From the August 2009 issue

Do black holes and their accretion disks always spin in the same direction? Does a supermassive black hole always spin in the same direction as its host galaxy?

Shanita Andrews, Baltimore
By | Published: August 21, 2009
As matter falls toward a black hole, it forms a spinning, whirlpool-like accretion disk. Gravity forces the infalling gas to rotate around the same axis as the black hole itself. The disk can rotate either clockwise or counterclockwise relative to the black hole.

The same is true for the direction black holes rotate: They can spin either clockwise or counterclockwise. Astronomers have seen no evidence that black holes have “handedness,” meaning they tend to spin one way or the other.

Most large galaxies appear to host central supermassive black holes. We can tell which direction such black holes spin by observing the jets of matter and energy they expel in opposite directions.

Theoretical studies indicate the jets align parallel to the spin axis of the black hole that generates them and also rotate in the same direction. But when we observe galaxies, it’s clear that they and their supermassive black holes don’t necessarily rotate in the same directions. — Eliot Quataert, Theoretical Astrophysics Center, University of California at Berkeley