Supermassive black holes at the centers of most galaxies, including ours, have an insanely strong gravitational pull. Besides voraciously consuming matter, they also produce jets of ultra-high-speed particles that blast outward. Now, scientists have found that magnetic fields -- which have always been considered to be a much weaker force -- play an unexpected, and impressive, role in those black hole dynamics.
After surveying dozens of black holes, a team led by Mohammad Zamaninasab of the Max Planck Institute for Radio Astronomy found that magnetic field strength matched the force produced by the black holes’ powerful pull. It’s comparable in strength to those produced inside MRI machines in hospitals -- that’s roughly 10,000 times greater than the field of Earth itself.
This is the first time anyone has systematically measured the strength of magnetic fields near black holes, according to study coauthor Alexander Tchekhovskoy of the U.S. Department of Energy’s Lawrence Berkeley National Laboratory in a news release. “This is important because we had no idea, and now we have evidence from not just one, not just two, but from 76 black holes.”
Previous models have suggested that a black hole could sustain a magnetic field that’s as strong as its gravity -- but there hadn’t been observational evidence to support this prediction. With the two forces balancing out, a gas cloud caught on top of the magnetic field would be spared from the gravity’s pull, and it should just levitate in place.
The magnetic field strength was confirmed by evidence from jets of gas shooting away from supermassive black holes; these jets are formed by magnetic fields, and they produce a radio emission. “We realized that the radio emission from a black hole's jets can be used to measure the magnetic field strength in the immediate vicinity of the black hole itself,” Zamaninasab explains in a news release.
By analyzing pre-existing radio-emission data -- previously collected using the Very Long Baseline Array -- the team created radio-emission maps at different wavelengths. The shifts in jet features between different maps allowed them to calculate the field strength near the black hole.
“The magnetic fields are strong enough to dramatically alter how gas falls into black holes and how gas produces outflows that we do observe, much stronger than what has usually been assumed,” Tchekhovskoy explains. “We need to go back and look at our models once again.”
The work was published in Nature this week.
Image: Alexander Tchekhovskoy, LBL