Free Astronomy Magazine January-February 2015

SPACE CHRONICLES T he dwarf galaxy Markarian 177 (center) and its un- usual source SDSS1133 (blue) lie 90 million light-years away. The galaxies are located in the bowl of the Big Dipper, a well-known star pattern in the constellation Ursa Major. [Sloan Digital Sky Survey] simulating recoils, or "kicks," in merg- ing black holes. "Astronomers search- ing for recoiling black holes have been unable to confirm a detection, so finding even one of these sources would be a major discovery." The collision and merger of two gal- axies disrupts their shapes and re- sults in new episodes of star forma- tion. If each galaxy possesses a cen- tral supermassive black hole, they will form a bound binary pair at the center of the merged galaxy before ultimately coalescing themselves. “This study was extremely interest- ing for our research group because we typically study star formation and AGN activity that happens dur- ing the galaxy merger, but now we are looking for recoiling black holes after the merger,” said David San- ders, an IfA astronomer involved in the study. Merging black holes re- lease a large amount of energy in the form of gravitational radiation, as explained by Einstein's theory of gravity. Waves in the fabric of space- time ripple outward in all directions from accelerating masses. If both black holes have equal masses and spins, their merger emits gravitational waves uniform- ly in all directions. More likely, masses and spins will be different, leading to lop- sided gravitational wave emission that launches the black hole in the opposite direction. The kick may be strong enough to hurl the black hole entirely out of its home galaxy, fating it to forever drift through inter- galactic space. More typi- cally, a kick will send the object into an elongated orbit. Despite its relocation, the ejected black hole will re- tain any hot gas trapped around it and continue to shine until all of the gas is consumed as it moves along its new path. While the object's unusual source of light in a galaxy some 90 S DSS1133 (bright spot, lower left) has been a persistent source for more than 60 years. This sequence of archival astronomical imagery, taken through different instruments and filters, shows that the source is detectable in 1950 and brightest in 2001. [NASA’s Goddard Space Flight Center/M. Koss (ETH Zurich)] million light-years away, its properties make it a good match for a supermassive black hole ejected from its home galaxy, astronomers can't yet rule out an alternative possibili- ty. The source, called SDSS1133, may be the remnant of a mas- sive star that underwent a re- cord period of eruptions be- fore destroying itself in a su- pernova explosion. "With the data we have in hand, we can't yet distinguish between these two scenarios," Koss said, now an astronomer at ETH Zu-