AUSTRALIAN FRONTIERS OF SCIENCE, 2008
The Shine Dome, Canberra, 21-22 February
Milky Way evolution with the square kilometre array and its Pathfinders
by Dr Naomi McClure-Griffiths
 Photo courtesy of Brian James |
Naomi McClure-Griffiths is a CEO Science Leader at the CSIRO Australia Telescope National Facility (ATNF) where she leads a research group aiming to better understand the structure and evolution of the Milky Way. Currently Naomi is the principal investigator on the Galactic All Sky-Survey, an international project to map all of the hydrogen in the Milky Way.
Naomi has a degree in physics from Oberlin College in Oberlin Ohio, USA and a PhD in astrophysics from the University of Minnesota in Minneapolis, USA. She started at the ATNF in 2001 as a Bolton Fellow and was later appointed as a Postdoctoral Fellow. She holds an Honorary appointment at the University of Sydney and supervises PhD students in Australia, the US and Japan. In 2006 she was the recipient of the Prime Minister's Malcolm McIntosh Prize for Physical Scientist of the Year for her discovery of a new spiral arm in the outer Milky Way. |
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I am going to talk particularly about my field of research, which is Milky Way evolution, and how the Square Kilometre Array and its Pathfinders are going to open up this field for us. You see here a nice picture of what we think one of the Pathfinders might look like, and I will tell you a little bit more about that later.
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Anne Green gave you a brief introduction to the Square Kilometre Array, which will be the 'next great thing' for radio astronomy. This is the thing that we are all excited about one million square kilometres of collecting area. That's an incredibly big bucket for collecting lots and lots of radiation from the universe. It will have continent-scale baselines to cover the entire continent (hopefully) of Australia, and be 100 times more powerful than any existing radio telescopes.
This slide gives you an idea of what we think it might look like. It will have some elements that are on the ground, working at very low frequencies, and dishes in the background here working at very high frequencies, all of that adding up to be one square kilometre.
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There are two possible sites shortlisted: Southern Africa and Australia.
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To focus on Australia: we want to have this telescope spread out across the country, with its core in Western Australia and then stations shown here as little dots spreading all the way across the country and, hopefully, over into New Zealand.
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The core will be out in the middle of nowhere, the Shire of Murchison town, zero; population, up to 160. This is a good thing for radio astronomy, because where people are, there is radio 'noise', and we want to get away from the radio noise. We don't want any FM radios, we don't want your mobile phones, we don't want your telephones, we don't want anything to do with you near our telescope.
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This is what the place looks like. This is what we hope will be the core site in 2020: a few thousand antennas sitting out in what looks kind of like Mars, a 100 petaflop computer, 250 kilometres away from the centre of the site, data transferring a long way, population density 3 nano-humans per square metre, and 10 megawatt power supply. We need a lot of power out there, and we want it green if you can do that for us, I'd like to hear from you.
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Before we get to that point, though, what we would really like to do is to build the Australian SKA Pathfinder (ASKAP). The SKA Pathfinder is about a 1 per cent SKA, and it is designed to be a mapping telescope it looks at the sky very quickly, it covers a lot of the sky very quickly, 50 times faster than any of the current facilities to be completed in 2012.
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The idea behind this is that it will demonstrate SKA technology and the site. Can we operate the telescope out in a place where temperatures can get up to 50°C, snakes crawl into your equipment can we do all of these things? And also can we do excellent science while we are demonstrating this site and the technology? So this is what we hope the telescope will look like.
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What do I want to get out of these telescopes? The main thing that I am interested in is the Milky Way, our own galaxy. In trying to understand our own galaxy, we use it as a laboratory for understanding how galaxies in the universe have evolved. How did we all come to be here, how did the galaxy that we are living in form? These are very basic, fundamental questions.
Gas in our galaxy provides a very good way of getting at those questions. It is the key to understanding the evolution of the galaxy. It acts as an atmosphere for a galaxy: information about pressure, about temperature, travels from one place to another within the galaxy through its gas. So if we did not have gas, we would not convey any information. It is the matter from which stars form and that they return to when they die. So we wouldn't have any stars, we wouldn't have any light, if we didn't initially have gas in the galaxy.
And the Milky Way, our own galaxy, must receive constant fuel from extragalactic space in order to keep itself alive. It is 'fed' by extragalactic space. So all these are big things that gas plays a role in, and radio astronomy has a unique place in showing us about the gas rather than the stars in galaxies.
