Podcast: Galactic Exiles

Artist illustration of a galactic exile. Image credit: CfA. Click to enlarge.
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Fraser Cain: Can you tell me about the stars you observed and how they’ve come to be kicked out of our galaxy?

Dr. Warren Brown: What we discovered are two stars in the far out regions of the Milky Way that are traveling at speeds that no one has ever really seen stars in our galaxy, at least stars outside of the galactic centre. Except that these stars are hundreds of thousands of light years away from the galactic centre. And yet, the only plausible explanation for their velocity is that they were ejected by the supermassive black hole at the centre of the galaxy.

Fraser: So they strayed too close to the supermassive black hole and were kind of kicked out?

Brown: Yeah, so here’s the picture. This scenario requires three bodies, and astronomers say that the most likely way that it happened is if you have a pair of stars. As you may be aware, something like half the stars in the sky are actually systems containing a pair, or sometimes more stars. And so if you have a tightly bound pair of stars that, for some reason, travel too close to the supermassive black hole, at some point the black hole’s gravity will exceed the binding energy between the pair of stars and rip one of those stars away. It’ll capture the one star, but the other star then leaves the system with the orbital energy of the pair. And that’s how you get this extra boost of velocity. It’s that the supermassive black hole is basically able to unbind one star, capture it, and leave the other one with the entire amount of energy that the pair used to have. And that star then gets ejected right out of the galaxy.

Fraser: Then if a regular, single star came too close, it wouldn’t have the energy to be ejected. I think I’ve seen some simulations where the star gets too close to the black hole and kind of changes the direction of its orbit, but it’s still continuing to orbit around.

Brown: Sure, you could imagine it’s like a spacecraft that gets slingshot around Jupiter or something. You can imagine that you might be changing the trajectory, and gaining some speed. But there’s no mechanism in the galaxy to gain this much speed for something that’s the mass of a 3-4 solar mass star. That requires a three body interaction to create the velocity we see. And what we observe is their motion with respect to us. They’re moving away from us at a velocity of about 1-1.5 million miles an hour.

Fraser: How fast would the stars have been going when they came in to meet their breakup?

Brown: I don’t know for sure. Probably something 10 times that, right before that moment when they’re swinging past the black hole. Of course, as you leave that gravitational potential well of the black hole, they slow down pretty suddenly. Their final escape velocity is what we observe now; it’s on the order of a million miles an hour. And that’s well over twice the velocity that you need to escape our galaxy altogether. These stars really are exiles. They’re being outcast from the galaxy and they’ll never return.

Fraser: And one star is kicked out. What happens to the other star?

Brown: That’s an interesting question. In fact there’s a theory paper that some theorists have written that suggested that these stars in very long elliptical orbits around the central massive black hole might be the former companions to these so-called hypervelocity stars that we’ve discovered. And that’s the sort of orbit you’d expect. Unless the star is so unlucky as to fall straight into the black hole, if it misses just a little bit, it’s going to just swing around and then be on a very long elliptical orbit around the central massive black hole.

Fraser: And where did the pair originate? Is this a fate that might affect some nearby binary stars?

Brown: Well, that actually gets to the bigger picture. The galactic centre is an interesting place. It has lots of young stars. Three of the youngest massive star clusters discovered in the galaxy come from right near the galactic centre. And they contain some of the most massive stars in the galaxy. So there’s lots of young stars orbiting around down there. The question is, how do you get a star to tweak its orbit so that it shoots straight towards the supermassive black hole, instead of just orbiting around it, like the Earth orbiting the Sun. And that’s an open question. And one thing that these hypervelocity stars we’ve discovered are starting to give us hints about maybe how that mechanism works. Because, for example, one idea is that with these star clusters we’ve observed. Perhaps by dynamical friction, as they encounter other stars, they can sink slowly down towards the galactic centre where there’s the black hole. And it that were to happen, you could imagine that suddenly there were a whole bunch of stars right by that massive black hole. You could get a burst of these hypervelocity stars. There’s all sorts of stars to eject. And yet the stars that we observe all have different travel times from the galactic centre. This is only suggestive, but already we’re starting to be able to say something about the history of stars interacting with the supermassive black hole. And what appears so far, is that there’s no evidence for star clusters falling into the galactic centre.

Fraser: There could be some kind of conveyor belt that stars are born and then they slowly sink down and then they’re kicked out as they get too close.

