X literally may mark the spot for astrophysicists hunting for colliding black holes. A report published online by Science on 2 August suggests that cross-shaped radio galaxies harbor massive black holes that suddenly flipped their spins, probably by absorbing black holes from other galaxies. Researchers combined the new model with a census of these distinctive galaxies to estimate that such titanic encounters happen about once a year in the cosmos.
Most galaxies host supermassive black holes with millions or billions of times the mass of the sun. The most active of these vortexes are thought to spin at awesome rates as they devour gas and stars. The incoming matter spirals into a raging disk that shoots powerful jets of energy into space. Astrophysicists don't yet understand this process, but they assume the jets mark a black hole's spin axis. Previous surveys showed that about 7% of active radio galaxies have X-shaped, or "winged," jets. Astronomers thought these features pointed to a precession of the central black hole, much as Earth's spin axis wobbles over time.
However, recent high-resolution radio images of some winged galaxies show sharp breaks where a pair of jets angles off in a new direction, rather than sweeping out gradual curves. "That's clearly not precession," says astrophysicist David Merritt of Rutgers University in Piscataway, New Jersey. "It has to flip over." The likeliest mechanism is the arrival of a second massive black hole during a galaxy collision, say Merritt and his colleague, radio astronomer Ron Ekers of the Australia Telescope National Facility in Sydney. According to their model, an incoming black hole with at least 20% of the mass of its partner will knock the main black hole off kilter, no matter how rapidly it spins.
"It's really hard to torque a black hole around by a large amount without having something as massive as another black hole slam into it," says astrophysicist Scott Hughes of the University of California, Santa Barbara, co-author of a forthcoming independent analysis that draws similar conclusions. Merritt and Ekers project that a typical large galaxy will undergo a black-hole-tilting crash once every billion years--enough for one such event to pop off somewhere in the universe each year. The result bodes well for planned studies of the ripples in space-time, called gravitational waves, that should cascade from such mergers.
Related sites
David Merritt's
home page, with movies of black hole mergers
LISA project, which may detect
gravitational waves from mergers
Detailed images of NGC 326 system
)
)
)
)
)
)
)
)
)
)
)