The image I have got up here is just three galaxies in optical image. When we look at them in optical they look sort of disconnected and, I think, not all that interesting, because I don't really care about stars.
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But when we look at the gas, particularly the atomic hydrogen, in these galaxies we can see that they are all connected, with blue streamers of gas pulled off from the galaxies. One appears massive, and out further we see a clump who knows where that came from, as there were no stars there. So the gas gives a very different picture of what is happening in a galaxy, and by putting that together with the stars we can start to understand how galaxies form, how they evolve and how they live out their lives.
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In order to try to motivate some of the science I am just going to show you a couple of interesting results. The current state of play for understanding gas in the galaxy, in the radio astronomy context, is the Galactic All Sky Survey, which is a project that we have recently completed with the Parkes telescope with the goal of understanding how gas flows around in our galaxy. How does it move? How does it move out of the disk of the galaxy where all the stars are, that wonderful band that you see across the night sky when you look up? How does it move off that out into the halo the spherical, blobby not-very-dense bit of gas that floats around our galaxy?
How did the Milky Way form? How did all the bits and pieces, the little clumps of gas that were originally sitting around, come together to form a galaxy that looks like our galaxy? And how do we interact with our neighbours? Do we play nicely, do we not play nicely? Do we rip them apart in various ways? How do those interactions happen? We use atomic hydrogen gas and radio telescopes to try to understand that, to get at those pieces.
The image that we have here is the entire southern sky. You can see the curved edge of the disk of the galaxy, you can see the Magellanic Clouds, which are little nearby galaxies, and gas that streams off them as they approach towards the Galactic plane.
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This is perhaps a more familiar view of the Milky Way. You get your nice disk; the southern hemisphere sees all the wonderful, beautiful stuff towards the centre; and you might be able to see the Magellanic Clouds on a nice clear night.
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If we look at this with atomic hydrogen gas, it looks a bit different but you still have the same disk-like structure and you can see that there are little filaments of gas floating up into high latitudes. Understanding how this gas moves up above the disk is a very important thing in understanding how information travels around in our galaxy.
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Another wonderful thing that we get out of radio astronomy is a semi-three-dimensional picture of the gas in the galaxy. The movie that we are stepping through here is, effectively, stepping through distance in the galaxy. (It is a bit more complicated than that, but we will just leave it at that for now.)
We are stepping through 'slices' of the galaxy, looking at what the gas looks like in a three-dimensional way. You can see that there are lots of bubbles and filaments and loops and wisps a lot of information tied up in there. Nearby Magellanic Clouds coming up appear, heading out into the very, very far reaches of the galaxy, where the emission is just dropping away and we are hardly seeing it.
The Australian SKA Pathfinder, the telescope that we are going to have in 2012, will be able to do this with 10 times as much resolving power. So every little detail that we see here looking like a blob is probably going break down into 10 blobs and give us a much better idea of what is happening in the galaxy.
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An example of the things that we can do with data like this is a project that we have recently done, looking at how we interact with the Magellanic Clouds near us. The Magellanic Clouds are nearby galaxies they are smaller than our own galaxy and dynamically associated with our galaxy, they have a wonderful stream of gas that carries off far behind them, and another stream of gas that leads ahead of them, the so-called leading arm. For a long time we have been wondering where that leading arm moves, what distance it is, how far away from us, because it gives an indication of the future orbit of those clouds. Are those clouds going to come spiralling into our own galaxy, or are they just cruising on past?
There has been a lot of controversy about this in the past few years, because people have been saying that in fact the Magellanic Clouds are just passing by and we will never see them again.
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Recently we have found evidence for an interaction that says that that is not actually the case. We have found, by looking at the atomic hydrogen gas with radio telescopes, that we can see a part of the leading arm structure which is poking into our own galaxy, about 80,000 to 100,000 lightyears from the centre of the galaxy. This is closer than anybody thought before, and it suggests that the Magellanic Clouds will eventually merge in with our own galaxy resolving the controversy.
So the atomic hydrogen data gives you a good idea of how we are interacting with our neighbours.
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In the ASKAP/SKA era we should be able to do much better. On this slide I have got the old image before the new data which we have just taken with Parkes, to the factor of 3 better in resolving power and detail, and you can see that it gets much better. But ASKAP is going to be a factor of 10 better, so understanding the physics of how gas interacts should be much easier when we can actually see it, instead of it being largely unresolved. It is like putting on glasses that actually work for you, as opposed to looking out with glasses that just leave everything a bit blurry so you can't tell what is happening.
