Black Hole Spinning at Half Light Speed
The most distant black hole whose speed has been measured is rotating at half the speed of light, University of Michigan astronomers have revealed in Nature. It is not yet clear whether this extraordinary speed is an outlier, or indicative of normal black hole behavior six billion years ago.
A year ago the spin of a supermassive black hole was determined for the first time, when the outer layers of the object at the center of Great Barred Spiral Galaxy or NGC 1365 was found to be turning at an astonishing 84% of the speed of light. NGC 1365 is one of the largest known galaxies, more than twice the size of the Milky Way and 56 million light years away.
As black holes spin they distort the space around them, imparting angular motion to space so it swirls around them like water into a plughole. Astronomers have theorized for a while that the rate of spin of a black hole can be determined by measuring the size of the Innermost Stable Circular Orbit, which the rest of us might call the point of no return. The faster the hole spins the smaller the area (for a given mass) around it in which matter has no chance of escape.
Since black holes only have two measurements, mass and spin, that define them knowing these two numbers is rather important. Black holes start small and grow as more and more matter is drawn inside. If the mass is pulled from all directions little spin is imparted. However, if the mass always comes in at a particular angle the hole’s spin accelearates, like a bicycle wheel getting sped up by a mechanic checking its alignment. “The black hole’s spin is a memory, a record, of the past history of the galaxy as a whole,” explained Guido Risaliti of the Harvard-Smithsonian Center for Astrophysics when he announced the measurement.
To calculate the spin detailed measurements were taken of X-Rays produced by matter getting close to the Innermost Stable Circular Orbit. In the year since a spins for a number of galaxies have been announced, but all have been in our patch of the universe.
Now however, the black hole at the center of quasar RX J1131-1231, which lies 6 billion light years from Earth, has been measured at half light speed. Normally such detailed measurements would be impossible at such a distance with current telescopes. However, in the case of RX J1131-1231 we have some help. A galaxy roughly half way between us and the quasar is beautifully positioned to bend the light from RX J1131-1231 so that a large proportion is focused on us while other locations miss out, a process known as gravitational lensing.
“This is the first time that we’ve been able to push out to this type of distance by using the gravitational lensing effect. We hope … to carry out similar studies on other (more distant) galaxies,” said author Mark Reynolds.
While RX J1131-1231’s hole’s spin is still a lot slower than that of NGC 1365 is also has had a lot less time to speed up, since we are seeing it as it was when the universe was not much more than half its current age. The speed is still high enough to show that the accretion is coherent, with most of the mass coming in at the same angle.
Galaxies form through the merger of smaller galaxies, but there are competing theories on how this occurs, with differing outcomes on how relatively young black holes should behave. “If we go back further, maybe they’ll all be maximally spinning because of more mergers and more things happening. Or maybe they’ll be less spinning. We can theoretically produce both scenarios at the moment,” Reynolds said. Only by collecting a larger sample of black holes at great distances will astronomers be able to establish which of the theories is correct. RX J1131-1231 proves this is possible, at least when we have a handy galaxy in between.
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