The Milky Way鈥檚 supermassive black hole is eerily quiet, rarely devouring anything, and now we may know why. Magnetic fields in the centre of our galaxy may be steering gas and dust away into orbit around the black hole rather than allowing it to drop into its gaping maw.
Darren Dowell at NASA鈥檚 Jet Propulsion Laboratory and his colleagues used a camera aboard NASA鈥檚 Stratospheric Observatory for Infrared Astronomy (SOFIA), which is a telescope on a high-flying airplane, to look at the infrared light from the centre of the galaxy.
This sort of light is completely blocked out by Earth鈥檚 atmosphere, so flying above the atmosphere or using a space telescope are the only ways to observe it. Dowell presented this work at the June 2019 meeting of the American Astronomical Society in St. Louis, Missouri.
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The infrared light they observed was emitted by grains of magnetic dust floating in the central 16 light years of the galaxy. Those grains align with their long axes perpendicular to the direction of the magnetic field, which leaves an observable signature in the light they emit as they spin.
Those signatures showed that the grains are arranged in a sort of spiral pattern similar to the spiral of the Milky Way itself. 聽鈥淔or many years we have studied the high resolution structure of the gas in this region 鈥� the mini spiral of the galactic centre 鈥� but we haven鈥檛 had the ability to probe the magnetic field on such fine scales,鈥� says Cornelia Lang at the University of Iowa.
Feeding a black hole
The strongest force in the galactic centre is the gravity from Sagittarius A*, the Milky Way鈥檚 central supermassive black hole. Even though the magnetic field is weaker than the gravity, the team found that it is strong enough to overcome the turbulence in the dust and gas and shepherd it into the spiral shape that they observed. That spiral eventually brings the debris into orbit around the black hole, Dowell says.
But it seems like the magnetic field isn鈥檛 funnelling the dust and gas directly into Sagittarius A*, which very rarely eats up anything substantial. We know that because we almost never see the bright outbursts that occur when matter falls into a black hole.
Instead, it seems to be channelling the matter away from the black hole鈥檚 event horizon 鈥� the line around a black hole from beyond which nothing can emerge 鈥� and maybe pouring it into orbit instead.
Magnetic fields are constantly shifting, so if the shape of the field at the centre of the galaxy changes, Sagittarius A* could start feeding again, says SOFIA team member Joan Schmelz.
That would shake up the Milky Way with huge amounts of radiation and powerful winds that could blow material right out of the galaxy, a phenomenon that we have seen in other, more active galaxies. Thankfully, Earth is far enough away from galactic centre that it wouldn鈥檛 affect us, says Farhad Zadeh at Northwestern University in Illinois.
鈥淭his could be a way to differentiate and explain why the black hole at the centre of the Milky Way is quite quiet, where the black holes at the centres of some other galaxies are quite loud and active,鈥� says Schmelz.
We can鈥檛 confirm for sure that the magnetic fields make all the difference, because we can only take these sorts of detailed measurements for our own galaxy. 鈥淎s we get better at making these kinds of measurements, we could learn about the magnetic environments around other galaxies,鈥� says Zadeh.
