Waltzing black holes and colliding galaxies

A dance of black holes is expected to occur in our own Milky Way galaxy in about 3 billion years, when astronomers predict it will collide with the Andromeda Galaxy.

Professional astronomers – members of the the American Astronomical Society – begin their semi-annual meeting today in Washington D.C.

Astronomers are announcing today that they have discovered 33 pairs of waltzing black holes in distant galaxies. The team clocked each black hole dance at a velocity of a few hundred kilometers per second (500,000 miles per hour, or 800 times the cruising speed of a jet airliner).

In each case, they measured the distance between the two black hole dancers to be 3000 light-years (1/8 the distance from the sun to the center of the Milky Way galaxy).

The waltzing black holes are located in galaxies at distances 4 to 7 billion light-years away from Earth. These distances correspond to a time when the universe was 7 to 10 billion years old.

Dr. Julia Comerford of the University of California, Berkeley, is presenting this result to the American Astronomical Society meeting, which begins today. She says this result is important because it shows that supermassive black hole pairs are more common than previously known, and because the black hole pairs can be used to estimate how often galaxies merge with each other.

Before this, astronomical observations showed that 1) 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 2) 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 what these astronomers are calling a black hole dance. Such a dance is expected to occur in our own Milky Way galaxy in about 3 billion years, when it is predicted to collide with the Andromeda Galaxy.

Astronomers expect there to be many such waltzing supermassive black holes in the universe, but until recently only a handful had been found. Dr. Comerford and her colleagues announce the discoveries of 33 new pairs of waltzing supermassive black holes, which help alleviate
the discrepancy between the expected and observed numbers of black hole pairs.

Dr. Comerford and her colleagues observed the waltzing black holes that have gas collapsing onto them, and this gas releases energy and powers each black hole as an active galactic nucleus (AGN). This lights up the black hole like a Christmas tree.

The team of astronomers used two new techniques to discover the waltzing black holes. First, they identified waltzing black holes by the velocities of their dances in the host galaxy. The host galaxy is the ballroom floor, and the astronomers measured redshifted light from a black hole dancer if it danced away from the telescope and blueshifted light if it danced towards the telescope.

By searching for the redshifted and blueshifted light that is a signature of black hole dances, Dr. Comerford and her colleagues discovered 32 waltzing supermassive black hole pairs in the DEEP2 Galaxy Redshift Survey, a survey of 50,000 galaxies observed with the Deep Imaging Multi-Object Spectrograph (DEIMOS) on the 10-meter (400-inch) Keck II Telescope on Mauna Kea, Hawaii.

Want to know more? Go here for images, and a link to the press announcement.

Deborah Byrd