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.
This is 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 a 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 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."
The EHT isn't a single telescope, but a world-spanning network of telescopes located at the farthest reaches of the globe that all must observe simultaneously. When combined, together they effectively form the Earth-sized instrument necessary to generate the highest-resolution images ever produced in astronomy. But, with only eight stations, the EHT must wait for the Earth to rotate to fill in the image, collecting radio waves throughout an entire night. Therefore, the rapid changes in the radio emission from Sgr A* presented a significant challenge for the research team - what is the "correct" image of a black hole that is changing even as it is being measured?
To address these challenges and obtain today's image of Sgr A*, new algorithms were required to overcome its variable nature. In addition to the imaging and analysis tools used with M87* three years ago, two new powerful methods developed by Broderick and his team proved critical.
The first method addressed the problem of Sgr A*'s variable nature. To reconstruct time-averaged images, similar to a long exposure photograph of a sporting event, the EHT simultaneously estimated the range of variations about it. The recovered statistical description of Sgr A*'s variability matched that anticipated by some super-computer simulations, providing new evidence for the existence of the turbulent motions that are believed to be key to driving the plasma toward the black hole.
The second, called THEMIS Bayesian imaging, is a new imaging algorithm, which has also been used to generate some of the first polarized images of M87*. Where other imaging schemes attempt to identify the image that best fits the EHT data, Bayesian imaging instead explores all of the images that are consistent with the data, assigning a probability to each. In this way, THEMIS provides an "error bar" on the image, identifying which aspects can be trusted and which are uncertain. Based on these probabilities, the EHT team confirmed the presence of an event horizon silhouetted against the bright surrounding plasma, the shadow of the black hole at the center of the galaxy.
"We are witnessing an evolution of radio imaging," Broderick says. "Sgr A* has forced us to contend with image variability, unlocking a new era in radio astronomy in which image dynamics is no longer an obstacle but a new way to learn about the physics of the source."
There is great scientific advancement in confirming a second black hole. "Where one black hole might be a chance occurrence, two is confirmation and contrast. With two black hole horizons laid bare, the EHT has thrown open a window onto the most extreme gravitational objects in the universe," Broderick says.
What is unique about Sgr A* is that it is our black hole - the centre of our spiral galaxy. Today's image opens the door for scientists to learn more about our galaxy's centre and learn more about black holes. Like M87* previously, this image will cement in the human imagination the terrible and awesome nature of black holes.
