Ep. 680: Rogue Black Holes

Last week we talked about rogue stars. This week we’re going to take things up a notch and talk about an even more extreme event. Rogue black holes. Astronomers recently discovered a supermassive black hole on an escape trajectory, leaving newly forming stars in its wake. It’s wonderful, terrible, nightmare fuel.


(This is an automatically generated transcript)

Fraser Cain [00:01:50] Astronomy Cast Episode 680 Rogue Black Holes. Welcome to Astronomy Cast, her weekly fact space journey through the cosmos, where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, I’m the publisher of Universe Today. With me is Doctor Pamela Gay, a senior scientist for the Planetary Science Institute and the director of Cosmic Quest. Hey, Pam. How are you doing? 

Pamela Gay [00:02:10] I am doing well. How are you doing? 

Fraser Cain [00:02:13] I’m doing really well. Like, I, you know, like, I know we talk about the weather a lot. This channel. Like, maybe this should just be the weather podcast weather cast, but. But spring arrived. Finally. Finally. And with a with a roar. So we had records broken all across British Columbia because we had such a warm spring day. 

Pamela Gay [00:02:35] I learned last week that asparagus is capable of growing four feet in one week. 

Fraser Cain [00:02:42] I got a plate of Fergus. I love asparagus and and it’s like easy to grow and it’s perennial. And yeah, I gotta get that going on. 

Pamela Gay [00:02:50] It’s a multi-year dedication to it. And this was supposed to be the first year we could harvest it, and it was just coming up above the ground at the beginning of the week. And it rained all week. And I went outside with scissors ready to harvest. And the staff was like. 

Fraser Cain [00:03:06] You still, you could still harvest. You just cut the top and then it’ll throw more up and then you cut those up. 

Pamela Gay [00:03:11] That’s what I did. 

Fraser Cain [00:03:12] Going. Yeah. Yeah. Last week we talked about rogue stars. This week we’re going to take things up a notch and talk about an even more extreme event rogue black holes. Astronomers recently discovered a supermassive black hole on an escape trajectory, leaving newly forming stars in its wake. It’s a wonderful, terrible nightmare. Fuel, right. So last week we talked about rogue stars and how they are, you know, they adorably fly out of the galaxy on escape velocities thanks to close encounters with rogue black holes. But black holes themselves, yes, can be ejected and probably are often ejected. 

Pamela Gay [00:03:52] So the thing about rogue black holes is, is the definition for rogue black holes incorporates so much more than rogue stars with with rogue black holes, it’s any black hole that doesn’t have a companion. And and there’s thinking that there could be a 100 million rogue black holes just randomly making their way through the Milky Way. And that is awesome. 

Fraser Cain [00:04:16] So, I mean, is just a black hole a rogue black hole? 

Pamela Gay [00:04:19] If a black hole has a companion, a disc, something that it can feed upon so that it is not alone in this universe, it is not a rogue black hole, but a black hole by itself, minding its own business, not eating anything, or that has been ejected out of a galaxy just to confuse things. Those are rogue black holes. 

Fraser Cain [00:04:44] Right? But like, does that mean like a star is a rogue star? No. 

Pamela Gay [00:04:47] No, because we can see a star line. 

Fraser Cain [00:04:51] So. So if we can’t see it, then suddenly it’s gone rogue. I guess that. Yes. 

Pamela Gay [00:04:58] Yeah. 

Fraser Cain [00:04:58] Right. All right. Okay. You know, I think that tracks fine. So so I guess what. But it really is. It does seem to be approaching that all black holes are rogue black holes, which is. Which is fine. 

Pamela Gay [00:05:10] I think such a star is not rogue. It has not gone rogue. It has gone evil. But it is not sure, sure. 

Fraser Cain [00:05:17] But like Cygnus X-1 has not gone rogue because it has a companion and we can see it. But it’s is rare. 

Pamela Gay [00:05:23] It is capable of maintaining relationships. They just aren’t healthy ladies. 

Fraser Cain [00:05:28] And as soon as it’s finished destroying its companion, it has gone rogue. Yeah. All right. Fine. I, I, I will accept your expanded terminology of what is a rogue object, which in this case is just any black hole. 

Pamela Gay [00:05:45] That isn’t. 

Fraser Cain [00:05:46] It isn’t tied down. But is it tied down? Yeah, exactly. Yeah. That’s fine. All right. Fine. So let’s talk about the ones that are drifting within the galaxy. 

