Black holes are dangerous. They have so much gravity that they pull everything inward that comes too close. And when something falls into a black hole, it can never escape again. Luckily, all black holes in the universe are very far away. We don’t need to worry about them.

But did you know that black holes also blow matter outward? Most black holes spew two jets of hot gas into space, in opposite directions. Of course, this gas does not come out of the black hole itself – nothing can escape a black hole, after all. Instead, the gas is blown into space just before it falls into the black hole.

No one knows exactly how this happens. What kind of force can be strong enough to overcome the fierce gravity of the black hole? Astronomers have always thought that strong magnetic fields might play a role. Now, thanks to ALMA, they have supporting evidence.

ALMA looked at a supermassive black hole, located in the core of a very distant galaxy. The submillimeter radiation from the jets of this black hole turned out to be polarized. This made it possible to measure the magnetic field.

To understand what polarized radiation is, think of you and your friend holding a rope. If you wiggle your end of the rope, you will see how it starts to show waves, like waves on a pond. If you wiggle up and down, the waves are vertical. If you wiggle sideways, the waves are horizontal. Light (or submillimeter radiation) also consists of waves.

Normally, those waves have all possible directions: vertical, horizontal and everything in between. But if the light is polarized, the waves have a preferred direction, just like the rope when you wiggle it only in one particular way.

Scientists know that polarized light is influenced by magnetic fields. The magnetic field produces a rotation of the polarization angle – the preferred direction of the waves changes. By measuring this rotation at different wavelengths, the astronomers were able to deduce the strength of the magnetic field close to the distant black hole.

Never before has it been possible to measure magnetic fields so close to a supermassive black hole. The field turns out to be tens or maybe even hundreds of times stronger than the magnetic field at the surface of the Earth.

The precise origin of the jets from black holes is still something of a mystery. But the new ALMA measurements indeed support the idea that strong magnetic fields may be responsible.

The supermassive black hole that was studied by ALMA is located in the core of a very distant galaxy, known as PKS 1830-211. This galaxy is billions of light-years away, in the direction of the constellation Sagittarius, the Archer. In fact, it is so far away that ALMA would have difficulty in observing it, if its submillimeter brightness had not been increased by the gravity of a foreground galaxy, located somewhere between PKS 1830-211 and the Earth.
The polarization measurements of PKS 1830-211 were carried out by Ivan Martí-Vidal, Sébastian Muller, Wouter Vlemmings, Cathy Horellou and Susanne Aalto. They all work at the Chalmers University of Technology in Göteborg, Sweden, which also runs the Onsala Space Observatory – a Swedish radio observatory south of Göteborg. The new results have been published in the 17 April issue of Science.