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Credit: S. Dagnello (NRAO/AUI/NSF)
History was made in 2019 when scientists with the Event Horizon Telescope (EHT) project released the first image of a black hole. Astronomers have now taken what they learned from the EHT and conducted more observations of the same supermassive black hole, M87* (pronounced “M87 star”). This time, they’ve zoomed out to observe the full scale of its roiling disk of superheated plasma and the jet of relativistic matter blasting into space.
The new project, the Global Millimeter VLBI Array (GMVA), relies on the same technique as the EHT. Instead of having one large instrument pointed at the distant M87 galaxy, the GMVA depends on a network of radio telescopes around the world that are synchronized to act as a single receiver. We also have an acronym within an acronym here—VLBI, or very-long-baseline interferometry, is the radio astronomy technique at the core of this research. As radio signals, like the ones emanating from a black hole’s super-hot plasma ring, pass by Earth, they are detected at different times by each part of the array. Scientists can use the detection time and distance between the receivers to combine the data into a single, synchronized image.
Initially, the project had about a dozen radio telescopes spaced around the east-west axis of Earth. To widen the GMVA’s “eye,” scientists added the Greenland Telescope up north and the Atacama Large Millimeter/submillimeter Array (ALMA) in the southern hemisphere. Astronomers on the EHT project zoomed in on the black hole in M87, capturing just the innermost parts of the accretion disk. This helped highlight the black hole’s event horizon, but the GMVA was tuned to look at the wider picture. The EHT’s network operated at a frequency of 1.3 millimeters, which is the equivalent of focusing on a grain of rice in California from Massachusetts, says MIT. The GMVA operates at 3mm, which could only resolve a pumpkin seed at the same scale.
The jet and shadow of the black hole at the center of the M87 galaxy together for the first time in this image from the GMVA project.
Credit: R.-S. Lu (SHAO), E. Ros (MPIfR), S. Dagnello (NRAO/AUI/NSF)
The wider image reveals some critical details about this distant monster. For one, GMVA can see a larger, “fluffier” plasma ring surrounding the event horizon. Some of this plasma also appears to travel up and away from the black hole, in what astronomers believe is the base of a relativistic jet. The base of the jet also appears connected to the central ring, marking the first time observations have concretely shown these two features are related. “The exciting thing is, we still see a shadow feature of the black hole, but we also start to see a more extended jet,” says Kazunori Akiyama from the MIT Haystack Observatory.
The team now plans to continue observing M87* in additional frequencies. This should be possible by re-tuning the EHT and GMVA to operate at different wavelengths. Eventually, we could have a multi-layered image of the black hole that helps scientists test many of our conceptions of how these extreme stellar objects work. It’s not going to happen overnight, though. The EHT project conducted observations in 2017 that led to the 2019 data release, and the GMVA captured this data the next year. It’s taken five years of painstaking image processing to turn the radio frequency data into a visual image for you to enjoy today.
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