New black hole image: 4 things we know
On Thursday, May 12, 2022, the Event Horizon Telescope (EHT) team of astronomers presented the 1st direct image taken of the Milky Way galaxy’s supermassive central black hole, Sagittarius A*, aka Sgr A* (pronounced Sajj-a-star). The image shows a glowing, lumpy donut. In fact, it’s a black hole with a mass some 4 million times that of our sun. EarthSky.org reporters covered the virtual event, and the EHT panel of experts specifically answered one of our questions. EarthSky.org’s full coverage of announcement details is here. A video of the entire press conference and follow-up events is available on ESO’s YouTube channel. Below are 4 things the new image confirms.
1. The black hole is face-on to Earth
During the conference, Dr. Christian Fromm, EHT’s Sgr A* Theory Working Group Coordinator, described the black hole as being face-on to Earth. EarthSky.org’s question sought elaboration on that statement. Following is a transcript of the question and the answer.
Bárbara Ferreira, Media Manager, European Southern Observatory (ESO) Department of Communications: OK, we have one from Dave Adalian reporting for EarthSky.org. I think Christian will be able to answer that one. When the orientation of Sagittarius A* is said to be “face-on,” does that mean one of the poles is directed towards Earth? Do we know how fast Sagittarius A* precesses? And, what is the nature of the particles ejected?
Fromm: OK, so let me start first, so we found that it’s face-on, so this is correct that one of the spin axes is pointing more or less towards us.
The other part was about the material or the emission actually that we see, and what we model is synchrotron radiation, so you have particles, mainly electrons, gyrating magnetic fields, and while they gyrate, the magnetic fields, they emit emission, and this emission is synchrotron, and this is what we actually see.
And there was a third part. Could you please repeat it?
Ferreira: Ah, so it was the precesses (sic).
Fromm: Ah, the, uh, it was– Ah, yes, so the precession. I think what you mean is spin. So what we have actually is, we have some kind of best-bet models. So we have not found an exact model which would explain everything, so we have best-bet models and best-bet regions. And there we could say that it’s spinning, and it’s in the same direction as the gas orbiting it, but the precise number of the spin has to be obtained in upcoming observations.
2. The black hole’s direct influence on Milky Way now is small
Later, during a live question-and-answer session featuring other researchers who joined the EHT Collaboration, the topic of black hole emission jets was covered in more detail. Emission jets from black holes originate along their spin axes, revealing why the orientation of Sgr A* may be important to understand galactic development. Following is a partial transcript of that Q&A session.
Juan Carlos Muñoz, host, reading a question from a viewer: Would it be possible to observe jets at this distance and in the specific case of this particular black hole (Sgr A*)?
Dr. Sera Markoff, co-chair of the EHT Science Council and a professor of theoretical astrophysics at the University of Amsterdam, the Netherlands: I would love to take that one, because I’ve been trying to find jets at Sgr A* for a long time. The point is, even if Sagittarius A* was launching very small jets … and we think there’s a pretty good chance, I mean at least the models we use predict there should be jets. But we have to remember that Sgr A* is basically 10s or 100,000 times less powerful than M87. So if you just turn the crank down, its ability to make powerful jets is hampered. And then you have this complicated mess in the galactic center, and trying to pick out some weak feature. We have known for quite a while that it would be quite difficult to see. So the question is whether when we have our upcoming data sets from the coming years, when we have more telescopes, we also fill in some of the baselines, will we be able to maybe connect this, you know, fuzzy sort of donut thing to maybe some kind of extended emission. It’s the same issue we’d like to do with M87.
3. The black hole might still help create stars
The Q&A session then turned to the importance of Sgr A*’s gravitational influence on the rest of the Milky Way, and specifically what would happen if Sgr A* was suddenly removed. It is a myth, the experts explained, that supermassive central black holes play a role in holding their galaxies together. From the discussion:
Dr. Violette Impellizzeri, Leiden Observatory, NRAO: The short answer is, if you were to remove it now, probably nothing. That’s what would happen. The question is what role it had in the formation of our galaxy, and in the fact it looks like it does now. But the sphere of influence of the black hole itself on its surrounding is not very large. It’s very small. If you removed that, it wouldn’t impact us directly, but maybe the evolution would change again.
The panel explained, however, that jets emitted by other supermassive central black holes might have an important indirect role to play in star formation that would be lost if Sgr A* suddenly disappeared:
Michael Janssen, Max Planck Institute for Radio Astronomy: Maybe more indirect effect. The jets have been mentioned before, right? Sera liked them very much in Sgr A*. Unfortunately we haven’t seen that yet, but we see it in other galaxies. In M87, there’s very, very clear indication of a jet. Another galaxy, for example, is Centaurus A, where we also imaged a jet.
And when these jets, when they plow through the galaxy, they actually impact the gas that is in the galaxy itself and maybe compress it, maybe heat it, and that might trigger – or actually hamper, it’s a critically active field of research – trigger or hamper star formation. So we may actually have the influence of the jet or other outflows from the central black hole – it’s not only accreting; it’s also putting stuff out there – and that might influence the whole evolution of the galaxy.
4. Our Milky Way had a violent youth
While the potential influence of the hypothetical jets perhaps produced by Sgr A* today is relatively mild, that wasn’t so in the Milky Way’s distant past. At one point, an emission jet from Sgr A* may have erupted with extreme violence, leaving remnants we still see today surrounding our home galaxy. From the transcript:
Ziri Younsi, UKRI Stephen Hawking Fellow, University College London: I just wanted quickly to add to that about the jet stuff, because I think it’s really interesting. We see evidence of a very violent past for Sagittarius A*. Throughout the galaxy we have these things called Fermi bubbles, which are these enormous radio lobe-like structures that extend above and below the plane of our galaxy.
So they speak to some very violent cataclysmic event that happened a very long time ago. And it’s curious that we’re in a period right now where everything is very quiet, and probably in some way a jet of some description, some enormous eruptive event, is responsible for that. And so I think what’s really cool about these observations is we don’t see a large-scale jet, as Sera and Michael have said, but the question is really: Why don’t we see one? And yet in the past we seem to have indications that there may have been something like that, and I think we’re having this rare opportunity to finally have a look at the very heart and maybe start to answer the questions in time.
Younsi’s comment brought a nervous moment of reflection from the other panelists:
Impellizzeri: A thought that occurred to my mind, and I wondered what would happen if Sgr A* had a jet like M87, right? What would that mean for us?
Markoff: Luckily, it’s pretty far away, so…
Muñoz: … We should be safe from it. Right? (laughs)
Bottom line: The Milky Way’s recently imaged central supermassive black hole is oriented “face-on” toward Earth, and the jets of particles such objects emit can influence galactic development.