Free Astronomy Magazine May-June 2016

SPACE CHRONICLES times more massive than they had predicted for a galaxy of this mass. Based on previous Hubble surveys of black holes, astronomers had de- veloped a correlation between a black hole's mass and the mass of its host galaxy's central bulge of stars — the larger the galaxy bulge, the proportionally more massive the black hole. But for galaxy NGC 1600, the giant black hole's mass far overshadows the mass of its rela- tively sparse bulge. "It appears that that relation does not work very well with extremely massive black holes; they are a larger fraction of the host galaxy's mass," Ma said. Ma and her colleagues reported the discovery of the black hole, which is located about 200 million light-years from Earth in the direction of the constellation Eridanus, in the April 6 issue of the journal Nature. Jens Thomas of the Max Planck Institute for Extraterrestrial Physics, Garching, Germany, is the paper's lead author. One idea to explain the black hole's monster size is that it merged with another black hole long ago when galaxy interactions were more fre- quent. When two galaxies merge, their central black holes settle into the core of the new galaxy and orbit each other. Stars falling near the bi- nary black hole, depending on their speed and trajectory, can actually rob momentum from the whirling pair and pick up enough velocity to escape from the galaxy's core. This gravitational interaction causes the black holes to slowly move closer to- gether, eventually merging to form an even larger black hole. The super- massive black hole then continues to grow by gobbling up gas funneled to the core by galaxy collisions. "To become this massive, the black hole would have had a very voracious phase during which it devoured lots of gas," Ma said. The frequent meals consumed by NGC 1600 may also be the reason why the galaxy resides in T he massive elliptical galaxy in the center of this image, taken by the Digitized Sky Survey, resides in an uncluttered region of space. A close-up view of the gal- axy, called NGC 1600, is shown in the inset image, which was taken in near-infrared light by the Hubble Space Telescope's Near Infrared Camera and Multi-Object Spec- trometer (NICMOS). [NASA, ESA, and C.-P. Ma (University of California, Berkeley)] a small town, with few galactic neigh- bors. NGC 1600 is the most domi- nant galaxy in its galactic group, at least three times brighter than its neighbors. "Other groups like this rarely have such a large luminosity gap between the brightest and the second brightest galaxies," Ma said. Most of the galaxy's gas was con- sumed long ago when the black hole blazed as a brilliant quasar from material streaming into it that was heated into a glowing plasma. "Now, the black hole is a sleeping giant," Ma said. "The only way we found it was by measuring the ve- locities of stars near it, which are strongly influenced by the gravity of the black hole. The velocity mea- surements give us an estimate of the black hole's mass." The velocity measurements were made by the Gemini Multi-Object Spectrograph (GMOS) on the Gemini North 8- meter telescope on Mauna Kea in Hawaii. GMOS spectroscopically dis- sected the light from the galaxy's center, revealing stars within 3,000 light-years of the core. Some of these stars are circling around the black hole and avoiding close en- counters. However, stars moving on a straighter path away from the core suggest that they had ven- tured closer to the center and had been slung away, most likely by the twin black holes. Archival Hubble images, taken by the Near Infrared Camera and Multi- Object Spectrometer (NICMOS), sup- port the idea of twin black holes pushing stars away. The NICMOS images revealed that the galaxy's core was unusually faint, indicating a lack of stars close to the galactic center. A star-depleted core distin- guishes massive galaxies from stan- dard elliptical galaxies, which are much brighter in their centers. Ma and her colleagues estimated that the amount of stars tossed out of the central region equals 40 billion suns, comparable to ejecting the en- tire disk of our Milky Way galaxy. n