'Seat belts' restrain galaxies' black holes during galactic mergers
18:13 30 May 2007
NewScientist.com news service
Maggie McKee, Honolulu
Many galaxies are fitted with "seat belts" that keep their colossal black holes from flying out into space after violent collisions, a new analysis suggests.
The work may explain why astronomers have so far failed to observe any black holes being hurled out of their host galaxies – a phenomenon predicted by gravitational theory.
When two galaxies collide, the supermassive black holes at their cores are thought to merge eventually. According to the general theory of relativity, this clash of black holes should release a powerful burst of gravitational waves, shaking the fabric of space-time. And if the burst is lopsided, it will also give the newly merged black hole a kick, like the recoil from a gun.
Detailed computer simulations performed in the last few months have shown that one in ten of these gravitational-wave bursts should kick the newly merged black hole up to speeds of more than 1000 kilometres per second – fast enough to evict the black holes from their galactic homes forever.
But observers have failed to find any evidence of "empty nest" galaxies missing their central black holes. Nor have they found any exiled black holes, which should be visible if they drag some of their galaxy's gas with them when they leave, reports a team led by Erin Bonning of the Paris Observatory in France. She and colleagues sifted through observations of 2600 potential black hole escapees made by the Sloan Digital Sky Survey, but came up empty-handed.
"It's an interesting non-discovery," she said on Tuesday at a meeting of the American Astronomical Society in Honolulu, Hawaii, US. She says it suggests some mechanism may be preventing the black holes from getting kicked out of their host galaxies.
Now, Tamara Bogdanovic and Christopher Reynolds of the University of Maryland in College Park, US, say the merged galaxies restrain the black holes by minimising the strength of the birth kick.
The kick depends on how the black holes are spinning. If their spin axes lie in the same plane as their orbit about each other, the merger gives them a strong kick. If the spin axes are perpendicular to the orbit, the kick is weak.
Handle on the hole
When the two galaxies begin to merge, the axes are likely to be oriented randomly, but they do not stay that way. "What we believe we've identified is a surprisingly powerful mechanism that can take black holes in [one] configuration and twist them around," says Reynolds.
A giant disc of gas forms around the still-separate black holes, roughly in the same plane as their orbit, and each black hole draws matter from this giant disc into a small "accretion disc" aligned with the hole’s equator. The accretion disc acts like a handle on the hole.
Over a million years or so the gravity of the larger disc pulls each accretion disc into line, and along with it the spinning black hole. Both black hole spins end up perpendicular to the disc, and therefore to the plane of their orbit. When these black holes merge, they are not kicked fast enough to escape their galaxy.
The researchers do not rule out the possibility that some black holes get evicted from their galaxies. Merging galaxies that have little gas, such as old, blob-shaped elliptical galaxies, may have discs with such low mass that they cannot affect the orientation of the black holes.
Future gravitational wave detectors, such as the proposed space mission LISA (Laser Interferometer Space Antenna), should be able to measure the orientation of spinning black holes that are close to merging.
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