly to the amount of mass in a galaxy's central bulge, an ancient and dense sphere of stars that populates the central regions of many galaxies, including our own
Milky Way. But M33, which like the Milky Way is a spiral galaxy, has little if any central bulge.
One of the new studies, led by Rutgers University Professor David Merritt, found that if M33 contains a black hole, it is not more than 3,000 times the mass of our Sun. The result appears today on Science Express, a recently launched online sister publication to the journal Science that allows for more rapid publication of science papers that have been reviewed and accepted.
The other group, led by Karl Gebhardt of the University of Texas at Austin, produced a paper that is being reviewed for possible publication in the Astronomical Journal. In it, Gebhardt and his colleagues present evidence that the black hole in M33 is not more than 1,500 times as massive as our Sun and that M33 more than likely has no black hole at all.
Merritt said in a telephone interview that "the data don't support the more stringent result" found by Gebhardt's group. In response, Gebhardt said his team had developed a "resampling technique" that infers a spatial resolution of the Hubble data that is improved by a factor of two, and he said he's confident in the study.
Both research teams detailed their methodology last year in the same issue of the Astrophysical Journal.
Because black holes
can't be seen directly, the teams study how fast stars in a galaxy rotate. That observation is then used to calculate the mass that must exist at the galactic center, exerting a gravitational tug that accelerates the stars.The battle of the bulges
Previous studies had already shown the method works for estimating the mass of black holes in galaxies with large central bulges. But the new findings help confirm a growing expectation that "when you form a bulge in a galaxy, you also form a black hole," Gebhardt said.
He and his colleagues have made preliminary studies of other galaxies with no bulges, "and it looks like they have no black hole as well."
Most of the regular mass in the universe is thought to be in galaxies that have supermassive black holes, simply because these are the
largest galaxies. But there may be many, many galaxies without bulges and without black holes which, because of their relatively low output of light and other energy, are hard to detect.Gebhardt refers to these possibly numerous groupings of stars as "very puny disk-only galaxies." Some researchers think there may be more of them in the universe than any other type of galaxy. And so if they in fact do not harbor black holes, then galaxies with black holes would be in the minority.
The first evidence that M33 might not have a black hole came in 1993, in a paper published in the Astronomical Journal by John Kormendy and Robert McClure. Kormendy is a co-author of the new Gebhardt paper. The 1993 work was based on ground-based observations that were not as detailed as the new Hubble data.
"The new work confirms what has generally been believed to be the case that supermassive black holes in local galaxies reside in bulge-dominated galaxies," said Amy Barger of the University of Hawaii's Institute for Astronomy.
Barger
studies more distant black holes using the Chandra X-ray Observatory. She said the possible absence of a supermassive black hole in M33 is reasonable based on her research into how black holes grow. Looking at ancient, faraway galaxies, Barger and her colleagues have found that black holes which are actively gathering mass, and therefore emitting large amounts of X-rays, lie within galaxies dominated by bulges.Mergers and acquisitions
The results should improve understanding of how black holes develop and evolve.
"This observation makes it clear that the mass of a black hole is not predicted by ... the size of the disk in the galaxy" or by other properties, Merritt said. "It shows very clearly that black hole masses are determined by bulge properties."
Many researchers think that large galaxies with big bulges are the result of
mergers, an ancient and sometimes continual process of feasting upon smaller galaxies. If this is so, then supermassive black holes might be thought of as a badge of honor pinned on a galaxy for winning the
merger wars. A galaxy that absorbs many others early in its life might have a greater chance of supplying the material, via the central bulge, needed to create a central monster with the mass of a million or more stars.This material would include not just stars, but interstellar gas and even other black holes.
"If you grant that mergers between galaxies occur, and if you also grant that the black holes have been there for a long time, then the mergers would bring black holes together," Merritt explained. "In that way, black holes would grow at the same time that galaxies grow."
This implies that M33 has not made many galactic acquisitions, which Merritt says is supported by M33's large disk.
"Disks don't survive mergers very well," he points out. In fact, one leading idea about galaxy evolution holds that elliptical galaxies are the evolutionary end point for spiral galaxies -- their flailing arms no longer outstretched, but their overall bodies grown huge.
For M33, all this will one day be moot, as the Milky Way is expected to eventually add M33, which is just 3 million light-years away, to its list of acquisitions, Merritt said.
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