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Wayward Gluttons: Galactic Black Holes Can Migrate or Quickly Awaken from Quiescence

A pair of studies of galactic supermassive black holes reveals surprising properties and behavior
M87 Supermassive Black Hole



NASA, ESA, D. Batcheldor and E. Perlman (Florida Institute of Technology), the Hubble Heritage Team (STScI/AURA), and J. Biretta, W. Sparks, and F.D. Macchetto (STScI)

MIAMI—Observations from NASA space telescopes have revealed new quirks about the supermassive black holes at the heart of two galaxies. In the supersize elliptical galaxy M87 some 55 million light-years away, for one, the black hole is not in the galaxy's center of mass, apparently having been pushed askew by some violent process. And in the Andromeda Galaxy, a neighbor to our own Milky Way just 2.5 million light-years away, the black hole appears to have recently—and very suddenly—awoken from a slumber. Two groups presented the black-hole findings Tuesday at the semiannual meeting of the American Astronomical Society being held here this week.

Daniel Batcheldor of the Florida Institute of Technology and his colleagues used high-resolution imagery from the Hubble Space Telescope's Advanced Camera for Surveys to determine the location of M87's galactic center, finding that the galaxy's black hole is off-center by about 22 light-years. (The black hole, though itself invisible, is relatively easy to detect, as the material falling into it gives off copious amounts of radiation.) "The black hole is not where it's expected to be," said Batcheldor, whose group says its finding has been accepted for publication in the Astrophysical Journal Letters

A number of processes could explain the displacement, including a push from the bright jet that is seen to emanate from the black hole region. But the jet does not appear to be powerful enough, and may not be one-sided in any event—that is, there may be a counterjet pointed in the opposite direction. A more likely mechanism, Batcheldor said, is a past merger between two black holes that generated a gravitational recoil, knocking the resulting black hole, which packs the mass of about six billion suns, out of whack.

Batcheldor added that a similar test could be carried out on other galaxies to see how common the phenomenon is. "To be honest, it's a very straightforward technique," he said. The problem is that the high-resolution channel of the Hubble instrument that his group used to look at M87 is no longer functional, so archival data will have to be mined to check whether other black holes are similarly offset.

The nearby Andromeda Galaxy, a spiral like our own Milky Way, is home to its own black hole intrigue. And like the black hole at the center of the Milky Way, the Andromeda black hole has been quiescent, said Zhiyuan Li of the Harvard–Smithsonian Center for Astrophysics. In fact, Li said, the x-ray luminosity of the Andromeda black hole, which is as massive as 140 million suns, is about one ten-billionth of its theoretical maximum. That indicates that it is accreting rather slowly, as rapid growth is usually accompanied by bountiful x-rays from the soon-to-be-devoured material swirling around the black hole.

At least, that was the case when Li and his colleagues started observing Andromeda's black hole with the Chandra X-Ray Observatory in 1999. In January 2006, though, things changed—the black hole suddenly perked up in x-ray luminosity, flaring up to roughly 100 times its typical levels. And that flare appears not to have been a fluke in the data—since that time, the activity of the black hole has hovered around an average x-ray luminosity roughly seven times its pre-2006 value. "The black hole appears to enter a relatively active phase," Li said, adding that he and his colleagues have a tentative explanation for the phenomenon. Episodic ejections of plasma blobs at nearly the speed of light, somewhat analogous to solar outbursts called coronal mass ejections, could be the culprit. 

The Milky Way's own supermassive black hole, a relative lightweight compared with those of M87 and Andromeda at a mere four million solar masses, occasionally flares up as well. But Li said that its luminosity generally returns to the quiet preflare level, making the lasting change in M87 rather unique. "This is really, to our knowledge, the first example of a supermassive black hole showing such temporal behavior," he said.

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