Event Horizon: A Q&A With the EHT Scientists Who Captured Images of Sagittarius A*

Researchers from the Event Horizon Telescope (EHT) have revealed the first direct image of Sagittarius A* (abbreviated Sgr A*, pronounced “sadge-ay-star”), the supermassive black hole at the center of our galaxy. EHT project scientists made their dramatic reveal in multiple simultaneous press conferences around the world. The one in Munich was hosted by the European Southern Observatory (ESO), whose ALMA and APEX telescopes played crucial roles in the discovery.

We sat down with the panelists and other EHT experts after the Munich conference, during a period set aside for Q&A. And we had a lot of questions! Here’s what we learned.

wfoojjaec would like to thank the panelists and project scientists who answered our questions, including Sara Issaoun, Christian M. Fromm, Mariafelicia de Laurentis, EHT project director Huib van Langeveld, J. Anton Zensus, Thomas P. Krichbaum, Jose L. Gomez, and Prof. Sera Markoff, as well Geoffrey Bower, who we spoke to individually.

There’s a lot going on here. Can someone just… explain the image to me?

Issaoun: The black hole resides inside the dark region at the center of this image, where its gravitational pull is so strong that light cannot escape, and only darkness remains. We call this region the shadow of the black hole. This region is surrounded by very hot gas, swirling around the black hole. This gas emits radio waves we observe with the Event Horizon Telescope. These radio waves create the glow we observe around the shadow of the black hole.

Today’s image might look familiar to some of you. We were all amazed that the image of SA* looked so similar to that of the famous black hole in the M87 galaxy — an image our team revealed, back in 2019! However, SA* is over 1000x less massive than M87*. If SA* were the size of this donut right here — tiny — M87* would be the size of the Allianz Arena, the Munich football stadium (just a few kilometers from where we are today). On top of that, SA* consumes gas at a much slower rate than M87*. And these two black holes also reside in completely different environments. SA* resides at the center of our small spiral galaxy — whereas M87* lives at the center of a giant elliptical galaxy, and ejects a powerful jet of plasma.

Despite all these differences, the images of these two black holes look very similar. Why is that? This similarity reveals to us a key aspect of black holes. No matter their size nor the environment they live in, once you arrive at the edge of a black hole, gravity takes over.

What do we know about Sgr A* now, following these observations? What open questions does this image address?

Issaoun: From our image, we measured the size of the shadow of SA* to be 52 micro-arc-seconds on the sky. This is about the size of a donut on the surface of the Moon, as seen from Earth. In reality, Sgr A* is about as big as the orbit of Mercury around the sun — but at a distance of 27,000 light-years. Because the size of a black hole’s shadow is proportional to its mass, our image tells us that the mass of Sgr A* is four million times greater than that of our sun.

Open questions that we can answer include — is the black hole spinning? Yes, it is. What is the black hole’s orientation with respect to us? We’re fairly confident that it is pointed more or less towards us, or face-on.

When you say “face-on,” does that mean one of its poles or spin axes is pointed toward Earth?

Markoff: When we talk about “pointing at us,” what we talk about is the axis around which the black hole is spinning. And it turns out that the net angular momentum is rather random, compared to the larger scale of the galaxy. Right now, for instance, we think the black hole is being fed by a couple random stars sitting around it, and that orientation has more to do with the stars’ orientation than the greater disk of the galaxy.

Fromm: Yes, it’s correct that one of the spin axes is pointed towards us. We don’t have an exact model that would explain everything. We can say that it’s spinning — and in the same direction of the gas orbiting it. But the precise speed of the spin has to be obtained in upcoming observations.

How do your observations compare with the predictions of general relativity?

de Laurentis: These observations confirm Einstein’s theory of relativity to within ten percent. Black holes — and in particular, the vicinity of the event horizon — are becoming an observational testbed for gravitational physics. This environment offers us the unique opportunity to determine where and how Einstein’s theory breaks down. And if it does, of course, it will transform our understanding of gravity, even the properties of space and time.

Can you explain what’s going on in this fly-through video you showed?

van Langeveld: So, we start out from the plains of Chile, where the ALMA telescope is located. We’re going to zoom in to Sagittarius, the Archer, which is high in the sky above northern Chile. And we will go and zoom in, first in the optical, but we have to switch to the infrared, because there we can penetrate all the way into the galaxy.

The video:

van Langeveld: We leave tens of millions of stars behind, and we get to the place where stars are in orbit around a dark spot. When we switch then to our radio eyes, what we see at last is the image of Sgr A*, the black hole at the galactic center.

What tech does the EHT Collaboration use to make these observations?

Zensus: I’ll start with ALMA, the Atacama Large Millimeter/submillimeter Array, at five thousand meters altitude in the Chilean desert. This was our game-changer. Its sensitivity, because of its huge collecting area, was the difference in measuring the weakest signals. IRAM has been our flagship for over two decades. It is essential for making the sharpest image. APEX, the Atacama Pathfinder EXperiment, is invaluable for precisely calibrating our signals. And NOEMA, in the French Alps, with its huge collecting area and super-sensitive receivers, has given a huge boost to the EHT.

Krichbaum: Imaging a distant black hole would not have been possible without a big telescope of very high magnifying power, or very high angular resolution, as we astronomers call it. To achieve this, astronomers combined radio telescopes located around the globe, to create a super-telescope which has the size of the Earth. This technique is called very-long-baseline interferometry, or VLBI.

