The Bubbles At The Center Of The Milky Way
The centers of many galaxies are active, even violent, places where massive black holes routinely expel X-rays and jets of plasma after consuming meals of gas and dust. And even our own galactic center, though relatively quiet, still experiences activity several times a day from its own black hole, a bright source of radio waves called Sagittarius A*.
Now researchers using the new MeerKat radio telescope array in South Africa have written in Nature that they have found new structures at our galactic center—twin bubbles inflating from a black hole outburst that may have occurred more than a million years ago. Northwestern University astrophysicist Farhad Yusef-Zadeh says these structures are a hint that our galaxy isn’t so different from the ones that gleam gorgeously in Hubble Telescope photos. Furthermore, the bubbles may explain another strange phenomenon: the much larger, 50,000-light-year-long Fermi Bubbles that were discovered emanating from the center of the galaxy in 2010.
Yusef-Zadeh takes Ira on a mental journey to the center of the Milky Way, explaining how this new picture of our galactic core might add to our understanding of our galaxy’s evolution, other galaxies, and even fundamental physics.
Farhad Yusef-Zadeh is a professor of physics and astronomy at Northwestern University.
IRA FLATOW: This is Science Friday. I’m Ira Flatow. A bit later in the hour, playing hide and seek with rats. We have a video of this. You have to see this. This is really cool. How do you teach them to do that? And we’ll talk about, of course, what neuroscientists can learn by studying the brain at play.
But first, I journey to the center of our galaxy, the Milky Way. There is a lot going on at the center of most galaxies. You start with a hungry massive or supermassive black hole. You add some gas and dust. And you get jets of plasma, X-rays, other great bursts of energy.
And yes, our galaxy, the Milky Way, has a big black hole at the center. And it’s emitting small burst of energy a few times a day. But researchers writing in the journal “Nature” this week say they have evidence of at least one big feeding frenzy over 1 million years ago– a pair of bubbles towering above and below that galactic center. Kind of mysterious.
Here to explain more about why this is exciting news for space science is Dr. Farhad Zadeh, an astrophysicist at Northwestern University’s Center for Interdisciplinary Exploration and Research in Astrophysics. He’s also co-author of the research on this. Welcome to Science Friday.
FARHAD ZADEH: Thank you for having me on.
IRA FLATOW: Can you give us a picture of what’s going on at the center of the galaxy? Take us on a tour, if you will.
FARHAD ZADEH: Sure. We’re about 25,000 light years away from the center of the Milky Way. And the Milky Way center is really an interesting region. There is obviously a massive black hole at the center of the galaxy. And then you have a very large concentration of stars in there. And these stars are orbiting around the black hole, the supermassive black hole. And they have these crazy orbits that you don’t find them elsewhere.
So just to compare the number of stars that we have in our solar neighborhood compared to the galactic center, you find that, visually, the closest star is about a few light years away from us. But at the galactic center, it is about something like 100 times smaller, the average distance between stars. So it’s a very crowded region of the sky. And these stars interact with each other.
And so it’s really the metropolis of the galaxy. And we’re more or less in the rural area of the Milky Way galaxy, way out there, 25,000 light years away.
IRA FLATOW: So we’ve got a lot of crazy stars bunching together toward the middle of the–
FARHAD ZADEH: That’s right, that’s right. The concentration of stars peaking right around the black hole is.
IRA FLATOW: And so what we now have these giant bubbles, from what I understand. What do you mean by bubbles?
FARHAD ZADEH: Well, it’s just a lob of material that is actually covering over 1,000 light years around the center of the galaxy. So this material is hot, and we think it’s expanding. There is, in fact, X-ray-emitting gas inside this bubble of gas that is actually pushing and making the material basically expand.
So that’s what we mean by bubble of gas. Just the material that is basically– is produced by an outflow or a wind from the center of the galaxy. And that’s what this bubble is telling us, that there must have been an outflow really of past activity in the nucleus of our galaxy about a few million years ago. And we actually see this. And that’s what this is about.
IRA FLATOW: Do we know why it might have suddenly started a few million years ago, what might have kicked it off?
FARHAD ZADEH: Well, we think that it could be due to an outburst from the supermassive black hole. So this kind of event happens quite often in external galaxies and supermassive black holes elsewhere. And we see, actually, structures like this in other galaxies.
So either the supermassive black hole produced a powerful wind that has actually, eventually led to this a large-scale structure around the center of the galaxy, or it could be just a large number of massive stars all basically blowing up at the same time. And then they produce an outflow.
So they’re two ideas, and people are debating about these two different possibilities. But a lot of people think that it’s actually the black hole that was active a few million years ago and produced this giant bubble of material.
IRA FLATOW: Is the center of our galaxy– that black hole and that star is around it– is it typical of other galaxies, or different than other galaxies?
