Intermediate Mass Black Holes: fact or fiction?

Here's two things that we know to be true about black holes. So, when a massive star dies and goes supernova, the core of the star collapses down under gravity and forms a black hole. For that to happen, the black hole that's formed at the end has to be heavier than three times heavier than the sun. The second thing that we know to be true about black holes is that there is a super massive black hole at the center of every galaxy, every island of stars in our universe. And those super massive black holes are anywhere from a million times heavier than the sun up to tens of billion times heavier than the sun. But those two facts that we have lots of evidence to support means that we have a gap. both a knowledge gap and a mass gap. We don't find any black holes between 100 to 100,000 times heavier than the sun. The only way that we know that a black hole can form that we have enough evidence to support is through a supernova when a star dies, which only makes what we call the stellar mass black holes from around 3 to 100 times the mass of the sun. So, if you've got super massive black holes in the centers of galaxies, then presumably they formed through a supernova and have grown from three times heavier than the sun to 3 million times heavier than the sun. But that means they'll need to slowly grow through that mass gap. So, why don't we find any black holes here? what's known as intermediate mass black holes, also known as IMBH's in astrophysics lingo. So, in this video, we're going to dive into this and chat first about how we find black holes, then chat about why intermediate mass black holes are particularly difficult to find, and then finally, the possible candidate intermediate mass black holes that we know of, including one that was very recently reported on by Changen collaborators using observations from the Hol Space Telescope. But before we dive into all of that, one of the most common questions I get asked by subscribers is if someone wants to become a scientist, what's the best thing that they can do to help them along that journey? And my answer has been the same for years. And that is learn how to code, which I admit is not an easy task, not least the fact that you have to selfotivate to do it. But this week's video sponsor, boot. dev, Dev makes learning to code just that little bit easier. By using tactics learned from modern game design to push you towards your learning goals. With Boot. dev's hands-on lessons that balance like theory with practice, you can gain the skills that you need to forge a successful career. Whether that is as a scientist or maybe it's as a back-end developer. Whether you choose to learn Python, the language of choice for many of us scientists, or JavaScript for web development, Boot. dev has you covered. with Boots, their AI wizard bear, trained on each lesson to help you if you get stuck. He doesn't just give you the answers. He asks you questions to deepen your understanding and guide you. But if you get really stuck, the solutions are still available. Now, coding is not something that you can learn overnight. You know, like any language, it takes time. And Boot. dev's curriculum reflects that, taking around about 12 months to go properly in depth on both the theory and the practice. All of Boot. dev's Dev's content is free to read and watch in guest mode, but a paid membership unlocks the interactive features like the hands-on coding, the AI assistance, and progress tracking. So, if you go to boot. dev, you can try out their courses for free first and then you can use my code drbecky to get 25% off your entire first year if you choose the annual plan or you can click on that link in the description as well. So, a big thanks again to boot. dev for helping people to learn to code and for sponsoring this video. And now let's dive in and chat first about how we find

