Radio waves passing through magnetized gas have their polarization slightly rotated, like a compass needle deflecting near a magnet. This rotation, called the Faraday effect, revealed the invisible environment around the supermassive black hole in galaxy NGC 1275.
Astronomers gathered data over nine years from three different radio telescope arrays. At all frequencies and in all years, the polarization rotation changed in the same way: the farther from the black hole, the weaker it was. Such stability means we are looking at a permanent veil of gas enveloping the galaxy's center.
They even managed to measure the magnetic field strength. At the edge of the gas cloud, about eight light-years from the black hole, the field is a few millionths of a gauss — similar to Earth's surface field. Closer to the black hole, at one light-year out, it grows to thousandths of a gauss, and right at the horizon it could reach a few gauss.
This is the first direct measurement of the magnetic field at such distances in an elliptical galaxy. Previously, the field strength was only inferred indirectly; now we have something like a map of the magnetic environment around the black hole.
The magnetic field at the black hole's horizon in NGC 1275 is a million times stronger than at Earth's surface, even though at the edge of the gas cloud it is as weak as the one beneath our feet.