A Black Hole 'Dance Party'

Team finds 33 merging galaxies with 'waltzing' black-hole pairs

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Hubble Space Telescope images of a galaxy forming from the collision of two smaller galaxies showed two bright nuclei near the galactic center. The nuclei could be two supermassive black holes engaged in a cosmic waltz, or they could be the result of a black hole fleeing the galaxy.

By Lisa Grossman, Science News

WASHINGTON—The universe is one big dance party for black holes. New observations from the W.M. Keck Observatory in Hawaii and the Hubble Space Telescope found 33 merged galaxies in which pairs of supermassive black holes are “waltzing” around the galactic centers.

“Our result shows that such waltzing black holes are much more common than we previously knew,” said Julie Comerford of the University of California, Berkeley. Comerford presented her results on January 4 at the winter meeting of the American Astronomical Society.

Finding pairs of black holes moving in a certain way can help estimate how often galaxy mergers occur in the universe. Observations have shown that nearly every galaxy has a supermassive black hole—a black hole with a mass of one million to one billion times that of the sun—at its center and that galaxies often collide and merge to create larger galaxies. Astronomers have expected to find many mid-merge galaxies by focusing on the two supermassive black holes, which should be orbiting each other in the middle. But so far, the dance floor has pretty much been empty.

“We expect the universe to be littered with these waltzing black holes,” Comerford said. “But until recently, only a few had ever been found.” Those missing black hole pairs posed problems for theories of how galaxies merge and grow.

Now, using two new observational techniques, Comerford and her colleagues have found 33 galaxies with dual supermassive black holes. The first technique found 32 black hole pairs in the DEEP2 Galaxy Redshift Survey conducted on the Keck II Telescope on Hawaii’s Mauna Kea, by determining whether each black hole is moving toward or away from Earth.

Black holes are visible only when they can accrete gas and other material from the surrounding environment. Energy from the black hole heats the gas, lighting the gas up in visible wavelengths. When the black hole is moving away from Earth, the light from the accreted gas appears to be at a longer wavelength, or redshifted. When the black hole dances toward Earth, its light is blueshifted—meaning it has a shorter wavelength. The team identified waltzing pairs by looking for instances when one black hole was blueshifted and the other redshifted.

“It’s kind of the disco ball that tells you where the party is, where the black holes are dancing,” Comerford said.

The waltz is quick—both black holes are moving at velocities of about 200 kilometers per second. But the black holes are keeping “a chaste distance,” Comerford said. They are separated on average by about 3,000 to 8,000 light-years, or one-eighth to one-third the distance from the sun to the center of the Milky Way.

For the population of galaxies Comerford and her colleagues observed, which were mostly gas-poor galaxies 4 billion to 7 billion light years from Earth, galaxy mergers occur three times every billion years, Comerford said.

The final black hole duo was found serendipitously in a Hubble image of a galaxy called COSMOS J100043.15+020637.2. The galaxy sports a tidal tail of stars, gas and dust, a sure sign of a recent galaxy merger.

“It’s like a black eye, a sign that this galaxy has recently gone through a collision with another galaxy,” Comerford said.

The galaxy also has two bright nuclei, each of which could be a supermassive black hole surrounded by glowing dust and gas. Follow-up observations with the Keck II Telescope showed the telltale velocity shifts of dancing black holes.

But the black holes might not be two waltzers. Instead, the data could point to one black hole that is fleeing the galaxy. When two black holes merge together, they produce gravitational waves that carry momentum away from the resulting larger black hole. Gravitational waves pointed mostly in one direction can “kick” the black hole in the opposite direction. Black holes could wander through their host galaxies, or, if the kick is large enough, leave the galaxy behind.

Observations of the same galaxy by Francesca Civano of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and colleagues suggest that the two bright sources are flying apart at a velocity of about 1,300 kilometers per second. Civano also presented her results at the American Astronomical Society meeting on January 4.

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“For a merger, that is a bit high,” Civano said. “But this number is completely normal for a gravitational wave kick.”

“Whether this thing is a dual pair of waltzing black holes or an ejected black hole, this is definitely a merger,” Comerford said. “It’s just whether you’re seeing it before the black holes merge [with each other] or after.”

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