FAR from holding sway over all other matter, the most massive black holes in
the Universe are being twirled around at random.
This celestial buffeting resembles Brownian motion鈥攖he jittery movement
of microscopic particles in a fluid, such as pollen grains suspended in water.
It was first described by the botanist Robert Brown in 1827 but only explained
by Einstein in 1905. According to Einstein, pollen grains move about randomly
because they are being bombarded by water molecules.
Now David Merritt of Rutgers University in New Jersey suggests the same
effect is happening on a much grander scale, in the hearts of galaxies. 鈥淵ou
often have a very massive black hole鈥攖ypically millions or billions of
times the mass of the Sun鈥攂eing jostled by the gravity of passing stars,鈥�
he says.
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Because we can鈥檛 observe black holes directly, it鈥檚 almost impossible to see
how they are affected by other objects. But there is one thing we can detect. As
some black holes spin, they eject thin jets of matter at high speed from their
poles, and the direction of these jets reveals the black hole鈥檚 axis of
rotation.
Large black holes form when two smaller black holes orbit each other, and
eventually merge. The plane of their orbit determines the axis around which the
resulting black hole spins. You would expect the plane of the black holes鈥� orbit
to start off parallel to the galactic plane. But as they circle, the gravity of
passing stars might apply a torque to the orbit, says Merritt, altering its
alignment. He predicted that if black holes were being repeatedly jostled by
passing stars as they spiralled inwards, the spin axis of the resulting hole
would end up bearing no relation to the orientation of the parent galaxy.
Merritt studied data on supermassive black holes across the Universe and this
is exactly what he saw. The jets鈥攁nd hence the spin axes of the
holes鈥攁ppeared to be randomly oriented. His research will appear in a
future issue of The Astrophysical Journal.