Pamela Gay [00:05:56] So one of the early thoughts of what dark matter might be is just rogue black holes and, neutron stars that we couldn’t see called white dwarfs. And so researchers set up two different long term observational programs, Ogle and Macho. One was pointed towards the center of our Milky Way galaxy, looking at all the stars in the bulge and looking for them to brighten and fade away as something passed in front of them and gravitationally magnified their light. The other one was pointed out, looking at the Large Magellanic Cloud, and was looking for the exact same thing, and the thought was if there are sufficient stellar remnants. Out there to be dark matter. We would, within a short period of time, be able to see all of them. And we did not. But over the years they have firmly seen one seven solar mass black hole that over a long period of time, gradually passed in front of a background star and said, hi, I am here with its gravity, right? And that that is our. Our prototype for rogue black holes in the Milky Way. 

Fraser Cain [00:07:18] And I mean, this is the same technique that astronomers have used to discover planets. They watch as a star moves in front of a background star, and the star flares up in a way that that has an almost like a double flare. And you can see the distortions not only of the gravity of the star, but the distortions from the planet itself, and they identify the planet. The downside of that is that you don’t get a second shot at it. Like once. 

Pamela Gay [00:07:43] This very. 

Fraser Cain [00:07:43] True. Once in a lifetime situation lines up. You get to see it for a couple of hours and then it’s gone. And so they they just like, stared at the entire galaxy hoping to see any of these alignments. And they saw one. 

Pamela Gay [00:07:59] Yes. 

Fraser Cain [00:08:00] And so dark matter, though, being 90% of the matter in the universe, you’d think you’d see more. 

Pamela Gay [00:08:08] Well, this just means the dark matter is not predominantly made of stellar remnants. It is stuff that we now define through its distribution of particles that don’t like to interact, that do not interact with the electromagnetic spectrum that may be sterile neutrinos and other similar particles. Yeah. So tiny bit of dark matter, red black holes, most of dark matter stuff. 

Fraser Cain [00:08:38] Particularly stuff. 

Pamela Gay [00:08:39] Particle stuff. 

Fraser Cain [00:08:41] They haven’t ruled them out. They just have ruled them out at a certain sizes. Right? From what I understand, they can either be a thousand times the mass of the sun, or they could be the mass of an asteroid, but you can’t have them in between because of these surveys that have ruled out the in-between stuff. And so the really, you know, the intermediate mass black holes, they’re not going to line up enough so they could still be out there while the teeny tiny ones, they’re not going to cause enough of a distortion that you’re going to be able to detect it. 

Pamela Gay [00:09:07] And and this is where there, there is still the potential that a noticeable amount of dark matter is going to be made of. These missing intermediate black holes is going to be made up of, microscopic black holes. Primordial black holes haven’t evaporated away, but. The more we see distant galaxies getting distorted by all of the intervening clouds of dark matter, the more evidence we have that the dominant part of dark matter, which is going to turn out to be a whole family of different things, working together to break our brains. 

Fraser Cain [00:09:49] Yes. 

Pamela Gay [00:09:49] It’s it’s going to be particles. 

Fraser Cain [00:09:51] Yeah. And just to put a pin in that like. Yeah, allow me to predict the future. Well, when people go like it turns out dark matter was. And then you list out these ten things, right? That it was it was neutrinos and gas and black holes and a kind of particle that we didn’t know about before, and a different kind of particle that we didn’t know before, and another different kind of particle that we didn’t know before. And we did nursing gravity at the largest distances. So yes. And yeah. All right. So we’ve talked about just like roaming black holes, ones that are just just gently drifting cosmos causing havoc in their wake. But what about ones that are on like regular stellar mass black holes? Can they get on an escape trajectory? 

Pamela Gay [00:10:43] Yeah. 

Fraser Cain [00:10:43] Like stars. 

Pamela Gay [00:10:45] Totally. And and again, it’s it’s going to come from that same sort of multi-object interaction. And it’s, it’s now starting to look like there, there are many systems out there that have multiple, massive objects in binary systems. And all they need to do is find a third friend to send somebody off flying where we’re seeing gravitational waves from black hole, black hole mergers. Now, if instead of having two that are merging, you have a third one come in. And there’s lots of multi star systems out there that’s now unstable. And so something’s going to get thrown away and can escape. And it’s not necessarily just going to happen with stellar mass black holes. Do we want to go there. Do we want to go to the paper. 