How did your team use VLBI to observe Sagittarius A*?

Gomez: The EHT is like a giant, Earth-sized telescope. But it doesn’t work like a regular telescope. Instead, the radio telescopes of the EHT work in pairs, with each pair collecting the information required to obtain an image. As the Earth rotates, the separation between telescopes changes, providing this extra data we need.

How did the team choose the colors when they colorized the image from IR to visual?

Bower: The color scheme is completely arbitrary. We used it to illustrate the gas around the black hole, which is very hot — billions of degrees! But the result is a black-and-white result. The (infrared) light we see is created in the sub-millimeter, and then received in the sub-millimeter. The color fade-off represents the intensity of the light as we detected it.

Is there any way to infer the luminosity or temperature of Sgr A* from this image?

Bower: Yeah, absolutely – so, the luminosity that we detect is actually not that much brighter than the total luminosity of the Sun. As some of the speakers emphasized, Sgr A* is on a “starvation diet.” It’s the cowardly lion of the galactic center/of black holes. It’s consuming very little matter and generating very little light.

As for the second part — it’s not a black body; a black body produces a very specific kind of light. At the really extreme temperatures of the particles, billions of degrees, orbiting the black hole — we do have a temperature of billions of degrees. But the particles have a different distribution than what you’d get from a black body.

What were the greatest difficulties your team faced in observing and analyzing Sgr A*?

Gomez: Many. My God, where do I start? Imaging Sgr A* was significantly more challenging than M87*, oh my. It was really a hard time. Our line of sight to the galactic center is obscured by matter, which has scattered the radio waves coming from the region around the black hole.

But most importantly, as shown in these computer animations, the gas in these two black holes is moving at the same speed — nearly the speed of light. But SA* is nearly a thousand times smaller than M87*. This means that the time that the gas takes to orbit Sgr A* is just a few minutes, while it takes days to weeks to orbit the larger M87*. This means that the gas around Sgr A* was actually changing while we were observing it. It was like trying to take a clear picture of a running child at night. You can imagine how crazy it drove us.

We needed to find a way to overcome these extreme changes. We’re observing these sources (M87* and Sgr A*) for eight to ten hours. And during those eight to ten hours, M87* doesn’t change. That’s exactly what Sgr A* refuses to do. So we’ve created these new tools that allow us to take a slightly blurry image, to which we then add a small amount of noise, to compensate for all the variability.

Gomez: Each one of these images is slightly different, but by averaging these images together, we are able to emphasize the common features, finally revealing the giant lurking at the center of our galaxy for the first time.

What are the biggest differences between M87* and Sgr A*?

Fromm: The primary difference is the central mass. The galactic center has roughly four million solar masses, and M87* has billions of solar masses. Another difference is a powerful jet generated by M87*. For the galactic center, we see it face-on, so the jet — if it exists — would be pointed toward us. But it’s very dilute, so we can’t image it. I would refer you to my colleagues to explain what we need to finally detect it. And then there’s also the accretion rate.

Gomez: In order to obtain that image, we’d need antennas of different separations, to measure both features that are very small and features that are very large. In order to capture a possible jet in Sgr A*, and to actually capture the jet that we do know exists in M87*, we need measurements from telescopes with short separation distances, to see these larger scales. We lacked that information during the 2017 observations. … But we have new antennas for the observations that follow the 2017 campaign. It’s still unclear how bright the jet in SA* will be, if it exists. But we’ll be chasing it.

How much matter is Sgr A* accreting?

Issaoun: We actually calculated this for a Twitter pool. If you had the same diet as Sgr A*, scaled to your mass, you would eat one grain of rice, every million years.

How did you feel when you saw these results?

Issaoun: I did my Ph.D. research on Sgr A*, trying to chase down this jet that may be real, may be a myth. I was working on images of SA* taken at a lower frequency than the EHT, trying to understand where the radio emissions come from near Sgr A*, trying to understand if we could constrain the inclination of the black hole from my observations. And until today, I was also the record holder for the best image of Sgr A*! (laugh) When I saw the image of the black hole’s shadow, it was a really exciting moment. Shivers and excitement.

Krichbaum: As a somewhat older person, I realized how it started. I think it’s been now more than thirty years, to get this image. When we started observing, our first observations at 1mm failed horribly. We had to step back to 2mm before we could do it at 1mm again. And over the years, the techniques have improved so much. It’s a very good feeling to see now that it has worked, and that we’re finally able to show this image.

Does the EHT Collaboration have any other targets in mind?

Krichbaum: Of course! During these observing runs of SA* and M87*, we also observed a number of AGNs (active galactic nuclei), some of which are relatively close by.

Any final thoughts?

Bower: This is a milestone that’s been decades in the achieving. SA* was detected almost fifty years ago, and I started working on it in the mid-90s when I was a student. And everyone who has worked on it has been motivated by this idea of getting as close as possible to a black hole. This is a realization of a dream that many of us have had for a very long time, to produce this image and share it with the world.

de Laurentis: The years ahead will transform our understanding of black holes, and of the fundamental nature of gravity. So, stay tuned, because the best is yet to come.

These remarks have been edited for length and clarity.

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