FARHAD ZADEH: I think it’s very typical. Any mature galaxy has a supermassive black hole at its center. And we actually see a lot of galaxies also showing these beautiful, beautiful structures on the same sort of scale size as what the bubble that we’ve seen in our own galactic center. And so the event is usually considered to be due to either an active black hole, or it could be due to a burst of stars, basically, producing these bubbles.
IRA FLATOW: Now, I know there is another bubble, or there are other bubbles we’ve seen in the Milky Way. Even bigger ones than these, called the Fermi Bubbles.
FARHAD ZADEH: That’s right, that’s right. So that makes it quite interesting, because the Fermi Bubble is a much larger scale structure than even the bubble that we’re talking about. The bubble we’re talking about might be a little bit more than 1,000 light years across.
The one that we see– the Fermi Bubble is about something like 28,000 light years. I mean, it’s just tremendous. It’s really the giant– the elephant in the Milky Way galaxy. Can’t find anything else. And that is a producing gamma rays.
And in fact, what’s really interesting is that these features that we see closer in to the black hole could be the base of the Fermi Bubble. And that’s one of the ideas that may be in– and it could be basically feeding the Fermi Bubble. Maybe there are ways that you can actually have multiple events that are constantly pushing the gas out into a much, much larger region of the galaxy.
So I think we don’t really know for sure that this is how they’re connected to each other. But I think both are relic of past activities in our galactic center. So it’s very likely that the two are really causally connected, in a way.
IRA FLATOW: Now, I know you’ve been studying the center of our Milky Way for what, 30 years? Something like that.
FARHAD ZADEH: [LAUGHS] Yes.
IRA FLATOW: Well, what makes you so excited? What is there so exciting? Tell us. Share with us.
FARHAD ZADEH: I mean, it is an exciting region. I never had a plan to really just study the same region. The universe is huge. You can do so many different things.
But I think one discovery leads to another discovery. And at the same time, you really want to understand what’s going on. So understanding something takes much more time than really even discovering something.
So in order to really figure out what’s going on, and there’s always fantastic instrumentation coming up with different telescopes, and you look at the same object, basically, in a different view. And you really get a big picture of what’s going on from different angles. So I didn’t really study just one wavelength. But I did different things and different structures.
But it is really interesting. You have a supermassive black hole. You have the highest concentration of stars orbiting around the black hole and further away. And you see these– a torus of gas cloud orbiting around the black hole. And we think that some of the gas is actually being captured by the black hole.
And so you learn a lot about the different structures, and how– the interplay between these different components of the Milky Way galaxy. And it’s so close by. It’s only about 25,000 light years away from us compared to the next distant galaxy, which is 100 times further away.
IRA FLATOW: Now, I’m going to ask you this question because I think I know the answer, but I’m going to ask it anyhow because this year, we saw the first ever picture of a black hole’s event horizon. Are we– not ours, and that’s what I’m going to ask about. Are we going to be able to turn that same instrument to the black hole at the center of our Milky Way and get a look at the–
FARHAD ZADEH: Well, I mean, absolutely.
IRA FLATOW: How much would you like to do that?
FARHAD ZADEH: Well, one of the main sort of motivation for building EHT, the Event Horizon Telescope, was actually to look at the black hole at the center of our own galaxy. And, of course, M87. And I was actually disappointed when I didn’t see the image of the black hole coming from our own galactic center.
So there are two objects that they were more proposed to be observed with the Event Horizon Telescope. And certainly they have been doing a lot of work on the black hole at the center of the galaxy. And we haven’t really got the result. And I think I can say why we haven’t been able to get a good image so far.
The black hole at the center of our galaxy is relatively active. I mean, it does flare, as you mentioned earlier, a few times a day. And these flares basically fluctuates the emission that comes from the black hole. So it makes it really difficult to image this object when it’s–
IRA FLATOW: It doesn’t want to stand still, is what you’re saying.
FARHAD ZADEH: Yeah, that’s exactly right.
IRA FLATOW: Can’t take its picture.
FARHAD ZADEH: Yeah, you have to really wait until it becomes really quiet. And then you have a better opportunity. I think that’s one of the main reasons. Whereas M87, that black hole is actually active. But the activity is not on an hourly timescale. It’s on a much longer time scale, and we can actually image those galaxies further away, in some ways, better because they’re very quiet. Whereas here, galaxy center is happening, in a way. And a lot of people are really interested.
IRA FLATOW: Yeah. Well, we’ll have you back on when we get that photo.
FARHAD ZADEH: [CHUCKLE] Absolutely, yeah.
IRA FLATOW: Comes back from the drug store. We’ll have you on to talk about it. Thank you very much, Dr. Zadeh, for taking time to be with us today. Dr. Farhad Zadeh, professor of physics and astronomy at Northwestern University’s Center for Interdisciplinary Exploration and Research in Astrophysics.
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