Finding our galactic centre

Thursday, May 12, 2022

Three years ago, history was made when the first image of a black hole inspired wonder and awe around the world as we glimpsed the shadow of light escaping from the supermassive black hole M87*. Today, history is being made again as the Event Horizon Telescope (EHT) Collaboration releases the image of a second black hole — Sagittarius A* (Sgr A*) — the one at the centre of our own Milky Way galaxy.

Among the international group of astronomers behind these images is Professor Avery Broderick from the University of Waterloo. Broderick is a founding member of the EHT, and has built theoretical models for the EHT’s most promising targets and demonstrated the capacity of the EHT to transform black hole science. As part of the EHT, Broderick’s research group has developed powerful new computational tools that analyzed and interpreted the unique data generated by the global array of radio telescopes. With these tools, the EHT has revealed in greater detail than ever before the astrophysical dramas that play out in the vicinity of a black hole’s event horizon.

A bright orange ring with three very bright spots almost equally spaced along the ring. Smudges of orange light extend from the ring towards the edges of the image

The first image of Sagittarius A* (Sgr A*), the supermassive black hole at the centre of our galaxy. The Sgr A* black hole image was captured by the Event Horizon Telescope and is an average of the different images.

“Our image of the first black hole, M87*, was a huge success in science, and has grown into not just a science story, but a human story,” says Broderick, also the Delaney Family John Archibald Wheeler Chair of Theoretical Physics at the Perimeter Institute for Theoretical Physics. “It’s estimated that within three months one in two people on Earth had seen our image.”

Despite both being black holes, M87* and Sgr A* are very different objects. Located at the center of the Milky Way, Sgr A* is 2,000 times closer than M87*. Due to its location, the radio waves must pass through the galaxy’s stars and gas, which causes scattering and blurring before they are ultimately detected by the EHT, much like viewing Sgr A* through frosted glass.  Additionally, unlike M87*, Sgr A* does not exhibit light-speed outflows — powerful “jets” that can extend the influence of black holes to well beyond their host galaxies.

Most importantly, Sgr A* is 1,500 times less massive and smaller than M87* — and thus matter and light orbit Sgr A* 1,500 times faster than in M87*.

“Imaging Sgr A* was like trying to take a clear photograph of an energetic puppy chasing its tail. By comparison, in this analogy, M87* would be like a stately old lion, asleep and still,” Broderick says.

“The faster motions in the gas results in significant variability in the data for Sgr A* and required new methods to make sense of the data we collected,” said Boris Georgiev, a PhD student at the University of Waterloo and Perimeter Institute who led a study of what super-computer simulations could tell us about the expected variable emission from Sgr A*. But one astronomer’s noise is another’s music. “By carefully studying the variations, we determined if they are caused by a handful of large moving features or many small ones, testing our ideas about how 100-billion degree plasmas behave near black hole event horizons.”

For details on how the researchers overcame the variability in Sgr A*, see the full story in Waterloo News.