Supermassive black holes explained

Astrophysicist Julie Comerford and her group explain what supermassive black holes are and why they're so awesome and mysterious.

Video source: Piled Higher and Deeper (PHD Comics) / YouTube.

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VOICE 1: A black hole is a region of space where the force of gravity is so strong that nothing, not even light, can escape. Anything with mass can become a black hole. Say you weight 150 pounds and I compress you down a size smaller than a quark, then you would become a black hole. Or take the sun. Compress the sun down to a radius of 2 miles and the sun collapses and becomes a black hole.

One way to make a black hole is through the death of a star. If you have a star that is 20 times the mass of the sun or more, then as that star evolves through its life it's going to explode as a supernova and then what's left after, after the explosion, is a black hole.

But then there are black holes that are much, much bigger, so a million to a billion times the mass of the sun. And those are called supermassive black holes. Not just massive, that's not good enough, it's got to be supermassive. And it turns out that every big galaxy has a supermassive black hole at the center.

What my group does is we ask the question, How did these super massive black holes build up their mass? We don’t understand that part.

VOICE 2: One of the great mysteries here is where do these objects come from? If we're observing these supermassive black holes everywhere, and how do they achieve these huge masses? These supermassive black holes, they don't suck in material. That's a very common misconception. If you see a black hole, you're not just gonna be sucked straight into it. You are going to orbit around it just like it's a point mass. For example, if our sun were to become a black hole, we would keep going around it.

What I first thought about was why can't you just have some galaxy that just has a ton of gas in it and just feed this black hole to make it into this billion solar mass black hole. But those kinds of processes of just directly feeding it, that's not enough material to make it grow to what we observe.

So we specifically as a group study super massive black holes that are often undergoing some sort of large merger between galaxies. And we think that maybe super massive black holes merging together is one of the contributing factors that could explain why they’re so big in the current universe.

So we look for what we call a pair of quasars.

VOICE 1: So a quasar is a supermassive black hole with this big accretion disk of swirling gas and is really, really bright. So a quasar is like a black hole that you can… see. The biggest paradox about black holes is that they're the blackest things in the universe but they're also some of the brightest things in the universe. And that's because when you have a black hole and some gas is being gravitationally pulled towards it, the gas doesn't just fall straight in. Just like if you fill up your bathtub with water and you pull the plug the water doesn't just fall straight down the drain. It kind of swirls around and then goes in.

So the same thing happens with gas around a black hole. Gas is pulled in gravitationally, it swirls around and it forms this thing called an accretion disc, it's just a disc of gas kind of waiting its turn to fall into the black hole. But the black hole is so much mass, you're so close to it, that gas is swirling really fast, it gets heated up and emits lots and lots of light. So that accretion disk can create more light than all the stars in the galaxy combined.

VOICE 3: Say you have two Galaxies each with its own super massive black hole at the centre. When these two galaxies merge together, the two supermassive black holes come together and form a black hole pair. The black hole pair then merges together to become one more massive black hole.

The galaxy merger itself also creates a huge disturbance in the stars and gas in those merging galaxies. It's like a train wreck of disturbed gas with a lot of the gas being funnelled toward the center of the galaxy and feeding the super massive black hole at the center. What we're trying to do is learn about how much galaxy mergers can change the way black holes and galaxies evolve together.

VOICE 4: It is fascinating to imagine that something the size of a penny (that would be the supermassive black hole), could have an impact on something as big as the Earth (that would be the galaxy).

The way that black holes evolve is tightly connected to the way that galaxies evolve. If I was plotting the mass of the galaxy against the mass of the black hole, you’d find that this is a tight line with the mass of the black hole increasing in step with the gas of the galaxy. So to understand how galaxies evolve, you have to understand how black holes evolve.

VOICE 1: I think we're unique as a species because we pay to build telescopes, to understand these big questions of our origins and where we came from. Do you think a monkey would give up one banana to understand more about where it came from? No. A monkey would hold on to that banana and say, "You can't take it away from me". So, you're not going to be able to go out and change anything about the way the universe is evolving and galaxies colliding, they're not going to impact you on a personal level. But our lives would change if we could think even on a little bit bigger context, like a global context and taking care of our planet, for example.

This is just part of pushing ourselves to look a bit bigger. Bigger than just what's right in front of our noses.

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