Date: July 11, 2010

Title: A Zoo of Active Galaxies


Podcaster: Chris Lintott and Kevin Schawinski


Description: Chris and Kevin celebrate Galaxy Zoo’s 3rd Anniversary by discussing a new paper published by the Galaxy Zoo team that studies active galactic nuclei.

Bio: Chris Lintott is an English astrophysicist. He is a post-doctoral researcher who is involved in a number of popular science projects aimed at bringing astronomical science to a wider audience. He is the co-presenter of Patrick Moore’s BBC series “The Sky at Night” and a co-author of the book Bang! – The Complete History of the Universe with Patrick Moore and Queen guitarist Brian May. Chris Lintott is one of the principal investigators for the Galaxy Zoo project, and runs Zooniverse projects which allow you to help scientists explore the Universe. Chris is now the Director of Citizen Science Initiatives at the Adler Planetarium in Chicago.

Kevin Schawinski is an Einstein Fellow at the Yale Center for Astronomy & Astrophysics. He is a co-founder of Galaxy Zoo and also contributes to the Galaxy Zoo blog.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by John Sandlin because a little astronomy illuminates the darkest nights.


CL : Hello, I’m Chris Lintott. I’m one of the principal investigators for the Galaxy Zoo project which has involved more than 300,000 people in classifying galaxies which were imaged by the Sloan Digital Sky Survey or Hubble Space Telescope. Each of those people turned up at and told us about the shape of the galaxies.

When we launched the project 3 years ago today I don’t think we had any idea that it would be so successful, or that we’d find out as much about the galaxies as we did. Joining me to discuss the latest results is Kevin Schawinski, now a researcher at Yale University but at the time that Galaxy Zoo launched my colleague in Oxford. Good morning

KS : Good morning.

CL : The latest results are all to do with AGN, active galactic nuclei. So what are AGN, and why are they important?

KS : An AGN is an Active Galactic Nucleus, the nucleus of a galaxy that seems to be emiting prodigious amounts of energy, radiation, light. AGN have been known for some time, but it took quite a few years of research before people understood that the only power source that could drive AGN were accreting supermassive black hole.

CL : We now know, don’t we, that most galaxies in the Universe have a big black hole at their centre. Where does the evidence for that come from?

KS : We’ve used observatories like the Hubble Space Telescope as well as ground based telescopes to look at the very centers of nearby galaxies and there we find evidence for huge amounts of mass concentrated in the very nucleus. We know this from the motion of stars around the centre…

CL : …we just measure how quickly they’re moving

KS : When you work out how much mass is contained in that very center you come up with a number that’s enormous, millions or even billions of times the mass of the Sun and there’s no way that could be anything other than a supermassive black hole. We even see this at the center of our own Milky Way where there’s an empty spot with stars whizzing round it at hundreds of kilometres a second, and we now know that this could be nothing other than a supermassive black hole.

CL : OK, so we have massive black holes at the center of the galaxy, but there’s something confusing here. You talked about these AGN giving out huge amounts of radiation, and light, but the only thing everyone knows about black holes is that they don’t emit light, so how can you get these huge amounts of energy from black hole.

KS : It’s an irony that the brightest objects in the Universe are actually black holes. The light that we see from black holes doesn’t come from inside the black hole because as we all know light can’t escape them – the light actually comes from material, from gas and dust as it is being sucked into the black hole. As it screams into the black hole it forms this dense disk of material as it spirals into the black hole…

CL : what we call the accretion disk…

KS : the material gets compressed, there’s friction and it gets heated up, and suddenly you have an object – this accretion disk – that’s extremely hot and extremely luminous, and if our line of sight is not blocked in any way that’s what we see. And in the case of large black holes feeding essentially as fast as they can this tiny little accretion disk can be brighter than the 100 billion or so stars in the galaxy around it, and all we see is the material falling into the black hole.

CL : So that’s the basic process, but does this happen with all galaxies that have black holes.

KS : If we look around us only a tiny fraction of galaxies, perhaps a few percent, are actively feeding today. However, we also known from large scale studies that most black holes must have grown in an active galactic nucleus, an AGN phase. So we believe that every galaxy went through an active, feeding phase that established the black hole at its center.

CL : So, for example, although the Milky Way is quiet today, in the past it might have been an active galaxy.

KS : That’s the idea.

CL : How does Galaxy Zoo come into this? How have the Galaxy Zoo results helped us understand AGN?

KS : SSo one of the puzzling things about black holes and galaxies is that even though we think of black holes as this destructive force, they’re actually really tiny compared to the galaxies they live in. Their gravitational pull doesn’t go much further than the very center of the galaxy, and so we wouldn’t naturally expect the black hole to be in any way linked to the rest of the galaxy

CL : When you say ‘linked’ do you mean they should have no influence on the rest of the galaxy?

KS : It shouldn’t have much influence, and what people have found is that for some mysterious reason the mass of the black hole is linked very tightly to the mass of the galaxy around it.

