EHT Collaboration
Physicists have been confident since the 1980s that there is a supermassive black hole at the center of the Milky Way, similar to the one thought to be at the center of most spiral and elliptical galaxies. It has since been dubbed Sagittarius A* (pronounced A-star), or SgrA* for short. The Event Horizon Telescope (EHT) captured the first image of SgrA* two years ago. Now the collaboration has revealed a new polarized image (above) that displays the black hole's swirling magnetic fields. Technical details appear in two new Leaves Published in The Astrophysical Journal Letters. The new image is strikingly similar to another EHT image of an even larger supermassive black hole, M87*, so this may be something all these black holes have in common.
The only way to “see” a black hole is to image the shadow cast by light as it bends in response to the object's strong gravitational field. As Ars Science editor John Timmer reported in 2019, the EHT is not a telescope in the traditional sense. Instead, it is a collection of telescopes spread around the world. EHT is generated by interferometry, which uses light in the microwave system of the electromagnetic spectrum captured at different locations. These recorded images are combined and processed to create an image with telescope-like resolution at distant locations. Interferometry has been used at facilities such as ALMA (Atacama Large Millimeter/Submillimeter Array) in northern Chile, where telescopes can be spread across 16 kilometers of desert.
In theory, there is no upper limit on the size of the array, but to identify photons that originated simultaneously at the source, you need very precise location and timing information at each location. And you still have to collect enough photons to see anything at all. So atomic clocks were installed at many locations, and accurate GPS measurements over time were created. For the EHT, ALMA's large collection space – as well as the choice of wavelength at which supermassive black holes are very bright – ensured enough photons.
In 2019, EHT announced First live photo taken A black hole at the center of an elliptical galaxy, Messier 87, located in the constellation Virgo about 55 million light-years away. Impossible just a generation ago, this image has been made possible by technological breakthroughs, innovative new algorithms, and (of course) the connection of many of the world's best radio observatories. The image confirmed that the object at the center of M87* is indeed a black hole.
In 2021, the EHT collaboration released a new image of M87* showing what a black hole looks like in polarized light — a signature of magnetic fields at the edge of the object — giving new insight into how black holes swallow matter and emit powerful jets. Of their essence. A few months later, the EHT returned with images of the “dark heart” of a radio galaxy known as… Centaur A-Enabling collaboration To determine the location From the supermassive black hole at the center of the galaxy.
SgrA* is much smaller but also much closer than M87*. This made it more difficult to capture an equally clear image because SgrA* changes on time scales of minutes and hours compared to days and weeks for M87*. Physicist Matt Strassler Compare previously This feat is “taking a one-second exposure of a tree on a windy day. Things get blurry, and it can be difficult to determine what the photo really looks like.”
