Astrum Space - Where Have All the Quasars Gone? | Black Holes Part 4

Episode Date: January 15, 2024

Join with me today as we explore the curious explanations behind this cosmic mystery.

Black holes are the densest objects in existence, with gravity so powerful that even light can't escape. As such, it comes as no surprise to find the largest black holes at the center of galaxies, where matter has been fed into them for billions of years, ever increase their mass. The very largest of these supermassive black holes can be billions of times the mass of our sun. However, it may come as a surprise to you to realize that some of the most massive mass of the most massive black holes we know of are actually the youngest.

You see, when we look at distant galaxies, we are also looking back in time, and the galaxy's billions of light years away often have the largest black holes. If the universe is only 13.8 billion years old, and light takes billions of years to reach us, that means the galaxy we are observing can only be a few billion years old at most, from our perspective. Pretty young for a galaxy. Surely though, it should be the case that nearer and thus older supermassive black holes are more massive, seeing as they've had so much extra time to consume matter falling into them. So what's going on here?

I'm Alex McColgan and you're listening to the Astroon podcast. Join with me today as we explore the curious explanations behind this cosmic mystery. The very largest supermassive black hole we know of is known as Ton 618, with an incredible mass of 66 billion solar masses. By itself, its mass is comparable to the Milky Way galaxy. However, Ton 618 is exceptionally far away, and it's taking light emitted by it 10.8 billion years to reach us, meaning we are observing it as it was 10.8 billion years ago. This means it can be at most around 2.8 billion years old.

By comparison, our own Milky Way galaxy is approximately 13.6 billion years old. Yet the supermassive black hole found at our galaxy's core, Sagittarius A-star, is only 4 million solar masses. The Andromeda galaxy's supermassive black hole, while bigger, is still only 200 million solar masses. One of the big factors to consider here is the difficulty in detecting and measuring black holes. This is still a really new field of research, as technology has only just allowed us to start observing black holes in the last few decades. Even then, we can often only observe the area surrounding black holes, that is, before the Event Horizon Telescope came along,

but even that telescope takes ages to image just one black hole, so our general understanding really is still quite limited. In fact, most of the distant black holes we know about can only be seen because they are quasars. Tons 618 is a quasar. Matter is pouring into the black holes accretion disk at an incredible rate, and because of this, it's erupted into a quasar. It's hard to fully grasp the physics of this disk, but it is believed that the friction here is so great, the accretion disk of a quasar by itself can produce thousands of times

more light than entire galaxies combined. 618 produces as much light as 140 trillion suns, completely outshining the galaxy it resides in to the point that we can't even see it from our perspective. However, because quasars are the brightest objects in the universe, they can be seen from very far away. So one reason for large black holes being far away is down to something known as Mountquist bias. This is where brighter objects further away appear more.

plentiful, when in reality we simply can't see the dimmer objects of that distance, implying there may be an argument that the largest supermassive black holes are actually distributed fairly evenly throughout the universe, and in between all those distant quasars are plenty of smallest black holes that aren't bright enough for us to see them. There could be plenty of large yet dark black holes much closer to us, and we haven't noticed them yet. The possibility is due to the nature of the universe itself shortly after the Big Bang. As you may know, the universe is ever expanding, and during the early universe, matter was

a lot closer together. Quasars need extreme amounts of matter falling into them to give off light, and there was a lot more gas around during the early stages of the universe. Not only has the universe expanded, but over time, gas gets converted into stars. Some of the largest types of stars eventually turn into neutron stars and black holes themselves, meaning that they never get recycled back into gas. Less available gas means less gas will fall into a supermassive black hole. One of the theories for the fate of the universe is actually based on this, called the Big Freeze,

where after some trillions of years, all the gas in the universe is eventually converted into black holes. Even now, we see some galaxies where their gas has been completely used up. meaning no new stars can form. These are called elliptical galaxies. Spiral galaxies still have gas and dust structures, and thus can still produce new stars. It is interesting that most of the largest supermassive black holes appear to be in elliptical galaxies where there is no gas left.

Gas needs to lose angular momentum to fall into the galaxy's central supermassive black hole, and one such trigger could be the gravitational influence of nearby galaxies, or even collisions with other galaxies. This would explain why elliptical galaxies have huge supermassive black holes, while spiral galaxies have smaller ones. In any case, there is less gas available in the universe now than there was during the early universe, so black hole growth may have occurred rapidly then, but a slowdown now. This might be why there is no quasar within 500 million light years of us, as the universe ages

and things become less chaotic and more spread out, the number of active quasars should decrease, which means the only quasars we see, which are also usually the biggest types of supermassive black holes, are the ones that happened a long time ago. So, why are the largest supermassive black holes often the furthest away? Well, although it may appear that way, it might not actually be the case at all. As studies continue and technology improves, we'll start to get to get a more complete picture of the universe around us. Well, that's all we have time for today.

I hope you've enjoyed listening to this podcast on Black Holes and Malmquist Bias. If you like what you've heard, please feel free to follow us for more podcasts on other fascinating space topics. But for now, I'm Alex McColgan, and this has been Astrum. All the best, and see you next time.