Date: July 9th, 2012
Title: Encore: An Introduction to Active Galactic Nuclei
Podcaster: Olaf Davis & Renee Hlozek
Organization: Cosmic Web: www.cosmic-web.co.uk
This podcast originally aired on February 21st, 2010
http://365daysofastronomy.org/2010/02/21/february-21st-an-introduction-to-active-galactic-nuclei/
Description: Active Galactic Nuclei (AGNs) are formed when enormous black holes consume material and spew out energy in jets many thousands of light-years long. This energy output, which can be up to a thousand times brighter than the galaxy itself, has a profound impact on the development of the host galaxy and its formation of new stars.
Bio: Olaf is a second-year PhD student in Oxford Astrophysics. His research involves computer simulations of astronomical phenomena – these include the behaviour of energetic particles around the jets of AGNs, and also the large-scale distribution of galaxies across the Universe. His blog, the Cosmic Web, is about astronomy and aimed at the layman.
Renee is in her second year, at Christ Church college Oxford, reading for a degree in Astrophysics. Her research interests include Dark Energy and decoding information contained in the Cosmic Microwave Background radiation, Baryon Acoustic Oscillations and Type-Ia Supernovae. She’s also interested in new methods of parameter estimation and forecasting. She’s passionate about outreach and public understanding of science.
Today’s sponsor: “This episode of 365 days of astronomy was sponsored by iTelescope.net – Expanding your horizons in astronomy today. The premier on-demand telescope network, at dark sky sites in Spain, New Mexico and Siding Spring, Australia.”
Transcript:
O: Hello, my name is Olaf Davis.
R: And I’m Renee Hlojek
O: And we are both PhD students at the Astrophysics group in Oxford. Now, today we’re going to be talking to you a bit about a type of object called an ‘Active Galactic Nucleus’ or AGN. And it’s called that because it’s in a galaxy; and it’s in the centre of the galaxy, the nucleus – it’s nothing to do with nuclear physics. And the active part means that these are galaxies that are senidn gout lots and lots of very high energy radiation.
R: We thought it would be a good thing to talk about because it’s first of all very important for star formation and for how we understand how galaxies evolve, but also it involves balc holes and kind of little-known things about them.
O: That’s right. Probably everyone listening has heard of black holes and is somewhat familar with them, but people tend to think of a black hole as something which just sucks stuff in; it sucks material into it and it disappears. And that’s right but there’s also an aspect of that process which causes material to shoot out from the area around a black hole. So the basic idea is this: if you have some object that’s orbiting round a black hole and it’s slowly falling in and is going to get sucked in, that object is going to lose a lot of energy in that process, right: if you imagine, say, a space rocket that’s orbiting round the Earth it takes a lot of energy to keep that rocket moving. And if that rocket wants to come down and land on the Earth it’s got to lose all that energy some how. Now you’ve probably all seen videos of a space rocket coming in through the atmosphere, and it starts to heat up and glow red-hot and release lots and lots of energy into the air. Now if you imagine a rocket falling into a black hole there’s no air for it to go through and heat up with. But if there’s lots of other objects – other rockets, or bits of gas, or stars, or dust or whatever things are floating round in space – they’re all falling down together, and they can all bump into each other and start to kind of rub together and heat up and produce lots of radiation and release their energy. And once they’ve release enough energy they will fall into the black hole.
R: OK, so we know about black holes being black because they suck everything in and it can’t get out, right – but now you’re saying that energy’s coming out of the black hole?
O: Yeah, so it’s not quite that it’s coming out of the black hole: you’re right that anything that falls into the black hole itself will disappear and we’ll never see it again. But it’s this area of stuff around the black hole that’s on its way inwards that’s heating up and outputting this energy. And the geometry of these things is actually quite interesting. Because all this material that’s falling inwards – you don’t just get a symmetric cloud of it. It tends to be in a kind of disc around the black hole that’s spiraling inwards, kind of like a doughnut shape that’s slowing falling inwards towards the black hole. And this energy which is produced by this infalling stuff comes out in a kind of jet. If you imagine a doughnut that you’ve just slid over a stick, and the doughnut is the stuff falling in, then the stick would represent where the energy is being fired out in two kind of beams that can shoot out a very long way from the black hole.
R: And they’re coming out perpendicular to the way the material’s coming in.
