Caption: An image of the galaxy COSMOS J100043.15+020637.2 taken with the Advanced Camera for Surveys on the Hubble Space Telescope. Image courtesy Dr. Julia Comerford.
Astronomers have discovered 33 pairs of merging black holes in cosmic dances around each other, a finding that was predicted or ‘choreographed’ by Isaac Newton. “These results are significant because we now know that these ‘waltzing’ black holes are much more common than previously known,” said Dr. Julia Comerford of the University of California, Berkeley, at the American Astronomical Society meeting in Washington, DC. “Galaxy mergers are causing the waltzing, can use this finding to determine how often mergers occur. The black holes dancing towards us are shifted towards blue light, and those moving away from us are shifted toward the red. So it is like a cosmic disco ball showing us where the black holes are dancing.”
The dances are occurring in dual black holes, which are different from binary black holes in that the distance between the two object is much larger for dual black holes.
“These black holes have a separation of a kilo parsec,” said Comerford. “You haven’t heard about lots of small binary black holes, because no one has definitively found any yet. But this is the next best thing. We know these duals are going to merge and can use models to find out how often they merge.”
The team was able to observe the black holes that have gas collapsing onto them, and this gas releases energy and powers each black hole as an active galactic nucleus (AGN), which lights up the black hole like a Christmas tree.
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Astronomical observations have shown that nearly every galaxy has a central supermassive black hole (with a mass of a million to a billion times the mass of the Sun), and also that galaxies commonly collide and merge to form new, more massive galaxies. As a consequence of these two observations, a merger between two galaxies should bring two supermassive black holes to the new, more massive galaxy formed from the merger. The two black holes gradually in-spiral toward the center of this galaxy, engaging in a gravitational tug-of-war with the surrounding stars. The result is a black hole dance. Such a dance is expected to occur in our own Milky Way Galaxy in about 3 billion years, when it collides with the Andromeda Galaxy.
The team of astronomers used two new techniques to discover the waltzing black holes. First, they identified waltzing black holes and their velocities by the disco ball of the red-shift or blue-shift.
The second technique for identifying waltzing black holes through a chance discovery of a curious-looking galaxy. While visually inspecting images of galaxies taken with the Advanced Camera for Surveys on the Hubble Space Telescope, the team noticed a galaxy with a tidal tail of stars, gas, and dust, an unmistakable sign that the galaxy had recently merged with another galaxy, and the galaxy also featured two bright nuclei near its center. The team recognized that the two bright nuclei might be the AGNs of two waltzing black holes, a hypothesis seemingly supported by the recent galaxy merger activity evinced by the tidal tail. To test this hypothesis, the very next night the team obtained a spectrum of the galaxy with the DEIMOS spectrograph on the 10-meter (400-inch) Keck II Telescope on Mauna Kea, Hawaii.
The spectrum showed that the two central nuclei in the galaxy were indeed both AGNs, supporting the team’s hypothesis that the galaxy has two supermassive black holes. The black holes may be waltzing within the host galaxy, or the galaxy may have a recoiling black hole kicked out of the galaxy by gravity wave emission; additional observations are necessary to distinguish between these explanations.
Comerford said these new techniques can be used to find many more waltzing pairs in the future.
Source: AAS, Dr. Julia Comerford’s website