First glimpse of a black hole's spin
Astronomers have measured the rate of spin of a supermassive black hole for the first time - and it is big.
Measurements undertaken with two space-based X-ray telescopes imaged the black hole at the centre of galaxy NGC 1365.
The spin measurement, published in Nature, gives precious clues as to how the black hole grew and achieved supermassive status.
That growth influences the evolution of galaxies, so this simple number stands to teach scientists a great deal.
Black holes are notoriously difficult to study, since so much in astronomy depends on the detection of light - and within a certain distance from a black hole, even that cannot escape.
Black holes are known to draw in material - gas and even stars - and to stretch the very fabric of space-time at their edges. As matter goes in and gathers into what is called an accretion disk, it heats up and emits X-rays.
Previous attempts to quantify black holes' spins have attempted to analyse these X-rays - accounting for the violent processes within that can stretch and distort the X-rays' energies.
Those studies have until now focused on a fairly low-energy X-ray range. But those lower-energy X-rays can be further distorted by layers of gas between the black hole and the Earth, and previous spin observations have been contentious.
In a spin
Now Guido Risaliti of the Harvard-Smithsonian Center for Astrophysics and colleagues have looked at markedly higher energies - less subject to absorption in those gas layers - using Europe's XMM-Newton telescope and the recently launched Nustar telescope.
Nustar is unprecedented in its ability to focus in on distant parts of the cosmos in these high-energy X-rays.
The results suggest a black hole more than 3 million km across, whose outermost edge is moving at a speed near that of light.
But as Dr Risaliti explained, "the black hole's spin is a memory, a record, of the past history of the galaxy as a whole".
Had the black hole grown in a series of small "feeds" of gas or stars from random directions, its spin would be low. The results instead suggest that the black hole grew in one or a series of large absorptions of matter, taking on the momentum in one or a few events.
And as Christopher Reynolds of the University of Maryland explains in an accompanying article in Nature, understanding the evolution of such supermassive black holes at galaxies' centres is crucial to our understanding of how galaxies themselves grow.
"The energy released by a growing supermassive black hole can be so powerful that it disrupts the normal growth of the host galaxy," Prof Reynolds wrote.
"In extreme cases, (it) can terminate all subsequent growth of the galaxy."
However, the measurement is that of just one galaxy, and Prof Reynolds notes that even more advanced X-ray observatories will be needed to unravel the riddle with so few clues.