Picturing the Heart of Our Galaxy

In May, the world got its first glimpse of the centre of our galaxy. The picture showed a blurry bright ring of light surrounding a dark core. The image was not world-shattering for its aesthetic beauty, but for what it represented. Visual confirmation of what 2020 Nobel Prize in Physics recipients Reinhard Genzel and Andrea Ghez had proven to exist in the late noughties – Sagittarius A*, our very own supermassive black hole.

Perfectly Synched by a Team of Over 300 People

This wasn’t a quick snapshot from a super telescope. Sgr A* was observed over five nights in April 2017 by eight radio telescopes at six sites dotted across the world that formed a planet-size virtual telescope – the Event Horizon Telescope (EHT). Once the data were recorded, they were shipped to Germany and Massachusetts, where they were perfectly synched to within trillionths of a second. Next, the team, consisting of over 300 people working at 80 institutions in various countries, got to work reconstructing the image.

Anne-Kathrin Baczko (Max-Planck-Institute for Radioastronomy) – who remotely attended the 2020 Online Science Days and 70th Lindau Nobel Laureate Meeting in 2021, and with the perspective of coming to Lindau for the 2024 Physics meeting – has taken part in several EHT observations since 2017, monitoring worldwide observations from her computer in Germany to check for anomalies, and conducting several tests to verify that data are combined correctly during correlation. But one of her most important tasks has been in helping to produce images. This involves working in one of several small groups that applies sophisticated algorithms to the data.

Baczko describes how this was done for the Sgr A* image: “From thousands of images from these different people and methods, we found four clusters of images and obtained one averaged image for each cluster, and in the end one single averaged image,” she explains. “We were trying to make really sure that what we showed is the true image, and I’m proud of the people who worked on that.”

A Gentle Giant

What these teams produced is uncannily similar to the only other picture of a supermassive black hole, M87*, also observed by the EHT in 2017. But the two black holes couldn’t be more different. M87* is highly energetic, producing strong plasma jets, whereas Sgr A*is known to be a more gentle giant. Moreover, M87* is about 1500 times more massive – the mass of roughly 6.5 billion suns.

“Einstein’s theory of general relativity tells you that the black hole size scales linearly with its mass and this bright ring shouldn’t change – that would never be true for any other physical source that we know of,” says 2019 Lindau Alumnus Ziri Younsi (University College London), who is a member of the EHT’s Science Council, in the Gravitational Physics and Theory & Simulation working groups. “This speaks to the validity of general relativity.”

An Eye on the Big Picture

For Younsi, attending the 69^th Lindau Nobel Laureate Meeting was a transformative experience. “It’s not an understatement to say that it definitely changed, not the course of my career, but my outlook and my perspective,” says Younsi. “What I learned was that it’s not about getting too lost in what you’re doing, but trying to keep an eye on the big picture and remember the context of your work.”

Younsi applies this philosophy to his work every day. As part of the EHT collaboration, he is tasked with physically describing and calculating how matter moves around black holes, how radiation is produced by that matter, and how this radiation travels from near the edge of the black hole to us. He is using this treasure trove of data from M87* and Sgr A* to better simulate black hole dynamics and improve our understanding of Einstein’s theory of general relativity.

Open Questions Left

But both images have left open questions for Younsi, Baczko and the rest of the EHT collaboration that only further observations can resolve. Do these black holes produce jets? Why does Sgr A* appear to be nearly face on instead of in plane with the Milky Way? Which way does it spin? And can we image other supermassive black holes?

“We have images of M87* and Sgr A*, but that’s not everything that you can do with the EHT,” explains Baczko. “Going into the future, there will hopefully be more emphasis made in the direction of other active supermassive black holes – there’s so many interesting things imaging these objects will teach us on the dynamics between the central supermassive black holes and galaxies, and the physics of how these objects work.”

Baczko also hopes that imaging at different frequencies will bear fruit. “There are currently tests that are going to even shorter wavelengths than we currently observe with EHT, below 1 mm wavelengths,” adds Baczko. “That means that we will be sensitive to another part of the electromagnetic spectrum where we can maybe image something differently that we don’t see at the higher frequencies.”

Sharper Images in the Next Years

Meanwhile, Younsi is keen to see sharper images of Sgr A*, M87 and other supermassive black holes over the course of the next few years. Three additional telescopes were added to the EHT in 2018, but there are still gaps in coverage, holes in the virtual telescope, that blur the final images produced. “We can obtain better images, sharper images, just by having more telescopes,” says Younsi. “Some are actually being built in Africa now, for example, that will help us observe for much longer and remove uncertainty because we’ll be covering the different scales that we need to resolve.”

“We will be making movies of black holes in the next five or 10 years, hopefully, and they’ll be much, much sharper too,” he adds. “So we’ll see how black holes actually feed in real time, which is amazing.”