The black hole at the centre of the M87 galaxy is like a giant fire-breathing dragon that spews enormous jets of energetic particles at near light speeds across some 5,000 light years of space.

A new view of this black hole in polarized light, released today by the Event Horizon Telescope (EHT) collaboration, will help astrophysicists understand just how those jets are launched by this monstrous black hole.

A team led by Avery Broderick, a member of the EHT collaboration who is an astrophysicist at the University of Waterloo and Perimeter Institute for Theoretical Physics in Waterloo, contributed to making this new view in polarized light possible.

“It is a breakthrough in radio astronomy to see the polarization structure on horizon scales around the black hole,” Broderick said.

The black hole known as M87* (for Messier 87 star) became famous in 2019 as the subject of the historic first-ever picture of a black hole ever taken.  

This is yet another first. It is the first time that astronomers have been able to measure polarization this close to the edge of a black hole. It enables scientists to trace the structure of the magnetic fields that drive the powerful jets extending far beyond the galaxy.

In the same way polarised sunglasses help us see better by reducing reflections and glare from bright surfaces, astronomers can sharpen their vision of the region around the black hole by looking at how the light originating from there is polarized.

“The observations suggest that the magnetic fields at the black hole’s edge are strong enough to push back on the hot gas and help it resist gravity’s pull. Only the gas that slips through the field can spiral inwards to the event horizon,” said Jason Dexter, Assistant Professor at the University of Colorado Boulder, US, and one of the co-ordinators of the EHT Theory Working Group, in a press release.

But getting accurate polarization data has been a long-standing challenge in radio astronomy. Broderick and his team played a part in finding solutions to those problems.

“Misaligned antennas, imperfect polarization signal separation, and wavelength-dependent optical elements all result in mixing polarized and unpolarized emission — this mixing has the unfortunate effect of creating false polarization signals and corrupting real ones,” Broderick said.

Broderick’s team made advances in several key ways.

One was the development of Themis, the flexible and powerful analysis software that helps extract information from the observational data much more efficiently, and thus improves the accuracy in modelling the physical processes around black holes.

“Themis is a suite of analysis tools. It's really an analysis framework for EHT data and it was a key component in the first M87 science results. But now, it's also expanded polarized image production,” Broderick says.

It helps the telescopes work “smarter, not harder,” Broderick says. It is also modular and can be adapted to fit new data sets over time, Broderick adds. “It's a LEGO set for building analysis of EHT data.”

Broderick also worked with collaborators to develop “closure traces,” a clever mathematical way of combining multiple telescope measurements in order to circumvent polarization errors.

They drew inspiration from the work of radio astronomers in the late 1950s, who developed other types of “closure quantities” to correct for errors in the phase and amplitude measurements of the light coming into telescopes.  “Closure traces are the first such quantities discovered in 60 years, and provide a direct window onto the polarized structure of M87,” Broderick says.

Broderick was also central to bringing the EHT’s new view into agreement with that from ALMA (the Atacama Large Millimeter/submillimeter Array) in northern Chile, on scales a million times larger.  This was done by simultaneously modeling the small-scale structures seen by the EHT and a large-scale component seen only by ALMA, placing limits on a key systematic uncertainty—Faraday rotation—that bedevils the interpretation of astronomical polarization generally.

The new polarized light images of M87* is a tremendous achievement, Broderick said. No longer are scientists merely looking at “blobs of light” moving in a halo around a black hole. “Now we’re studying actual structures.”

The jets launched by supermassive black holes have an enormous impact on the formation and structure of galaxies and clusters of galaxies, he adds. “If you wanted to understand why galaxies and galaxy clusters look the way they do, you absolutely have to understand how black holes do this magic trick.”

Seeing how black holes interact with their surroundings can also help inform physicists about gravity in extreme environments. Over time, it will also help them “build out the dictionary” to relate the astronomical measurements taken now and in the past, to what black holes are doing.

In a sense, M87 and how it launches its jets can be like a “Rosetta Stone” in understanding the cosmos, Broderick says.  The jets launched by black holes shape their environment on intergalactic scales, expanding the influence of the black hole well beyond its immediate vicinity.  “Every piece of evidence that tells us a little bit more about where these jets come from is settling key questions in why the universe looks the way it does.”

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