CL : So the bigger the black hole, the bigger the galaxy

KS: The bigger the black hole, the bigger the galaxy, so somehow they must have grown together, so somehow the material falling into the black hole controls the growth of the stars in the galaxy around it. Now what we did with Galaxy Zoo was that we wanted to know what kinds of galaxies have currently feeding black holes in the Universe around us, and what this tells us about how this link between black holes and galaxies is established.

CL : When you say kinds of galaxies, what do you mean?

KS : The biggest question that Galaxy Zoo let us address is what kinds of galaxies are actually spiral, and what kinds are elliptical galaxies – football or rugby ball shaped galaxies. So what we wanted to know was whether growing black holes are most likely to occur in spiral or elliptical galaxies.

CL : And that’s what the paper you recently published directly addresses, so what’s the answer? Where are the active galaxies in our local Universe?

KS : Most of the active galaxies – up to 90% – are actually in galaxies with some sort of disk, about half of them in beautiful grand design galaxies like the Milky Way. One of the things we learnt from the Galaxy Zoo results is that the type of galaxy that is most likely to be growing its black hole today is actually a galaxy like our own Milky Way – the kind of mass, and the kind of disk we see in the Milky Way

CL : You sound quite surprised by that…

KS : It is surprising because when we look at our own Milky Way, in the centre, the black hole is virtually invisible. We see stars going around it but we don’t see it feeding. What the x-ray community has known from quite a while is that when you look at the clouds of molecular gas and material around the black hole of the Milky Way, you see an echo of past activity. Maybe a few hundred or a few thousand years ago the black hole has been much more active.

CL : Does that mean that these things can switch on and off? That maybe a galaxy isn’t always active or always quiet, but can switch between the two.

KS : That’s right, and it gives more context to that finding that our Milky Way has been active recently. It now makes much more sense because it’s galaxies like our Milky Way that have been active recently.

CL : What about elliptical galaxies? Do they have AGN?

KS : They do. 10% of all AGN sit in elliptical galaxies, and this is where the big surprise came in. When you look at spiral galaxies it is preferentially the most massive spiral galaxies with the biggest black holes that are feeding their black holes today.

CL : So the bigger the spiral galaxy, the greater the chance that it will be active.

KS : That’s right, but in ellipticals it’s exactly the opposite. It’s more likely for an early-type galaxy will be feeding its black hole today the smaller the elliptical galaxy and the smaller its black hole is.

CL : So it’s the small elliptical galaxies that are active. Does that mean that they’re behaving in a fundamentally different way from the spirals, that something else is going on, or is that just the way things turned out?

KS : That’s what we concluded in the paper. There seem to be two very different ways in which black holes can grow and co-evolve with the galaxy they live in, depending on whether you have a spiral galaxy or an elliptical galaxy.

CL : So what are the different modes?

KS : We believe that galaxies like the Milky Way are pretty stable systems – they have these large rotating disks that are very difficult to disrupt. We know our own Milky Way hasn’t been through any major changes probably within the last 10 billion years or so. So whatever process is feeding the black hole and leading to energy being released by the black hole doesn’t seem to have much of an effect on galaxies like the Milky Way.

CL : OK, so the black hole does its own thing, and we sit out here and we’re perfectly happy and the galaxy remains stable. But in ellipticals something else is happening.

KS: When we look at the galaxies that have growing black holes that are ellipticals, we find that in almost all cases they’re galaxies that have recently shut down their star formation so they used to be forming lots of young stars, but by the time we see a growing black hole they’ve stopped forming stars. So somehow in elliptical galaxies, black hole growth and AGN phases seem to be associated with shutting down star formation to go from an actively star-forming blue galaxy to a passively evolving dead and red galaxy.

CL : But that’s remarkable, because you said earlier that the black hole should have very little effect on the galaxy. How can the presence of an active black hole have such a dramatic effect on star formation throughout the whole galaxy?

KS : The hypothesis is that the energy liberated as material falls into the black hole giving you this bright powerful AGN or quasar phase, that if you just take a little bit, a small fraction of that energy and have that energy heat up the gas and dust that forms stars, then the jets, the wind, the radiation from the black hole could sweep the galaxy clear of its gas and dust and remove the fuel from star formation. The black hole doesn’t just starve itself because it’s blowing out any further material that could trickle down to the centre, but it’s also preventing any more stars forming in that galaxy. Even though its gravitational reach doesn’t go beyond the very centre, the energy liberated by the black hole as it feeds can effect the galaxy as a whole.

CL : You’ve talked mostly about the local Universe, but the process that you’re describing that sometimes goes by the name of AGN feedback, we think that was more important in the early Universe.

KS: So this is where the surprise that it’s the least massive early type galaxies that are still feeding their black holes and stopping their star formation comes in. We know that the most massive galaxies in the Universe – which are also elliptical – they formed in the early Universe, so what we’re seeing here is an anti-hierachical, an inverse growth. It’s the most massive early-type galaxies that formed first and went through both the black hole growth phase that shuts down star formation in the early Universe, while their small cousins are going through this process today.

CL : Excellent, so by studying the local Universe we can find out about our Universe’s early days. Kevin, thanks for joining us.

Galaxy Zoo is currently taking a look deep into the Universe with images from the Hubble Space Telescope, to try and address some of the questions that Kevin was describing. To take part, go to

End of podcast:

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