Fraser Cain [00:11:43] Night. Which one. The but supermassive not. Yeah. So so one thing I want to talk about like just regular you know we talked about this last week. We talked about how you can have. They can have a close encounter with a supermassive black hole. They can be part of a binary star system. We talked about how one star can detonate as a supernova and the other star can be thrown out. But yes, but could one of could one explode and then the other one. It’s been turned into a black hole. 

Pamela Gay [00:12:11] So the the. The idea that you can have a system where both stars become black holes and they fling themselves apart, they aren’t going to have a sufficient difference in mass for conservation of momentum to make one of them a high velocity star. Part of the magic of the rock stars we talked about last week is you have high mass black hole, neutron star, really dense object. It’s going away in one direction. And then the other lightweight thing is like, shoot. So imagine that you decide you’re going to put a. Heavy weight ball. I’m cricket a cricket ball on your pool table. And you hit your cricket ball with that little white normal cue ball from pool. It’s going to move a little bit, and the other star is going to come flying back at you. And and so that’s how we get rogue high velocity stars. Now if you just have two black holes, they’re going to wander apart from each other. But they’re going to still have that more normal 45km per second. But one might end up going the wrong direction. So it’ll still seem to be moving at a faster velocity. But it’s not as exciting because we can’t see them until they decide to try and kill us, in which case we won’t see them until they encounter the Oort Cloud. And that’s always. 

Fraser Cain [00:13:42] Fun right now. UMass together. Croquet, cricket and pool bear, I think. 

Pamela Gay [00:13:47] Yes, yes I did. 

Fraser Cain [00:13:49] Okay. But, but, you know, I remember when we talked about the different kinds of supernovae, there’s one called the pair instability. Yeah. Supernova, which is the largest stars where they just explode completely. So could you have, say, one star with a merely 20 times the mass of the sun and then another one with 100 times the mass of the sun orbiting one another, and then 100 mass. One goes first and now it’s apparent stability. Supernova was just gone. And now that other star is now thrown out, and then it later decides to go off as a supernova. 

Pamela Gay [00:14:25] So I’m trying to figure out how that mass would get flown out preferentially in one direction, so that the other star with. 

Fraser Cain [00:14:36] One star just disappears, right? Yeah, the 100 100 solar mass star just vaporizes. No remnant left behind. 

Pamela Gay [00:14:44] Right. 

Fraser Cain [00:14:44] And the other one, the 20 solar mass star, is now. 

Pamela Gay [00:14:47] Hanging out, going at the velocity it was going before. 

Fraser Cain [00:14:51] But wouldn’t it be? It was in an orbiting around the the center of mass of 100 mass star. And so now it’s just careening off like regular stars. Right. They’re they’re busting around each other. One now is no longer exists. The other one is thrown out like a like a slingshot, but it’s still the kind of star that’s going to go supernova in the future. And so it’s heading out on an on an escape trajectory, and then it explodes as a supernova. And now you’ve got a black hole. 

Pamela Gay [00:15:17] So. So the explosion is going to carry away the angular momentum of star one because boom star 2nd May have. Well, now it will have a change in its velocity, but it’s not going to be huge because you don’t have enough angular momentum to fling it off. Right. Unless you do something to the other star. 

Fraser Cain [00:15:44] Well, one star is no longer there once it disappears. 

Pamela Gay [00:15:47] But but it’s it’s the case of you have a ball on a string, and when you let go, yes, it flies off, but it’s it’s not getting terribly huge amount of velocity. Whereas with the rogue star’s situation, you have one object in motion in one direction and the other, that it’s that extra component that you get right, that gives it the extra kick. 

Fraser Cain [00:16:16] Got it. Yeah, yeah, yeah. It’s just like it’s it’s not enough angular momentum tied into the interaction that when it lets go it’s it’s a more serene escape. Right. Right. So this idea that of a rogue black hole. Because people ask me this question all the time, they’re like, oh, if a rogue black hole came into the solar system, so play that out for me. What would happen if a rogue black hole made its way through the solar system? What would happen? 

Pamela Gay [00:16:42] So it all depends on the crossing time. So it turns out that if a black hole goes through our system super, super fast, everything’s going to go, wait, I’m being pulled. Oh, I’m not being pulled on anymore. Right. And so things get jiggled around and that could be truly catastrophic. But it doesn’t necessarily have to be. Where it really sucks is if it’s coming along and it’s approaching from behind, and it’s just going a few kilometers per second faster than us through the galaxy, and it meanders slowly through the system. And as it meanders slowly through the system, it’s able to pick up everything within the system and cause them to enter orbit around it and potentially eat everything as it goes, depending on how much friction it creates. 

