One of black holes' best-kept secrets discovered

Polarisation of light in the black hole M87* explains how these objects eject matter at full speed

2 min
The new image of the black hole

BarcelonaIt's called M87* and it became famous on April 10, 2019, when a photo of it went around the world. It is a black hole that is 40 billion kilometres in diameter and has a mass equivalent to 6.5 billion suns and is at the center of a galaxy located 55 million light years from Earth. After presenting it in society two years ago with the first image captured of one of these cosmic monsters, the scientific team of the Event Horizon Telescope (EHT) has just published a new image that contains more information and that has allowed exploration of its inside.

"In addition to showing light, this image shows how it is polarised, which allows us to see the black hole in a new way", explains Ivan Martí-Vidal, researcher at the Universitat de València and coordinator of the polarimetry group at the EHT. The polarisation of light allows astrophysicists to obtain information about the magnetic field in the vicinity of the black hole. "In this region," says Martí-Vidal, "the magnetic field is related to how these objects feed on matter and how they produce the so-called relativistic jets, two processes that are still largely unknown".

Most of the material that spins around the black hole next to the point of no return (the event horizon) ends up swallowed by the gap. But some of it, instead of falling into the abyss, is ejected at full speed in a jet that is even longer than the galaxy containing the hole. The dynamics of these jets has always been a mystery. Now the new image suggests that the magnetic field next to the hole is so strong that it cannot only prevent some of this matter from falling out, but can propel it outwards at high speed. "The observations are compatible with the fact that, somehow, the magnetic field helps to extract energy from the black hole and injects it into the surrounding matter to generate the jets," explains Martí-Vidal. "This result also ties in with the fact that the most distant black holes known rotate faster than nearby ones," he adds. Since in astronomy looking far away is looking into the past, this means that these monsters rotate more slowly as time goes by. The rotational energy they lose is what would be used to propel the jets.

The results, obtained thanks to the collaboration of more than 300 researchers, have been published in two articles in the journal The Astrophysical Journal Letters. This image "has required a brutal instrumental calibration work," says Josep Miquel Girart, an astrophysicist at the Institute for Space Studies of Catalonia (IEEC). "And it shows for the first time that at these scales there is a magnetic field with the necessary properties to create these jets," he concludes.