Snapping a cosmic selfie — the first image of the Milky Way’s black hole

The Event Horizon Telescope (EHT) Collaboration, an international collaboration aimed at gathering direct images of black holes, has now set its sights close to home.

In a May 12 announcement, EHT researchers presented the first image of Sagittarius A*, the supermassive black hole at the center of the Milky Way, Earth’s home galaxy. The announcement comes three years after EHT made headlines around the world by producing the first photograph of a black hole — M87 in the center of the Virgo A galaxy, located 55 million light years away.

Sagittarius A* — also called Sgr A* — is 27,000 light years from Earth.

A black hole is a part of space that has such intense gravity that nothing, not even light, can escape it. The boundary surface between space and the inside of a black hole is called the “event horizon.”

EHT generates images by linking eight synchronized radio telescopes around the world, including at sites in Chile, Spain, Hawaii, and the South Pole, to create a fundamentally new instrument.

Mislav Baloković, a postdoctoral fellow at the Yale Center for Astronomy and Astrophysics and the Department of Physics, and an EHT member, spoke with Yale News about the new image and what it means for our understanding of our home galaxy.

Why was it important to study the black hole in our own galaxy, as opposed to other galaxies?

Mislav Baloković: The center of our Milky Way galaxy has been an interesting puzzle for over a hundred years, and especially since astronomers discovered about 50 years ago that it shines brightly in radio waves. The Milky Way's supermassive black hole (SMBH) is the closest one to us and can be studied in greater detail than any other. In comparison with the giant SMBH in the galaxy M87, whose image the EHT published three years ago, Sgr A* is much smaller and situated in a more typical galaxy. This provides us with grounds for comparison. Despite the different mass, size, and environment, they appear remarkably similar. It will certainly be interesting to see if this holds up for other galaxies and their SMBHs, but these are the only two that we know of that the current EHT can study at the event horizon scale.

What does the visual nature of this result bring to the project — both scientifically and in terms of public impact?

Baloković: The visual nature of this effort has always been appealing to me, and the popularity of both M87* and Sgr A* results in the general public shows that this aspect makes our science attractive to a very broad audience. Media professionals estimate that EHT's image of M87* reached billions of people and is widely recognized. Sgr A* also boasts the status of “our black hole,” just like our star, our planet, our solar system, and our galaxy, which makes it special and I think will contribute to its popularity.

On the scientific side, although having the image is certainly important, a lot of the important information that leads to new discoveries is in other forms of data. Unlike M87*, Sgr A* exhibits rapid changes during the EHT observations, consistent with its much smaller mass and size. This temporal variability and the changing broadband spectrum from the radio to the X-ray band teach us about the sources of emission and physical conditions close the event horizon, therefore constraining our astrophysical models.

What was your role in the work?

Baloković: My scientific interest is in understanding the broadband emission spectrum of the SMBHs' environment. For M87* and its galaxy-spanning jet, we published a study separate from the initial image; however, for Sgr A* the multi-wavelength data from a range of supporting facilities was included in the set of 10 papers published this week. My expertise is in X-ray observations, which were contributed by NASA's Chandra, NuSTAR, and Swift telescopes in the case of Sgr A*. I also contributed to EHT's public communications effort, which stems from my previous experience with leading the global announcement of the M87* results and public outreach more generally.

Were other Yale researchers involved?

Baloković: I am the only EHT Collaboration member currently at Yale. However, some of my colleagues who are Yale undergraduate and graduate alumni have made significant contributions to the new results: Richard Anantua ’10 and Angelo Ricarte Ph.D. ’19. Also, Priyamvada Natarajan, the newly appointed chair of the Department of Astronomy at Yale, is an external principal investigator of Harvard's Black Hole Initiative, which supports a large contingent of EHT scientists. Richard and Angelo are in that group now, as was I for three years right before coming to Yale as a YCAA Prize Postdoctoral Fellow.

What surprised you about this result?

Baloković: I am actually surprised by how sharp the image of Sgr A* is! I know it may seem the exact opposite to most people, which doubles the surprise, I guess. As a colleague pointed out at the May 12 press conference in Washington, D.C., EHT's images are some of the sharpest ever made and any other (non-simulated) image zoomed-in as much would appear far more blurry. The image of the Sgr A* black hole is based on observations of a constantly and rapidly changing target seen through about half a galaxy's worth of turbulently stirring hot gas, and I am amazed that despite this the EHT was able to arrive at results that are as robust as they are.

How will EHT follow-up on this latest work?

Baloković: The EHT collaboration is already hard at work with existing data, for example, to reveal the structure of magnetic fields close to the event horizons of black holes and along the jets of quasars. While there is more to come from the observations conducted in 2017, the EHT also collected data in 2018, 2021, and as recently as in April this year. These more recent observations include additional telescopes and improved capabilities, both of which will enable us to characterize variability in greater detail. We are also actively searching for the next-best SMBH candidate for imaging at the event horizon scale, as well as looking into optimizing the EHT array to enhance its capabilities in the future.