Free Astronomy Magazine July-August 2024

JULY-AUGUST 2024 S een here in polarised light, this side-by-side image of the supermassive black holes M87* and Sagittarius A* indicates to scientists that these beasts have similar magnetic field structures. This is significant because it suggests that the physical processes that govern how a black hole feeds and launches a jet may be universal features amongst supermassive black holes. The scale shows the appar- ent size on the sky of these images, in units of micro-arcseconds. A finger held at arm’s length measures 1 degree on the sky; a micro-arcsecond is 3.6 billion times smaller than that. In context, the images of these black holes have an apparent size similar to that of a donut on the surface of the Moon. [EHT Collaboration] Light is an oscillating, or moving, electromagnetic wave that allows us to see objects. Sometimes, light os- cillates in a preferred orientation, and we call it ‘polarised’. Although polarised light surrounds us, to human eyes it is indistinguishable from ‘normal’ light. In the plasma around these black holes, particles whirling around magnetic field lines impart a polarisation pattern per- pendicular to the field. This allows astronomers to see in increasingly vivid detail what’s happening in black hole regions and map their magnetic field lines. “By imaging po- larised light from hot glowing gas near black holes, we are directly inferring the structure and strength of the magnetic fields that thread the flow of gas and matter that the black hole feeds on and ejects,” said Harvard Black Hole Initiative Fellow and project co-lead Angelo Ricarte. “Po- larised light teaches us a lot more about the astrophysics, the properties of the gas, and mechanisms that take place as a black hole feeds.” But imaging black holes in polarised A new image from the Event Horizon Telescope (EHT) collaboration has uncovered strong and organised magnetic fields spiralling from the edge of the super- massive black hole Sagittarius A* (Sgr A*). Seen in po- larised light for the first time, this new view of the monster lurking at the heart of the Milky Way galaxy has revealed a magnetic field structure strikingly simi- lar to that of the black hole at the centre of the M87 galaxy, suggesting that strong magnetic fields may be common to all black holes. This similarity also hints to- wards a hidden jet in Sgr A*. [ESO] light isn’t as easy as putting on a pair of polarised sunglasses, and this is particularly true of Sgr A*, which is changing so fast that it doesn’t sit still for pictures. Imaging the super- massive black hole requires sophisti- cated tools above and beyond those previously used for capturing M87*, a much steadier target. EHT Project Scientist Geoffrey Bower from the Institute of Astronomy and Astro- physics, Academia Sinica, Taipei said, “Because Sgr A* moves around while we try to take its picture, it was difficult to construct even the unpolarised image,” adding that the first image was an average of multiple images owing to Sgr A*’s movement. “We were relieved that polarised imaging was even possi- ble. Some models were far too scrambled and turbulent to con- struct a polarised image, but Nature was not so cruel.” Mariafelicia De Laurentis, EHT Deputy Project Scientist and profes- sor at the University of Naples Fed-