How was our galaxy formed?
Rosemary Wyse surveys thousands of stars searching for clues.
Rosemary Wyse is a professor in the Department of Physics and Astronomy of the Johns Hopkins University.? Her research interests include galaxy formation and evolution. In February of 2006, Dr. Wyse spoke with Earth & Sky’s Jorge Salazar about her recent work as part of an international team of scientists called RAVE, the Radial Velocity Experiment, surveying a millions of stars in our Milky Way galaxy.
Salazar: Thank you for your time today, Dr. Wyse. Would you boil down what you’re attempting to do with the RAVE experiment?
Wyse: What we’re trying to do is figure out how our galaxy was formed. And one of the ways of doing this is to study how the stars in our galaxy are moving, is to figure what the chemical makeup of those stars is, and how that varies as a function of the type of star and the distance of the star from us in the galaxy.
So first, how do we do any of this, and why does it tell us anything. The way that we measure the motion of a star – well there’s two components to the motion. One is along the line of sight to us. And the other one you can think of as being perpendicular to that, which is in the plane of the sky.
The one that we’re measuring is along the line of sight to us, and we use the familiar Doppler Shift for that, which I think most people are familiar with. You hear a police car behind you, you hear the siren, and as the police car passes you, because we’re all law-abiding citizens, you hear the pitch change. And that’s because the wavelengths that you measure depends on the speed of the object towards you. And we basically do the same thing for the stars. Except we don’t listen to the sound that they’re making, except that we don’t listen to the sound that they’re making. But Instead we take the light from the star and we disperse it into its different elemental wavelengths, and then we compare the measured wavelength of a line that we identify with what it would be if the star wasn’t moving along the line of sight to us. So, that’s how we get its motion.
And why that’s important is for in fact a variety of reasons. One of the projects that you can do is to figure out how fast a star would have to move before it escapes from a galaxy, and that tells us what the overall potential well depth of the galaxy is, that’s a complicated phrase, which really just tell us the weight of the galaxy. And if we know the weight of the galaxy, we can tell something about this thing that you’ve probably heard of, dark matter. We want to know what the dark matter content of our galaxy is, and we want to know what dark matter is.
And trying to figure out the motions of the stars in our galaxy tells us about its the overall mass. And that tells us about dark matter, because we know there should be dark matter, and in fact, one of the early results of RAVE is to find lots of very quickly moving stars, which we wouldn’t have found if it weren’t that they were bound into our galaxy from this large amount of dark matter.
But the other thing that we can do with the motions is to look for stars that are all moving together. And that’s a signature, perhaps, that these stars have been accreted into our galaxy, that they used to be part of an external galaxy, which is why they’re all moving together, but that external galaxy, at some point in the past, it could have been a long time ago, but not that long on a cosmological time scale, but say a few billion years ago, could have merged with our Milky Way galaxy, and its stars they could still be moving coherently, and we’re looking for that signature.
Because the current theory of how our galaxy formed is indeed that we started out with a lot of small galaxies, and they then merged, and were accreted into our galaxy to form a big galaxy. And the theory says that this should still be happening. And in fact we see very strong evidence for this in terms of the Sagittarius dwarf galaxy, which is one that was discovered about 10 years ago, and is actually inn the process of being disrupted and eaten by our galaxy. But the theories say that there should have been a lot more in the recent past, and there should be more happening now. So, we’re looking for those signatures by having this extremely large survey. So, this is the largest survey that’s ever been attempted. And it’s basically sheer numbers that’s going to provide be the answer.
Salazar: Would you give our listeners a sense of just how stars, such as our sun, move through the galaxy?
Wyse: Our galaxy is a normal, spiral galaxy. And that means that a significant component of that galaxy are stars and gas that are in a disc that are going around the center of the galaxy in orbits that are close to circular. So, that means that they’re all going around together moving approximately the same way, in a fairly regular orbit about the center. And our sun is one of those stars. And most of the stars close to the sun are stars like that too, they’re all just going around the center, together. So, that means that most of the stars won’t have a very large velocity compared to the sun. But the disc isn’t the only component of the galaxy. There’s also a stellar halo, and those stars are moving extremely differently from the stars like the sun. So, although we’re sitting here, orbiting the sun, and the telescope is on the Earth orbiting the sun, and we’re going around with the sun around the galactic center, there are all these other stars that are on very “high-velocity,” we call them, orbits, with respect to the sun. And that’s probably where we’re going to find most of these stars that are moving together. Now because it’s only a small fraction of the stars that are close to the sun that are moving like this, that’s why we need a very large sample.