black holes. Unsurprisingly, it's quite difficult to find black holes. It's in the name. Black holes are black. They're regions of space that are so dense, they have so much matter squished there that the gravity becomes so strong that not even light traveling at the fastest speed there is can escape the pull of the black holes gravity. We call the edge of this region where we don't get any light from the event horizon. It's a sphere in space where we know nothing about what's going on. That means that unlike stars which emit light so we can see them or planets which reflect light so that we can see them, black holes are completely dark inside that event horizon. We get no information and no light from them. So it's very tricky to know that they're there. But we can be clever about this. So there are three main ways of us detecting a black hole of knowing that it is there. The first is from their gravity. So they might not give out light themselves, but they exert such a strong pull gravitationally on the stuff around them that we can then detect their influence on how things are moving that we can see. That's how we know that at the center of our island of stars, our galaxy, the Milky Way, that there's a super massive black hole 4 million times heavier than the sun. We've observed the stars right in the center on stable orbits around the black hole with the closest one in just 12 times the Earth's sun distance from the event horizon. It takes 12 years to make one orbit traveling at about a percent of the speed of light. And from calculating all the orbits of these individual stars, we can then work out, okay, well, if that's their orbits, how heavy is the thing that they're orbiting around? And from that, we know that there's a super massive black hole there. Another trick of gravity is called lensing. Massive objects curve space itself, meaning that the light travels on that curved space and changes the direction that it's traveling in. That means a black hole can actually act as a lens. So say a black hole in our galaxy passes in front of a star in the background, it can actually brighten that background star, lens it so that we know the black hole is there. The second way that we can tell that black holes are there is through gravitational waves. Ripples through space itself caused by a big change in that curvature of space. So if two black holes are orbiting around each other that constantly changes the curvature of space and sends out ripples. If they then finally merge together then that's a big cataclysmic event and you get a very big ripple sent out. We've detected many of these events for stellar mass black holes using gravitational wave detectors here on Earth like LIGO and Virgo. And the biggest black hole that has ever been found using gravitational waves was an event that produced a black hole 225 times heavier than the sun. But we've also used pulsars out there in the universe at fixed positions in a big array that send out these pulses at very regular intervals. So if you get any changes in those regular intervals, you know a gravitational wave has passed through. And it's using this pulsar timing array method that we've detected much bigger gravitational waves that could be from the merges of super massive black holes. And the third main way that we find black holes is because matter spiraling around them lights up like a Christmas tree. If gas gets funneled towards a black hole, it gets accelerated up to huge speeds by the black hole's gravity, which means it has a lot of energy, so much that the material starts to glow. And I know this idea of like energetic glowing gas often throws people a little bit, but it is an idea that we're all familiar with. So, for example, if you take a tube and fill it with gas and then bend that tube into, I don't know, say a word that says bar and then run electricity through that tube, give the gas energy, it will start to glow. That's what neon signs are. And yeah, okay, with a black hole, this process occurs much more energetically. You're likely to get light in X-ray light and UV light, but also in visible light and infrared as well. So we can detect everything from stellar mass black holes in our own galaxy to super massive black holes in other galaxies by looking for the light from the material accelerated and superheated around the black hole. And the key thing is that the energy of the x-ray light that we see will be roughly proportional to the mass of the black hole that is doing the accelerating of that material. So if we've got these three main ways of finding black holes and you know we've found so many of them with these methods, why then are

intermediate mass black holes particularly difficult to find? Well, half the problem with finding intermediate mass black holes is that we haven't really found any. So we don't know what the best places to look for them are. We know that super massive black holes are found in the centers of galaxies and we know that the mass of the super massive black hole tends to be correlated with the mass in stars of a galaxy. So if we follow that correlation down then shouldn't we find intermediate mass black holes in dwarf galaxies perhaps? And if we wanted to look for intermediate mass black holes starting from the other side of the mass gap, then we know that the best place to look is where there are more stars because then that's where more stars have died and you're more likely to find a black hole. And if those stars are close enough together, maybe those black holes can merge to grow an intermediate mass black hole. So we also look for intermediate mass black holes in big star clusters where the stars are really dense. Basically, you either need a region of space where you've got a whole lot of stars or a lot of gas that should be able to feed a black hole and grow it up from a stellar mass black hole through intermediate mass black hole up to super massive black hole. So having said that, let's look at our three options of finding black holes again. Starting with gravitational effects. And the big problem here is that big star clusters are very chaotic places. So it's not as clear-cut as looking towards the center of our galaxy. The influence of an intermediate mass black hole is very slight in comparison to a super massive black hole. So you can't really pick out its influence on the stars orbits, which are more influenced by the other stars around it. The same is true for the center of dwarf galaxies as well, which are very diffuse and don't always have ordered rotation around the center. What about gravitational waves then? Well, the problem here is that our detectors are built to be sensitive to the stellar mass black hole merges. I've gone into this in more detail in a video before, but essentially, if you want to detect the gravitational waves from the merger of two black holes, then the length of those arms of your detector is proportional to the mass of the black holes involved. Currently, LIGO and Virgo have detectors that are roughly four kilometers long. To detect the merger of super massive black holes, the European Space Agency is planning to build the Lisa Observatory, a gravitational wave detector in space 2. 5 million km in size. So, we just don't have the tech yet to detect the gravitational waves from the merges of intermediate mass black holes if they're there and happening in these super dense star clusters. Okay, so what about X-ray emission from the material around the black hole? Well, this is probably our best chance of spotting them. But if intermediate mass black holes are forming in big star clusters, that's usually a very gas poor environment because all of the gas went into forming the stars in the first place. There's nothing there to feed the black hole to cause this material to spiral around it so that it glows so that we can know the black hole is there. That means any X-ray radiation given off would be much fainter, making them much harder to detect. And this is why we don't think we found a big population of intermediate mass black holes in big X-ray surveys of the sky that image, you know, the entire sky. If they're there, they're just not standing out above the noise. But having said that, slowly but surely over the years, there have been more and more candidate intermediate mass black holes that have been found.