Brown: Yeah, that’s sort of one idea. For that conveyor belt to work, you need some kind of massive place like a star cluster for that conveyor to work. To be able to sink something down towards the massive black hole. As a massive object encounters lots of massive objects, it turns out the less massive objects will tend to give off a little more energy. As the massive object, in this case a star cluster, loses energy, its orbit decays and it gets close to the galactic centre.

Fraser: With the few number of stars that you’ve found, and the large number of stars in the galaxy, it must have been a pretty difficult job to track these guys down. What was the method that you used?

Brown: Yeah, that’s actually one of the exciting results of this time. The first discovery, a year ago, after the first hypervelocity star, it was something of a serendipitous discovery. And this time we were actively looking for them. And the trick was that these things ought to be very rare. Theorists estimate that there’s perhaps a thousand of these stars in the entire galaxy. And the galaxy contains over a 100 billion stars. So we had to look in a way that gave us a pretty good chance of finding more of them. And our strategy was twofold. One is that the outskirts of the Milky Way contain mostly old, dwarf stars. Stars like the Sun, or less stars that are red. There’s no young, blue massive stars, and that’s the kind of star that we decided to look for; stars that are young, and luminous so that we can see them far away, but where there shouldn’t be these stars like that in the outskirts of the galaxy. And the other part of the strategy was to look for faint stars. The further out you go, the less background galaxy stars you have to contend with. And the more likely you’ll come across these hypervelocity stars, as opposed to another star that’s just orbiting the galaxy.

Fraser: And what’s the method you use to actually tell how fast that the star is moving?

Brown: For that we had to take a spectrum of the star. Using the 6.5 MMT telescope in Arizona, we pointed the star at one of our candidate stars and we take the light from that star and we put it into a rainbow spectrum and take a picture of that spectrum. And the elements in the stellar atmosphere serve as a fingerprint. You can see absorption lines due to hydrogen and helium and other elements. And it was using the motions, the Doppler shifts – in this case the red shifts – of those wavelengths told us how fast the stars were moving away from us. And most of the stars in our sample were normal galaxy stars; they were moving fairly slow velocities, and then two of these happened to be traveling quite fast, and that’s the two that we announced just now.

Fraser: And what do you think this tells us about the formation of stars, or the centre of the galaxy, or…

Brown: Well, that’s actually an interesting part of the story this time around. Now that we actually have a sample of these, these are really a new class of objects, these hypervelocity stars, we can start to say something about where they come from, which is the galactic centre. These stars are uniquely suited for telling us the story about what’s been happening at the galactic centre. Their travels times tell us something about the history, what’s been happening, but also the kinds of stars we’re seeing. In this case, these young, blue stars – these 3-4 solar mass stars – which astronomers call them B-type stars. The fact that we’ve seen two in our survey region, which we’ve carried out for about 5% of the sky, is consistent with the average distribution of stars you’d see in the galaxy. But inconsistent with what a lot of these stars clusters you see in the galactic centre. So just the fact of the type of stars you’re seeing is starting to tell us about the population of what’s been shot out of the galaxy. In this case it doesn’t look like it’s these supermassive clusters of stars, but rather your average star that’s wandering through the galaxy.

Fraser: And if you had some kind of super Hubble telescope at your disposal, what would you want to look for?

Brown: Oh, we’d want to look for the motion of these stars in the sky. So all we know if their minimum velocity. The only thing that we can measure is their velocity in the line of sight with respect to us. What we don’t know is there velocity in the plane of the sky, the so called proper motion. It’s possible to do that with Hubble, if you have 3-5 year baselines with which to see these stars move. It should be a very small motion. If you had a super Hubble, maybe you could see it in a year. So that would be very interesting to know. Not only would that tell you for sure that these really are coming from the galactic centre, and not from some place else, but also their trajectories. If you knew exactly how they’re moving out, any deviation off a straight line from the galactic centre tells you about how the gravity of the galaxy has been affecting their trajectory over time. And that’s also very interesting to know.

Fraser: Right, so that would help with plotting out the distribution of dark matter.

Brown: Exactly, exactly. So astronomers infer the presence of dark matter. We see stars orbiting the galaxy faster than they should be just because there appears to be mass that we can’t account for holding them in their orbits. And this dark matter, it’s hard to get a handle on how it’s distributed around the galaxy. But these stars are already at the outskirts of the galaxy, and as they pass through it, this perturbation, this gravitational pull of dark matter as these things travel through the galaxy slowly adds up as they go. So they’re actually measuring the distribution of this dark matter, just on their orbits. So if you could measure their motion, of a sample of stars, it actually starts giving you a handle on how the dark matter is distributed around the galaxy.