Fraser Cain [00:17:39] Right or cause through body interactions, it causes things to do. Things crash into other things than those things being squished out added to the accretion disk. So? So the distance, like how close does it get to the sun and how? Slowly. Does it make this journey. 

Pamela Gay [00:17:56] Exactly. 

Fraser Cain [00:17:57] Right? Yeah, but. But what I love is that we don’t see any examples of it in the in the solar system has been here for 4.5 billion years. And so everything like we do not look like the kind of place that a black hole passed nearby at any point in the last almost 5 billion years. 

Pamela Gay [00:18:16] So so the thing to think about is we cannot historically tell the difference between a large star passing near solar system and a black hole passing near our solar system. What we can say is, to the best of our understanding, there hasn’t been a high mass object that passed through our solar system, but we do think that there has been some sort of gravitational disruption that has flung large amounts of comets in at various points. So we could have taken a glancing blow, and we wouldn’t have the capacity to tell the difference between a glancing blow from a black hole and a glancing blow from a star. And that’s one of the most wonderfully weird parts of science, is you can have a whole variety of different things, different masses, different speeds that have the exact same effect on our solar system. We just know nothing destroyed us, which is good. 

Fraser Cain [00:19:24] All right. We’ve been waiting. Let’s talk about it. Let’s talk about these. So we’ve talked about just regular old black holes. The common variety black holes. But supermassive black holes can go rogue, too. Yeah, a star. A black hole with millions or even billions of times the mass of the sun can be on a escape trajectory. Yeah, causing mayhem in its wake. 

Pamela Gay [00:19:51] And. And what I love is we have periodically found, galaxies that don’t appear to have a supermassive black hole in their core. And that makes no sense by any of the dynamics we know. And we now may know why. There was a paper posted to archive several months ago that you discovered in the original passing. 

Fraser Cain [00:20:14] Yeah, I reported it three months ago. Yeah. And now everybody’s. Yeah. I’m not bitter. 

Pamela Gay [00:20:21] Thank you. And and this particular paper, what happened was researchers, first noticed that there was this fairly dwarf ish galaxy that resembled dead bugs on a windshield. And they looked at it, and there was a stripe coming off of it. Just this, like straight line filament of light coming off of the dwarf galaxy. And we see stuff like that out there. Galaxies have jets, galaxies, all sorts of different stuff happens. But when you have a jet, it’s just like a, water fountain. The water starts out tightly collimated and then widens out as it moves away from the source. A galaxy has a jet. It starts out tightly collimated and then widens out as it moves away from the galaxy. But this this stripe that they saw in their Hubble images turned out that it started out wide and got tighter and tighter as it moved away. So they had some time coming up on CAK. They used that time to look at the system, and they looked at the galaxy, they looked at the system, and it turned out that the stripe, the line was baby stars, and they had the same general age as as that dwarf galaxy. And so you have a dwarf galaxy and a line of stars, same age, same redshift. What can cause that? And so they got to imaging and they got to thinking and they got to thinking and they got to imaging. And what they realized is, well, when you end up with these dead bug shaped dwarf galaxies, it’s usually because mergers have taken place and the physics of three galaxies coming together, having all three supermassive black holes begin interacting could lead to two supermassive black holes going in one direction, one supermassive black hole going in the other direction, and that runaway singleton colliding through the intergalactic media was able to trigger star formation as it went. So basically, it’s going along. And as it’s going, it’s going come together gas come together, guys. 

Fraser Cain [00:22:49] Right. 

Pamela Gay [00:22:50] And this gas coming together is forming stars. And then over time they drift apart. 

Fraser Cain [00:22:55] And we always talk about that, that you need some some trigger to begin star formation. And we talk about it like a supernova shockwave maybe the gravitational fact of a nearby star. But in this case, a supermassive black hole was passing nearby and causing ripples of gas and dust to come together and cause over densities and begin this process of star formation in its wake. 

