Waltzing Black Holes Take Center Stage at Astronomy Meeting

WASHINGTON D.C.—Astronomers using the W. M. Keck Observatory have discovered 33 pairs of black holes in distant galaxies. The new results verify that these waltzing black holes are more common than previously observed.

Nearly every galaxy has a central, supermassive black hole, typically with a mass of a million to a billion times the mass of the Sun. Galaxies also commonly collide and merge to form new, more massive galaxies. Astronomers therefore expect that many “waltzing” supermassive black holes exist in the Universe.

The new results provide some confirmation of this expectation, said Julia Comerford of the University of California, Berkeley, during the 215th American Astronomical Society meeting in Washington, DC.

Comerford and her colleagues studied 33 dual black holes, which appear to waltz in a dance choreographed by Isaac Newton’s laws of physics. Many galaxies will eventually do such a dance, including the Milky Way and the Andromeda galaxies when they collide in about three billion years.

The light used to observe the waltzing pairs comes from the gas collapsing onto the black holes. The gas releases energy and powers each black hole as an active galactic nucleus (AGN), lighting it up like a Christmas tree, Comerford said.

Thirty-two of the black holes in the new study were identified in the DEEP2 Galaxy Redshift Survey and are located in galaxies at distances 4 to 7 billion light years from Earth. The researchers used the Deep Imaging Multi-Object Spectrograph (DEIMOS) on the 10-meter Keck II Telescope on Mauna Kea, Hawai’i to measure the redshifted light from a black hole as it moved away from the telescope and blueshifted light as it moved toward the telescope.

Each black hole’s velocity was measured to be a few hundred kilometers per second or “800 times the cruising speed of a jet airliner,” Comerford said. She added that the distance between the two black holes is roughly 3,000 light years, or roughly one-eighth the distance from the Sun to the center of the Milky Way Galaxy. The Sun sits roughly 26,000 light years from the Galactic Center.

The researchers identified one galaxy, COSMOS J100043.15+020637.2, however, in an image taken by the Advanced Camera for Surveys on the Hubble Space Telescope. The galaxy is located four billion light years from Earth.

Comerford explained that the galaxy’s tidal tail of stars, gas and dust—an unmistakable sign that the galaxy had recently merged with another galaxy—as well its prominently featured two bright nuclei near its center, led the team to become “smitten” with the galaxy.

To determine whether the two bright nuclei might be the AGNs of two waltzing black holes, the researchers then observed the galaxy with the Keck II telescope and its DEIMOS spectrograph. The spectra confirmed that the two central nuclei in the galaxy were both AGNs and were most likely on a path toward merging, she explained. The measured distance between the two black holes is 8000 light years—roughly one-third the distance between the Sun and the Galactic Center.

Francesca Civano of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. also presented findings on the same COSMOS galaxy during the conference. Civano, however, argued that instead of observing a pair of waltzing a black holes, astronomers are seeing one of the black holes as it is recoiling and being kicked out of the galaxy.

Either scenario—waltzing or recoiling black holes—hint at the merger of supermassive objects, Comerford explained. The researchers, however, need additional observations to distinguish between a pre-merger waltz or a post-merger recoil. Both researchers plan to use the Keck telescopes to collect future data, the astronomers said.

The W. M. Keck Observatory operates two 10-meter optical/infrared telescopes on the summit of Mauna Kea on the island of Hawai’i and is a scientific partnership of the California Institute of Technology, the University of California and NASA. For more information please call 808.881.3827 or visit http://www.keckobservatory.org.