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H1821+643: Chandra Shows Giant Black Hole Spins Slower Than Its Peers

  • Astronomers have gauged how fast a supermassive black hole is spinning inside a quasar 3.4 billion light years away.

  • Using Chandra data, they found it is rotating at about half the speed of light.

  • This remarkable speed is still much slower than many less massive black holes, providing clues to how this black hole grew.

  • Scientists think that nearly every galaxy, including the Milky Way, has a giant black hole at its center.

H1821+643 is a quasar powered by a supermassive black hole, located about 3.4 billion light years from Earth. Astronomers used /about/ to determine the spin of the black hole in H1821+643, making it the most massive one to have an accurate measurement of this fundamental property, as described in our press release. Astronomers estimate the actively growing black hole in H1821+643 contains between about three and 30 billion solar masses, making it one of the most massive known. By contrast the supermassive black hole in the center of the Milky Way galaxy weighs about four million suns.

This composite image of H1821+643 contains X-rays from Chandra (blue) that have been combined with radio data from NSF's Karl G. Jansky Very Large Array (red) and an optical image from the PanSTARRS telescope on Hawaii (white and yellow). The researchers used nearly a week's worth of Chandra observing time, taken over two decades ago, to obtain this latest result. The supermassive black hole is located in the bright dot in the center of the radio and X-ray emission.

Because a spinning black hole drags space around with it and allows matter to orbit closer to it than is possible for a non-spinning one, the X-ray data can show how fast the black hole is spinning. The spectrum — that is, the amount of energy as a function wavelength — of H1821+643 indicates that the black hole is rotating at a modest rate compared to other, less massive ones that spin close to the speed of light. This is the most accurate spin measurement for such a massive black hole.

Why is the black hole in H1821+643 spinning only about half as fast as the lower mass cousins? The answer may lie in how these supermassive black holes grow and evolve. This relatively slow spin supports the idea that the most massive black holes like H1821+643 undergo most of their growth by merging with other black holes, or by gas being pulled inwards in random directions when their large disks are disrupted.

Supermassive black holes growing in these ways are likely to often undergo large changes of spin, including being slowed down or wrenched in the opposite direction. The prediction is therefore that the most massive black holes should be observed to have a wider range of spin rates than their less massive relatives.

On the other hand, scientists expect less massive black holes to accumulate most of their mass from a disk of gas spinning around them. Because such disks are expected to be stable, the incoming matter always approaches from a direction that will make the black holes spin faster until they reach the maximum speed possible, which is the speed of light.

A paper describing these results appears in the Monthly Notices of the Royal Astronomical Society and is available at The authors are Julia Sisk-Reynes, Christopher Reynolds, James Matthews, and Robyn Smith, all from the Institute of Astronomy at the University of Cambridge in the UK.

NASA's Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.


Fast Facts for H1821+643:
Credit  X-ray: NASA/CXC/Univ. of Cambridge/J. Sisk-Reynés et al.; Radio: NSF/NRAO/VLA; Optical: PanSTARRS
Release Date  June 30, 2022
Scale  Image is about 6.4 arcmin (5.6 million light-years) across.
Category  Black Holes, Groups & Clusters of Galaxies
Coordinates (J2000)  RA 18h 21m 57.40s | Dec +64° 20´ 37.0"
Constellation  Draco
Observation Date  5 observations Jan 17 to Feb 9, 2001
Observation Time  158 hours 16 minutes (6 days, 14 hours, 16 minutes)
Obs. ID  1599, 2186, 2310, 2311, 2418
Instrument  ACIS
References Sisk-Reynés, J. et al., 2022, MNRAS, 514, 2568; arXiv:2205.12974.
Color Code  X-ray: blue; Radio: red; Optical: red, green, blue
Distance Estimate  About 3.4 billion light-years (z=0.299)
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