19 April 2008
Galactic clusters
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Clusters of galaxies are the largest gravitationally bound systems in the universe. Some are 1015 solar masses. That's 1,000,000,000,000,000 times the mass of the sun. Christine Jones describes how and why galaxies cluster. One famous cluster is the bullet cluster.
Transcript
This transcript was typed from a recording of the program. The ABC cannot guarantee its complete accuracy because of the possibility of mishearing and occasional difficulty in identifying speakers.
Christine Jones: Chandra was launched in July 1999, so getting up there in age. We have a middle-aged satellite now, not a brand new one anymore.
Robyn Williams: And it's been going strong ever since. How far up is it?
Christine Jones: It actually has an orbit that is fairly elliptical. It comes at sometimes fairly close to the earth and then at its furtherest it's about one-third of the way to the moon.
Robyn Williams: And you're mainly concentrating on galactic clusters, is that right?
Christine Jones: Yes. Clusters of galaxies are basically the largest gravitationally balanced systems we know about in the entire universe. They are simply enormous. They are 1015 solar masses. A billion is 109 , a trillion is 1012. A quadrillion solar masses of stuff of them.
Robyn Williams: It's a gazillion.
Christine Jones: A gazillion, okay.
Robyn Williams: Beyond imagining.
Christine Jones: Beyond imagining. So most of that stuff is stuff you don't see at any wavelength. You see galaxies in small groups. You see a few galaxies. In rich clusters you can see a thousand galaxies. But the galaxies themselves make up only a couple per cent of the mass that we know is there. And we've know about this mass for quite a while. There was as guy named Fritz Zwicky who was before our time, who measured how fast these galaxies were moving around and found that in the rich clusters they're moving around at a thousand kilometres a second. This is pretty fast. So if you just had these galaxies moving around they wouldn't be concentrated any more. Over time they'd be spread out over the sky, but they're not, they're in these concentrations. And you need gravity to keep them in these concentrations.
Robyn Williams: Yes, but why do they cluster?
Christine Jones: It's because of how the universe forms. And it's basically a cosmic web with filaments of galaxies, long strings of galaxies, and where those strings cross at their nodes you get these rich clusters. So you have bubbles or voids where there aren't many galaxies and then you have these filaments where there are groups and things, and you have these rich concentrations where the filaments cross.
Robyn Williams: Christine Jones, a senior astrophysicist at the Smithsonian Observatory at Harvard. Mission control for Chandra. And yes the numbers are gigantic, the scale unbelievable and the results from Chandra quite startling. And there are also bodies colliding and merging and perpetrating violence on a scale Christine Jones really relishes. The thing that amazes me is some of the work you've been doing watching them collide as if they pass through each other as well.
Christine Jones: Now when you have these clusters, when they form and it used to be thought that clusters were sort of the oldest things around and they weren't doing anything. And then some of the very first X-ray observations we did they were imaging with the Einstein Observatory showed that in fact they weren't these nice old relaxed systems, in fact there was a lot going on in them and they had a lot of structure in their images. So as I was saying the galaxies make up a few per cent of the mass. The X-ray gas makes up maybe about 15%, 10 or 15% of the total mass in the clusters. So it's much more than the galaxies. It's four or five times in the rich clusters, the mass and the gas. But we only see that if we look in X-rays because the gas is very hot. It's about 100 million degrees. So it emits in the X-rays, not in the visible or the radio or other wavelengths.
You asked me about colliding. So when the clusters grow by having other groups of galaxies or other clusters fall in along these filaments into this really deep gravitational well that's holding all the gas, holding the galaxies, and then attracting the other clusters. So one of the most famous examples that's come from Chandra is what we call the Bullet Cluster. And we call it that because one of the clusters is very much shaped, it looks like a bullet or a shuttlecock. And it has flown through the other cluster at a velocity of a few thousand kilometres a second. And people have looked now at both where the hot gas is, which again it's most of the mass, and also where the dark matter is, which is really most of the mass. Because you've got the visible mass, a little bit in stars, a little bit more in the gas, but most of it is this dark matter, this stuff that we don't know what it is.
Robyn Williams: I can see one great big red cloud and then to the right there is as you say the bullet shaped thing that has zotted through it, and either side of them like a kind of halo effect is this dark matter. Now how long did it take the Bullet to shoot through the big one?