So let's go through some of these possible candidates that we know of. Starting with the less convincing candidates. So 47 Tucani, which is a globular cluster in the Milky Way itself, which can actually be seen with the naked eye in the southern hemisphere. And trying to do the analysis of the movements and the orbits of the stars in 47 Tukini have led some to claim there's a black hole of around 2,000 times heavier than the sun. But again, the movements of the stars are so chaotic that there are other models that don't need a black hole to explain that data at all. The same could also be said for my 2 around Andromeda, also known as G1M31, which people have claimed has an intermediate mass black hole 20,000 times heavier than the sun. And as we just heard, these methods of detection are very difficult. Hence the doubt over their candidacy as intermediate mass black holes. The stronger candidates come from X-ray detections. Specifically, X-ray detections that are found, you know, off center from a galaxy because if it was at the center, then it's probably the super massive black hole. If you can find them off center, then you think, okay, maybe this is an intermediate mass black hole. But these are very rare and again therefore difficult to find. The most famous of these is a very bright X-ray source detected right on the outskirts of the galaxy ESO24349 back in 2009. The energy and the brightness of the X-rays suggest that it's coming from material spiraling around an intermediate mass black hole. And follow-up work showed there was evidence of a young star cluster surrounding that X-ray emission and also a detection in radio light that helped constrain the mass of the black hole to around about 10,000 times heavier than the sun. Similarly, there's also M82X1, a very bright X-ray source that was detected on the outskirts of the galaxy M82 in 2003 that also fits the bill for an intermediate mass black hole. And really, the bulk of all our intermediate mass black hole candidates come from similar X-ray detections like this. The latest of those comes from this work by Changen collaborators who've used X-ray data from 2009, 2012, and 2023 to once again study a very bright X-ray source on the outskirts of a galaxy. this time NGC6099 which Chang and collaborators refer to as NGC 6099 HLX1 or just HLX1 for short. And what's fun about this object is that it's X-ray emission is variable. We've detected a different amount of X-ray every time it's been observed over the past 15 years. So, it got brighter from when it was first spotted in 2009, reaching that peak in 2012, and then fading again to much fainter in 2023. Once again, Hubble has spotted that there's a star cluster in the same spot as the X-ray is coming from. So, to explain both the fading and the energy of the X-rays that's been observed. And considering the fact that most star clusters don't have a lot of gas hanging around, the most likely explanation Chang and collaborators think to explain what they've seen is that a star got too close to the black hole, got shredded, and now the black hole is lighting up the remains of that star. and we're slowly seeing it fade as it runs out of the gas to feed on. Now, more observations should be able to provide, you know, more evidence or less evidence for that hypothesis, but also should be able to confirm the mass of the suspected intermediate mass black hole, which is currently estimated at somewhere between a,000 and 10,000 times heavier than the sun, putting it smack bang in the middle of that elusive mass gap between super massive black hole and stellar mass black holes. But there's one place that we haven't talked about when it comes to detecting intermediate mass black holes and that is the very early universe when the first galaxies and their super massive black holes were forming and presumably going through an intermediate mass black hole phase. So perhaps the next intermediate mass black hole discovery will come from the James Web Space Telescope which has been designed and built to see what's going on in the early universe. So, I've got my fingers crossed that with JBST, we can fill in this mass gap, this knowledge gap that we still have about black holes, but only time will tell. There's an airplane going overhead, so

I'll just wait while it goes overhead. Sam's sending me pictures of Pip while I film. She's so cute. Look how cute she is. Oh, you can't see cuz the reflection. And then she closes her That's the doorbell going. I presume that is our skip being delivered. Tukini tukane. Oh, baby girl, you look so cute there. Excuse me while I just look up how to pronounce this. You can't see her. Hang on. Look how cute she looks. Anyway, I'm going to put that there so that you can see her and I'll move over here. Let's see. You can see your little face.