MIAMI — Supermassive black holes are shiftier beasts than astronomers suspected. A new analysis reveals that the giant black hole at the core of the highly studied galaxy M87 somehow got displaced about 22 light-years from the galaxy’s center.
“This result signals a necessary shift in the supermassive black hole paradigm,” said Daniel Batcheldor of the Florida Institute of Technology in Melbourne. The prevalence of off-center black holes “could represent a significant change in our understanding of supermassive black holes, galaxies and the ways in which they may interact with each other,” he added.
Sorting through old Hubble Space Telescope observations of M87’s core, Batcheldor and his colleagues found that the giant black hole there doesn’t lie smack-dab at the galaxy’s center, as would be expected for an object weighing the equivalent of about 6 billion suns. The displacement could be a signpost of a merger with another as-yet-unknown supermassive black hole in the galaxy. Or the black hole might have been pushed aside by one of the twin central jets of gas and dust that emanate from the core of M87, Batcheldor reported May 25 at a meeting of the American Astronomical Society.
The finding, which will be described in an upcoming Astrophysical Journal Letters, has also been posted online at arXiv.org.
The result is plausible, says Karl Gebhardt of the University of Texas at Austin. But finding the true center of M87 is difficult because the light associated with the black hole, which isn’t entirely symmetrical and pointlike, could be confused with the bright blobs of material ejected by the galaxy’s jets.
“The data need to be carefully scrutinized,” Gebhardt says.
Theorist Avi Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., notes that there are several ways the jets could have confounded efforts to determine the galaxy’s center. Jets plowing into surrounding gas can generate excess light or even create new stars by shocking the gas.
Astronomers hadn’t realized earlier that the giant black hole was off-center because there was little reason to search for such an offset, Batcheldor said. Sophisticated computer simulations indicating that black holes may experience a kick when they merge have been in place for only a few years.
And most searches for off-center supermassive black holes have focused on objects that are traveling at thousands of kilometers per second. A fast-moving black hole is relatively easy to spot because its high velocity has either carried it to less-crowded environs far from its host galaxy’s center, or has modified its light spectrum so it readily stands out from other sources, or both.
Supermassive black holes that are ejected from a galactic center with lower velocities of a few hundred kilometers per second — like the one in M87 — are harder to detect, even though they may be more common, Batcheldor said.
Astronomers determine the center of a galaxy by measuring the diffuse light emission from stars, which is itself centered on a galaxy’s gravitational bull’s-eye. In contrast, the location of a black hole is marked by a bright, concentrated point of light near or at the galactic center.
The team originally used Hubble to search for off-center black holes in spiral galaxies, but soon switched to examining the elliptical galaxy M87 because it is known to harbor a supermassive black hole and very little obscuring dust. “It presented the perfect test case,” said Batcheldor. “Little did we know it had a well-kept secret.”
The most likely explanation for the offset is that the collision of two older, less-hefty supermassive black holes at some time in the past 10 billion years kicked the newly merged supermassive black hole out of the galaxy’s center, Batcheldor and his team suggested. “Computer simulations [of a merger] indicate that there is a preferred direction for the kick which coincides with the observed offset,” Batcheldor said. In addition, several properties of M87 indicate it coalesced from the merger of two separate galaxies, each of which may have had its own supermassive black hole.
Once it’s kicked, a supermassive black hole can take millions to billions of years to oscillate about the galactic center before coming to rest. The displacement of the hole can therefore reveal the merger history of the galaxy, says team member David Merritt of the Rochester Institute of Technology in New York.
However, the team can’t rule out another scenario, in which one of M87’s oppositely directed central jets pushed the black hole from the center. For that to have happened, one of the jets had to have been much stronger in the past than the other.
Loeb says he favors the jet explanation, but noted that a jet would have to last about 100 million years — 10 times longer than the usual age estimate of the M87 jets — in order to push the supermassive black hole far enough away from M87’s center. Regardless of which explanation is correct, the finding opens up several new avenues of study, including the effect a moving supermassive black hole would have on its galactic surroundings, Batcheldor said.
Previously, Loeb and his collaborators calculated that supermassive black holes pack on much less mass if they don’t lie exactly at a galaxy’s center, because the gas they would ordinarily gobble up instead flows into the galaxy’s center. Off-center supermassive black holes would therefore be much punier than their centrally located relatives.