O: Yeah, exactly. So if we look out into the distant universe and we see one of these Active Galactic Nuclei in a galaxy we won’t see the black hole itself, but we’ll see this jet of material coming out and we might see this glowing disc around the black hole.
R: OK, so these are the ‘hot rockets’ that are all trying to smash their way into the black hole.
O: Yeah, that’s right.
R: Good.
O: And I said at the beginning these things are called Active Galactic Nuclei: and that’s because these huge black holes which we’re talking about tend to live in the very centre of a galaxy. Most galaxies that we know about in the universe have a really massive black hole in the centre – we’re talking about maybe a milllion times the mass of the Sun. And lots of things in the galaxy – stars, clumps of gas and dust – will slowly fall down into this, and the amount of energy released by all these trillions and trillions of tons of material falling into this enormous black hole is astronomical, literally. So, although we can’t see the black hole itself, in a lot of these galaxies we can see this very bright radiation being emitted from just outside the black hole.
R: What I really like about the idea is that you can see things heating up and moving together, and you talked about Active Galactic Nuclei being at the centre of galaxies and of course that is really important for things like star formation.
I suppose if I am going to explain things I should kind of go back to the beginning and just quickly summarise what we mean by star formation. We know that we have huge, dense clouds of molecular gas and dust that are cooling to form stars, and once they reach a certain mass, then they collapse under their own gravity, and that is how stars form, but if we kind of go up a scale, then we also know that galaxies themselves have a lot of gas at the centre, and we can work out how long it would take for the gas at the centre to cool. The strange thing is if we calculate how long it would take for the gas to cool, it is much shorter than the lifetime of the galaxy. So it seems as though galaxies should just be really, really cold, but we see that that is not the case. The cool thing is that we think that Active Galactic Nuclei have a role in this: they have a way of heating up the galaxy again, and also, kind of pushing material out from the centre, so that they provide a way to feedback heat and the material back into the galaxy and stop it from just getting really, really cold.
O: So without the Active Galactic Nucleus pushing all this energy out into the galaxy, all this gas and stuff would just have collapsed together and there would be nothing left except stars?
R: Exactly, there would not be any of the gas that we still see around today.
O: And no new stars would be able to form.
R: Exactly. To pick up on something you said earlier, we know that all galaxies have supermassive black holes, but we also know that not all galaxies have Active Galactic Nuclei.
O: Yeah, that’s right, and it depends basically on whether there is this reservoir of material that is falling in, in this disk. For example, our galaxy – the Milky Way, does have a supermassive black hole at the centre, but it is not currently accreting material, there is not this disk of stuff falling onto it, so there isn’t this energetic jet coming out. But it is quite likely that some point in the in the past, in the history of the galaxy there was a jet because there was material.
R: We have been talking about Active Galactic Nuclei and what we know about them, but why don’t we focus a little bit on how we know things about them and the history of the observations.
O: So, our first observations of them were actually relatively recent. In the first half of the twentieth century, people saw these things, where we could see the energy coming out in this jet – but at first people thought that they were stars in our own galaxy, and it was only in about the fifties that people realised that this bright thing that we could see was always aligned with a galaxy. And they realised that it wasn’t a star close to us in our own galaxy, but this amazingly bright jet, much, much further away in a distant galaxy. And observations of them at first were quite difficult to understand, because it makes a big difference to what they look like, which angle you happen to see them at, right? These ones where we happen to be looking exactly down this jet just by chance, look very, very bright, and they were first mistaken for stars, but some of the other ones that you see edge on, or at some other kind of angle will look very different: you won’t be able to see the jet coming straight towards you and you will be able to see the signature of its energy being put out into the galaxy on either side of it.
R: So that is when you see the doughnut and the stick rather than just looking down at the centre of the stick?
O: Exactly, and that gives you a very different picture. It was quite a while before people realised that these were actually the same kind of object that we are observing differently, rather than two completely different types of galaxies.
R: Well, we hope you enjoyed our discussion about Active Galactic Nuclei, how they work and what they do for our galaxy.
O: Yeah, if you have any questions, or comments or anything, you can go to my website, which is cosmic-web, linked next to the podcast.
R: And you can find me on the Oxford Astrophysics website. Thanks for listening!
O: Thanks, bye-bye.
End of podcast:
365 Days of Astronomy
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