The SKA is going to be even better. It is going to be 100 times better than what we can do right now, and maybe even more. So that is going to be pretty impressive.
This should help us understand how it is that gas comes and goes in the Milky Way what goes up, what comes down and how it moves around how we interact with our neighbours, and the really important question of how the Milky Way was formed: how did gas came together to form our galaxy?
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But it's not just gas. There is also the big question of magnetic fields. We know that magnetic fields are an important thing everywhere. Our aeroplane wouldn't have got here from Sydney this morning if we didn't have a magnetic field and a compass to get us here. (It was pretty cloudy.) Magnetic fields are a very important component of the universe, but we know very little about them.
We can all think about magnetic fields in the way that little children do, where we have a magnet, we have north and south poles, and if you put iron filings around you get wonderful arcs of filings where they follow the lines of force.
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We see that type of thing out in space. The sun and the solar prominences that we see in this slide, and that maybe a lot of you have seen pictures of, are showing magnetic field type structures where the gas is following along lines of magnetic force that have pulled away from the sun.
Those exist in interstellar space as well. In fact, in interstellar space, magnetic fields vary by 20 orders of magnitude across various different places huge variations in magnetic fields from very, very strong to some that are very, very weak. We know they must be doing something: they must be controlling how the galaxies evolve, how stars form. But we just don't know how. So we need to get at that, and radio astronomy and the SKA are really going to get us there.
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This slide gives us a picture of what we know the magnetic field looks like in a galaxy in the M51 Group. These vectors here are those iron filings, effectively, that we saw in the previous picture, showing that the magnetic fields follow along the spiral arms of this galaxy.
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So what does it look like in our own galaxy? Well, we don't really know.
We know that there are loops and filaments that are magnetically formed, so this is sort of like a picture of the sun, but this is our own galaxy. The disk of the galaxy can be seen in this picture, and up in the halo there are loopy things which look like those solar prominences and are believed to be magnetically formed, but we don't know exactly how they get there and what they mean, or how they transfer gas around the galaxy. And we don't understand what the magnetic field structure of our full galaxy looks like.
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So what we do is to use background sources sources where they have polarised radio light, and magnetic fields in some intervening gas rotate that. It is as if you have light bouncing off a lake and you rotate your polaroid glasses, you cut out some amount of the light, depending upon its angle. Well, we're looking at the angle changing of the polarisation.
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By looking at millions of background sources scattered across the sky, we can get a feel for what the Milky Way's magnetic field structure looks like, using these background probes to light up the magnetic field of the intervening gas.
At present this is how many sources we have across the sky that we can do this with. We can put together crude models for what we think our galaxy looks like the spiral structure shown here, with the arrows showing the direction of the magnetic field but it is a very crude model and we don't know if it is right. There is a lot of guesswork involved, and we are basically limited by not having enough sources in the image shown at the top here. There is a lot of interpolation, a lot of information that is missing and that we can't do anything about. And that is where ASKAP and the SKA are going to come in, by filling in completely what you see in that image, so that there are sources everywhere to give us a full background grid to light up the magnetic field structure of the galaxy.
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To wrap up: the Australian SKA Pathfinder will help us to get a lot more information about the Milky Way, about how it is formed, how it lives, about its magnetic field structure, and that will, hopefully, tell us more about how the universe works. We will use this as our backyard laboratory for understanding galaxy evolution.
So the ASKAP telescope will be 50 times faster at mapping the sky than any previous telescope that we have, and for the Milky Way this is vital, because the galaxy covers the entire sky.
Among the scientific goals will be to try to measure the full magnetic field strength and geometry of the Milky Way, and to study the evolution of the galaxy and its interaction with its halo.
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We should take advantage of having the Milky Way galactic centre go directly overhead, and have this telescope here in Australia. We are going to have it in the southern hemisphere, but we need to use our galaxy and our natural resource of the interesting bits of the Milky Way being straight overhead instead of looking out into the galactic outback, as you do from the northern hemisphere. And we'll use that to better understand how the Milky Way came into being and evolved, and how magnetic fields work, what they do
in galaxies.
All of that can be done to a certain extent with the ASKAP, but with the SKA it is going to be even better.