Salazar: Where does the Radial Velocity Experiment fit in here?
Wyse: What RAVE is doing, is that it’s an international collaboration that has managed to take over a small telescope which would otherwise have been closed down, and is managing to really use the capabilities of that telescope to do a unique experiment, which is to measure the radial velocities, which is the motions along the line of sight, of up to a million stars. Most surveys up to now have done perhaps a few thousand. And it’s one of the things that I think could be is interesting in my interest to listeners is, astronomy is moving towards larger and larger telescopes, and these telescopes are extremely expensive. So one of the things that’s happening is that smaller telescopes tend to be closed down. But there really is a role for these smaller telescopes to play if you can find the right niche for them and if they can be dedicated to one experiment. Because it’s really expensive if you have to change instruments to support all sorts of different projects.
So, what we’ve managed to do is take over a 1.2 meter telescope, a Schmidt telescope, it’s in Australia. One of the advantages of a Schmidt telescope is that it has an extremely large field of view. It has a field of view that has an area that’s about 150 times the area of the moon. And this is much larger than most large telescopes. And the unique capability that this telescope has is that field of view puts light into a multi-object spectrograph, and that means that we can measure this radial velocity, this line-of-sight Doppler velocity, of not just one star at a time, but up to 150. And this very large field of view is actually extremely well-matched to the surface density of bright stars on the sky.
So with these 150’ers, we get a very significant fraction of the bright stars that are on the sky. And we can measure their velocities all together. And so with this very large sample, were going to be able to study in unprecedented detail how the stars like the sun are moving., because I just probably just made you think it was very boring the way stars like the sun are moving, but it’s not at all. Because in fact, although I said earlier that we’re more likely to find stars that have been accreted in the halo, there are some theories that are saying that stars are even accreted into the same disc, and they could be moving like the stars in the disc, they could be hiding there.
And so, we couldn’t be able to see them so much in terms of the kinematics, that’s the way they’re moving. And that’s why we need the chemical signatures as well. We want to measure the metallicities, as astronomers call it, of the stars. The reason we want to measure the metallicities is that, you probably know the saying, and it’s true, that we are all star dust. The Big Bang, the earliest phases of evolution of the universe, the hot dense phase, the have created hydrogen and helium, but essentially nothing else. Everything else has been created in stars, the carbon, the oxygen that’s around us, the iron. and it’s created in the centers of stars as they’re shining, as the sun is, and they’re sent out into the interstellar medium, when those stars die.
The most massive stars live a very short time, and they put back a significant number of elements, and massive elements, back into the medium out of which successive generations of stars form. And this goes on in different galaxies, and as time goes by, stars that are born later are more enriched in these heavier elements. And that means that by measuring the chemical elements in a star, you can trace the previous history of star formation in the gas out of which that star formed. And each galaxy has its own rate of star formation, its own rate of enrichment with heavy elements like this.
So, stars that came into our galaxy from outside would be expected to have a chemical signature as well. And we’re looking for the substructure, tracing the history of how our galaxy formed in two ways. One by looking at the motions, and the other by looking at the chemical abundances in the stars.
Salazar: Would you mind explaining again what radial velocity is?
Wyse: The Sun is going around the galactic center with the speed that’s about half a million miles an hour, I think it is something like that. So if I had a star in the halo, which wasn’t moving at this speed, then if it was moving perpendicular to the disc, what I’m trying to say is that we would measure that it was moving, relative to us, with a radial velocity that’s about the amplitude of the sun’s motion around the center. So the radial velocity is just the component a speed along the line of sight connecting us to that star.
Salazar: Thank you for sharing your time today with us. Is there anything else you’d like to add?
Wyse: I would like to share with you that we have made the first year’s data public, and that was what the press release was about on Friday. And so, there’s a web page where anybody can download the radial velocities of about 25,000 stars. And you get here, actually, not only the radial velocities, but from other surveys we’ve put in the other component of the velocity, which is the proper motion on the sky. And you can get this from a web page, www.rave-survey.org, and that gets you directly to the data.
And I would just like to say that the National Science Foundation does support my research, and is supporting RAVE, and it’s also supported by the national funding agencies of the other main people in RAVE. So, it’s a 10-country project, it’s a major international project. Essentially all of the funding agencies are putting in small amounts, because it’s not a huge project, but it is very important that the funding agencies get the recognition.
