To zoom in, Event Horizon Telescope wants to add more radio dishes to its network—and go to space.
A new image of the black hole in nearby galaxy M87 from 2018 observations (right) finds a familiar shadow—and a rotation in the brightest part of the ring that surrounds it.EHT Collaboration
A familiar shadow looms in a fresh image of the heart of the nearby galaxy M87. It confirms that the galaxy harbors a gravitational sinkhole so powerful that light cannot escape, one generated by a black hole 6.5 billion times the mass of the Sun. But compared with a previous image from the network of radio dishes called the Event Horizon Telescope (EHT), the new one reveals a subtle shift in the bright ring surrounding the shadow, which could provide clues to how gases churn around the black hole.
“We can see that shift now,” says team member Sera Markoff of the University of Amsterdam. “We can start to use that.” The new detail has also whetted astronomers’ desire for a proposed expansion of the EHT, which would deliver even sharper images of distant black holes.
The new picture, published this week in Astronomy & Astrophysics, comes from data collected 1 year after the observing campaign that led to the first-ever picture of a black hole, revealed in 2019 and named as Science’s Breakthrough of the Year. The dark center of the image is the same size as in the original image, confirming that the image depicts physical reality and is not an artifact. “It tells us it wasn’t a fluke,” says Martin Hardcastle, an astrophysicist at the University of Hertfordshire who was not involved in the study. The black hole’s mass would not have grown appreciably in 1 year, so the comparison also supports the idea that a black hole’s size is determined by its mass alone.
In the new image, however, the brightest part of a ring surrounding the black hole has shifted counterclockwise by about 30°. That could be because of random churning in the disk of material that swirls around the black hole’s equator. It could also be associated with fluctuations in one of the jets launched from the black hole’s poles—a sign that the jet isn’t aligned with the black hole’s spin axis, but precesses around it like a wobbling top. That would be “kind of exciting,” Markoff says. “The only way to know is to keep taking pictures.”
Although incredibly massive, black holes like M87 are relatively small: One could fit within the Solar System. To see it, the EHT relies on radio emissions, which penetrate the gas and dust shrouding the black hole. The array generates a sharp enough image by combining data from telescopes spaced as widely apart as possible, creating, in effect, an Earth-size dish. So far, the EHT team—some 300 researchers, also spread across the world—has observed for a couple of weeks each year, using up to a dozen observatories from the South Pole to Greenland and Hawaii to France.
Now, the team wants to add more telescopes to the network, which would further sharpen its images and enable it to see black holes in more distant galaxies. Last month, the team submitted a proposal to the National Science Foundation for a $73 million next-generation EHT (ngEHT). It calls for building 9-meter radio dishes in four locations—Wyoming, the Canary Islands, Chile, and Mexico—and adding the 37-meter dish at the Massachusetts Institute of Technology’s Haystack Observatory. This “fills in the holes beautifully,” says EHT founding director Shep Doeleman of the Center for Astrophysics | Harvard & Smithsonian (CfA).
The ngEHT project would also deploy new hardware and software to speed up data processing. The team could produce results in days instead of years, creating the opportunity for “black hole cinema,” Doeleman says. Sharpening the resolution in both space and time will help researchers unravel one of astrophysics’ most enduring mysteries: how a black hole’s spin, magnetic field, and swirling disk of material conspire to launch the powerful jets of particles far out into space.
Theorists also predict that within the fuzzy bright ring in the EHT images are sharper circles of light emitted by photons trapped in orbits as close as it’s possible to get to the black hole’s boundary, or event horizon. To zoom in on these so-called photon rings will require a virtual radio dish extending well beyond Earth. “We are reaching the limits of what can be done from the ground,” says CfA’s Michael Johnson. He is leading a mission proposal called Black Hole Explorer that he aims to submit to NASA in 2025. To be launched in 10 years into geosynchronous orbit, the spacecraft’s 4-meter dish would expand the baseline of ngEHT to about 35,000 kilometers, enough to see the photon rings. “With just one satellite we could pick out these orbits,” Markoff says.
Theory predicts multiple, nested photon rings, and the size and shape of the innermost one would help pinpoint the black hole’s mass, spin, and even the inclination of its swirling disk. By imaging multiple black holes with different life histories, researchers could find out how their spins are affected by mergers or a sudden feast of surrounding material.
The ngEHT and its orbiting outrigger should also be able to test Albert Einstein’s theory of gravity, general relativity, in the most extreme conditions ever, Doeleman says. “[It will] bring us as close to the edge of a black hole event horizon as we are likely to be for many years to come.”