Christine Jones: It's not so long astronomically speaking. But what happened here is the reason that the blue is here in front of the red, and the object that's gone through, is because this is the dark matter and it doesn't interact much. So it went on through. The X-ray gas was pushed back like smoke off a train as it's moving along. So this is moving along like a train and the train just ploughed through the dark matter. But the gas got pushed back by the other gas that's around here. It interacted.
Robyn Williams: Sure. Why I would expect it would be a bullet or a train is that when you've got such immense objects slamming into each other I'd expect some single huge mass of stuff, like the train or the bullet breaking up but they seem to remain intact.
Christine Jones: Yes, it's being pushed off, there's a lot less of it here, but the core of it is still pretty dense, it's still pretty much intact. The stuff from the sides gets pushed off and there's a big shock happening out here in front of that.
Robyn Williams: An amazing thing. Now how is it that you can see the dark matter but you don't know what it is or what it's made of?
Christine Jones: I guess you can see it the same way that we're here on the Earth. We feel the effects of gravity. If I drop something it's going to land on the floor, but I can't see that, I'm not aware of that gravitational potential that's holding us all down here. So we can see the effects of gravity. The only way we know to make gravity is to have enough mass there to cause that. So we see the effects, it's just all gravity.
Robyn Williams: Now to your pictures of recent discoveries like one of the black hole that burped. And so what's this?
Christine Jones: Okay. This is pictures in the X-ray and in the radio and in the optical of the biggest galaxy in a fairly nearby cluster of galaxies. It's called M87, because it was picked out as Messier's 87th object. And it's a huge elliptical galaxy. That means it's mostly a nice spheroidal bulge of stars. It doesn't have any of those pretty arms to it like the Milky Way or Andromeda. And when you look at this object in the radio it's fairly complicated. It has both a radio as well as an optical jet in the centre of it where energy in mass is being spewed out of this super massive black hole at the centre, which is about 3x109
solar masses, so a three billion mass black hole at the centre. It's a big one. And as it spews out this matter it carries it out in the radio plasma in these lobes of material that rise up. And surrounding all of this is a very hot gas of about a few times 107
degrees, so 30 million degrees.
Robyn Williams: How far away, you say it's quite near, is it that near?
Christine Jones: Yes, it's quite near. It's at a distance of about 16 mega-parsecs, so not so far by astronomical standards.
Robyn Williams: But still zillions of light-years?
Christine Jones: Yes, quite a zillion light-years. So when we look at it in the X-ray what you see is, you also see this jet and you see a fairly bright region at the centre, and you see these beautiful X-ray arms that extend out near where the radio ones are. But you also see a lot of holes in the gas. And this picture shows you some of these cavities that are in there. And these are almost certainly caused by sort of when the black hole burped and sent out this radio plasma that filled these holes.
Now if you look at it in different X-ray energies this picture shows these beautiful arms and all these cavities. And this picture doesn't show any cavities or arms and instead just shows a couple of rings around here. And these are the same object just taken at somewhat different X-ray energies. So what we're seeing here is the gas that's being dragged out and pushed out by these burps from the super massive black hole. But those burps, especially the larger ones, are also making shocks in the gas. And these rings are where you see that shock and how far it's gone. So by looking at the shocks and at the bubbles you can actually measure how much energy was in that outburst from the super massive black hole.
It's pretty straight forward from the bubbles that you see because you know that there was gas there that the bubble then pushed out of the way. So you just have to figure out how much work it took to move that gas and that tells you how much energy came out of the super massive black hole at the time. If the rims of the bubbles are hot that energy came out really pretty quickly. But if they're cool, which is what we generally see, that energy came out fairly slowly. So we can learn a lot about the history of the super massive black hole and how it spewed out all of these enormous amounts of energy.
Robyn Williams: And the bubbles presumably are larger than would occur in my bath.
Christine Jones: Oh much larger, much much larger, yes. The smallest ones here they're about one kiloparsec in size. But there are some enormous clusters that have these very large bubbles that are bigger than the Milky Way. Okay, so you could fit an entire galaxy in some of these bubbles. So unless you have a very large bathtub it's much much bigger, much bigger.
Robyn Williams: Christine Jones at Harvard with burping black holes and bubbles bigger than the Milky Way. It's difficult to cope with all that, isn't it? And fret not you can see some of those pictures yourself on the Chandra website. That's at www.chandra.harvard.edu
Guests
Christine Jones
Senior Astrophysicist Smithsonian Astrophysical Observatory Cambridge Massachusetts USA
http://cfa-www.harvard.edu/sao/
Presenter
Robyn Williams
Producer
David Fisher
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