Dazzling dispatches from the heart of a galaxy

Mislav Baloković, a postdoctoral fellow at the Yale Center for Astronomy and Astrophysics, has a prime viewing spot for the most famous black hole humans have ever seen.

That would be the supermassive black hole in the galaxy M87, located 55 million light years from Earth.

Two years ago, the Event Horizon Telescope Collaboration (EHTC) — a global collaboration of astrophysicists and observatories that created a virtual, Earth-sized telescope, released an image of M87. It was the first photograph of a black hole and a technical achievement lauded by scientists around the world.

Balcković, who came to Yale last fall, is a member of EHTC. He is a core contributor and co-author of a study published April 14 in The Astrophysical Journal Letters that sheds new light on the cosmic environment surrounding M87’s black hole.

EHTC data will allow scientists to conduct new lines of investigation into some of the most challenging areas of astrophysics. For example, scientists will use the data to improve tests of Albert Einstein’s theory of general relativity. Currently, the main hurdles for these tests are uncertainties about the material rotating around a black hole and being blasted away from the black hole in jets that produce light spanning the entire electromagnetic spectrum, from radio waves to visible light to gamma rays.

The new study also provides information about the origin of cosmic rays — energetic particles that continually bombard the Earth from space. The huge jets launched from black holes are thought to be the most likely source of the highest energy cosmic rays, but scientists have many questions about how this occurs.

Baloković talked with YaleNews about the new findings and what they mean for further study of the universe.

How do black holes influence our lives and the cosmos around us?

Black holes reside at the heart of nearly every galaxy, including our own Milky Way. These “supermassive” black holes contain the combined mass of a million to a billion stars within a volume smaller than our solar system. Despite their relatively small size on the cosmic scale, supermassive black holes manage to influence the evolution of their entire host galaxy. Huge amounts of radiation are emitted from their immediate environment and they launch jets of fast-moving particles that can escape the galaxies' boundaries. Though details of these processes are not well understood, we already have ample observational evidence that supermassive black holes play an important role in shaping galaxies over cosmic timescales.

What, for you, are the most important findings from the new study?

We assembled a dataset gathered from nearly all observable parts of the electromagnetic spectrum, matched in time to the now iconic image of the black hole in the galaxy M87. Studying this dataset, we established that the emission pattern of the jet is structured so that low-energy emission mostly originates from the immediate surroundings of the black hole, while high-energy emission was predominantly generated farther out along the jet during the observations in 2017.

This insight helps us to better understand how particles in the jet get accelerated to nearly the speed of light, producing some of the highest-energy cosmic rays that pervade deep space. It is also important that we defined a baseline for studies of changes in the following years and enabled more detailed comparisons of theoretical models to real data in the near future.

What was your role in the research?

I participated in the work of the core team leading the study and I coordinated consolidation of the data between teams of experts for each of 19 participating observatories. I specialize in observations in the X-ray portion of the electromagnetic spectrum, using NASA's space-borne X-ray observatories NuSTAR, Chandra, and Swift.

I also participated in observations performed in 2018 and in planning for the 2021 observing campaign that is going on at the moment. These additional observations will further improve the data quality and let us examine in detail how images and emission patterns change over time.

How long have you been a member of the Event Horizon Telescope Collaboration and what is it like to be part of such a large team?

I joined the EHTC after finishing my Ph.D. thesis in 2017, several months after the first observations were performed. I quickly met many new colleagues from all over the world and I remember how excited everyone was that the 2017 observing campaign was a success. Having even a small role in such a complex and ambitious undertaking can be very rewarding.

Until recently, I worked as a coordinator of the group for communication with the public, having covered the world-wide announcement of the first black hole image in April 2019. I love the global character of the EHTC and the diversity of both culture and expertise that it contains — one learns something new in each group meeting. I firmly believe that the different perspectives collaboration members bring into our discussions make it possible to deliver scientific results that are both impactful and reliable.

Do you remember your reaction when you saw the first image of a black hole?

Naturally, it was very exciting to lay eyes on something so few have ever seen. This was during a workshop at Harvard, where I worked at the time, attended by many members of the EHTC. I remember thinking what a remarkable crowning achievement this result must be for my senior colleagues who steadily worked towards it for as long as two decades.

It was also a relief that so much previous work on black holes was validated and strongly propelled forward. Many astronomical phenomena, starting with the discovery of the first quasar in 1963, have been explained assuming that supermassive black holes exist.

How will this new information guide or inspire future research?

The first image of the black hole in the galaxy M87, and the more recent polarized image, are groundbreaking in their own right. However, in order to improve our understanding of the environment in which the black hole resides and further sharpen tests of the General Theory of Relativity, we needed to complement them with information uniquely available in other portions of the electromagnetic spectrum.

The newly published dataset will enable many research groups around the world to test whether their theoretical models can reproduce the pattern of emission from M87 at the time the iconic image was taken. Complex theoretical models based on our cutting-edge understanding of black hole environments will help us gain new insight into how supermassive black holes launch jets of fast-moving particles into the galaxies that surround them, consequently influencing their evolution.