Pamela Gay [00:23:20] And and at the furthest and tightest, pulled together part of this line, they’re seeing unusual emission. That’s just enough different from everything else that it seems to be consistent with that supermassive black hole. And in the other direction, they see hints of what appear to be shocks in the intergalactic medium not enough to trigger star formation, but something indicating something in this way past. And left its mark. So now we have. Dead, bug shaped galaxy with no supermassive black hole and. Three rogue supermassive black holes, two flying together, one by itself, passing between the galaxies. And this raises the interesting idea that at any moment, a supermassive black hole could emerge from the darkness and begin interacting with with the space around our galaxy or any other galaxy. And it might not be easy to see, but it would sure be cool if it happened. 

Fraser Cain [00:24:33] Would you need that three body interaction? I mean, is there any way you can get a like a two body interaction with a supermassive black hole and less massive black hole? 

Pamela Gay [00:24:44] In that situation, they’re just going to merge, right? It’s you have to have that unstable third system to fling something away. 

Fraser Cain [00:24:52] But when you think about even the history of the of our local group, we’ve got the supermassive black hole, the heart of the Milky Way, the one in Andromeda. There’s probably supermassive black holes in several of the dwarf galaxies around us. 

Pamela Gay [00:25:03] Yeah. 

Fraser Cain [00:25:04] Triangulum doesn’t have one. Which way back to that? You know who knows. 

Pamela Gay [00:25:09] Everything that happens there? Yes. 

Fraser Cain [00:25:11] And so you can imagine two dwarf galaxies merging together there. Supermassive black holes haven’t merged yet. That combination reaches the center of the Milky Way and Andromeda, and. 

Pamela Gay [00:25:23] Then it can get flung. 

Fraser Cain [00:25:24] Yeah, and then one gets thrown out. Yeah. Yeah. And in those big galaxy clusters, the one seen early on in the universe, you’re seeing. Like just so much material coming together in these gigantic galaxy clusters. I mean, there aren’t many of them out there. There. There are dozens, maybe, that astronomers can see of the ones that are truly into the billion stellar mass black hole range. Right? And those are just the ludicrous, the forces involved in the kind of mayhem that’s going on in those places. 

Pamela Gay [00:25:59] The thing that this gets me thinking about is, for decades, it seems like every other American Astronomical Society meeting they’re announcing a new shredded dwarf. Galaxies star stream falling into the Milky Way has been discovered. And there is a chance that the majority of those stellar streams were, once upon a time, a dwarf galaxy associated with a intermediate mass black hole, a baby supermassive black hole. And so now those masses are somewhere. Somewhere. 

Fraser Cain [00:26:38] Yeah. Over there. Somewhere. 

Pamela Gay [00:26:40] Yeah. 

Fraser Cain [00:26:41] Could be here. Could be there, you know. All right, well, on that note, thank you mama. 

Pamela Gay [00:26:46] Thank you, Fraser. And thank you so much to our audience out there. You are making this show possible by supporting us through patreon.com slash astronomy cast. And if you aren’t a supporter and you want ad free versions of our show, consider joining our Patreon. This week I would like to thank Tim Baron, Paul Esposito, Bob Czapski, Arthur Lots Hall, Ron Thorson, Karen, careen, dump truck, Philip Walker, Hal McKinney, Bruno Latz, Jimmy. Bergeron, Jean-Baptiste. Le Martin. Ng. Timelord. IRA, Daniel. Donaldson, Frank. Stewart, Wil. Hamilton. Christiane. Golding, Michael. Pro toda Galactic president. Super. Oh, my, I this one doesn’t even fit on my screen. Galactic president scooper star mix scoops a lot. I love that name. 

Fraser Cain [00:27:42] It’s amazing. 

Pamela Gay [00:27:44] And shine phase. Thank you all so much for letting us pay the folks that make us look good. 

Fraser Cain [00:27:52] Thanks for all. We’ll see you next week. 

Pamela Gay [00:27:54] Bye bye. Astronomy cast is a joint product of Universe Today and the Planetary Science Institute. Astronomy cast is released under a Creative Commons Attribution license. So love it, share it, and remix it, but please credit it to our hosts, Fraser Cain and Doctor Pamela Gay. You can get more information on today’s show topic on our website. Astronomy. Cars.com. This episode was brought to you thanks to our generous patrons on Patreon. If you want to help keep the show going, please consider joining our community at Patreon.com Slash Astronomy Cast. Not only do you help us pay our producers a fair wage, you will also get special access to content right in your inbox and invites to online events. We are so grateful to all of you who have joined our Patreon community already. Anyways, keep looking up. This has been Astronomy Cast.