Astronomy Cast Ep. 118: Sky Surveys

Introduction to sky surveys

0:00It's the 365 Days of Astronomy podcast, coming in 3, 2, 1. Astronomy podcast, episode 118 for Monday, December 8, 2008, Sky Surveys.

0:31Welcome to Astronomycast, our weekly facts-based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. My name is Fraser Cain. I'm the publisher of Universe Today, and with me is Dr. Pamela Gay, a professor at Southern Illinois University, Evertsville. Hey, Pamela. Hey, Fraser. How are you doing? Well, I was sick. Yeah. Yeah. Public transit in foreign countries should be accompanied by Purell. Right. So, yeah, you know, when we've got two kids, people are always sick around here in the

1:06wintertime, so I know that feeling. So just ignore, if you hear kind of a clicky noise, that's just Pamela with a lozenge, so just ignore it.

1:16All right, well, let's go on to this week's show. So in the old days, astronomers had to beg for telescope time. They put together a proposal, convinced observatories to gather data, crunch the data, and release the results. No telescope, no science. But everything's different now. Fleets of robotic telescopes constantly scan the skies, building up a vast database of raw data about the universe. Anyone who wants can access the information through the internet, download what they need to do real science. No telescope necessary.

1:46Let's look at the development of sky surveys and how they're changing how astronomy gets done. Was that a little over the top, do you think? No, I think sky surveys really are actually changing how everything is done. Me too. And that's kind of cool. Okay, so why don't you then regale us with a story of you attempting to get science done the old way? Well, I think the best example is probably my doctoral dissertation. I was working on a project called the TechSox Survey where we were following up on a radio

2:20survey actually looking for places on the sky where we thought maybe there was a higher probability of finding galaxy clusters. So we put in for observing time at McDonald Observatory, first on the 30-inch telescope, then on the 107-inch telescope. Hundreds of nights were spent out at the observatory. When the weather went bad, we didn't get our data. I tracked clouds. There was much crying and sadness. We got somewhere, but we didn't get where we wanted due to clouds and forest fires and all

2:54that sort of stuff affecting the amount of time we had at the end of the day. And it was sad. It still kind of happens now. Like I know you're telling me about some projects that you're working on where you're having to scrape together telescope time. You're trying to convince amateurs to let them do some observations for you. You're distracting telescope operators so you can sneak in and quickly move the telescope. Well, there's two different ways of needing to get data. There's the, I have a question that requires significant coverage on the sky.

3:31I want to look at a whole bunch of different objects and try and prove something in a statistically significant way. To do that, you need a ton of telescope time scattered all over the sky. And the data that I take might also be good for somebody else. And that's the type of stuff that leads naturally to the type of surveys we're going to talk about in the rest of the show. But the other type of thing you need is, oh, I found a really cool object. I need a bazillion hours on this one really cool object.

4:02And that's where you start begging people. That's where you need the dedicated time to look at just your object. And this is why the stable of astronomical equipment needs to include both telescopes that are dedicated to doing surveys, looking at the whole sky night after night after night, and also telescopes that are set aside for astronomers to follow up on pet projects. All right. So let's then segue into the surveys.

The Palomar Observatory Sky Survey

4:31So can you give us an example of sort of what was one of the first sky surveys put together and what's kind of involved? Well, the first really big survey that people have paid attention to, even today, is the Palomar Observatory Sky Survey. It used glass plates, two different Kodak emulsions, one sensitive to red, one sensitive to blue, to look at pretty much the entire northern hemisphere of the sky all the way down to minus 30 south.

5:01So it had significant coverage of the sky. The idea was, let's just catalog what's out there. This survey has been used to look for galaxy clusters. It's been used to get statistics on what different types of objects are out there. It's used even today where some cool something or other happens and you pull out the digital sky survey and you look to see, well, what was there before? It's a historic record of what was where on the sky when and it's a map of the sky and it's a way to do big science,

5:35admittedly, on the 1940s technological scale. Right, so you've got a really nice telescope every night moving, just taking a picture, moving a little bit, taking a picture, moving a little bit, right? And just slowly cataloging every single little piece of the night sky. And I guess, as we talked about before, to really get good science about an object, you want to point Hubble at it for 100 hours and get every stray photon that's coming from it.

6:09So this is the opposite, right? This is quick and dirty. This is click, move, click, move, click. And so, as you said, you know, you're able to count up galaxy clusters, you're able to count stars, you're able to, you know, you're able to get a general sense of what's out there, but you're not able to really dig deep and see, you know, the same thing, pointing at one object for 100 hours. No, and in fact, with these old glass plate surveys, it took them years to get coverage of the entire sky.

6:40Even today, we've moved on, and today's new version, perhaps new version is too strong a word, but our new optical survey of the sky is the Sloan Digital Sky Survey. It's probing the southern galactic pole. It's looking at a very focused region of the sky, and it's studying it extremely deeply. It's doing it in five colors, whereas the Palomar Sky Survey looked at the sky originally in only two colors. And it's getting huge swaths of the sky every night, but to get as deep as it does, it does take significant amounts of time, and it takes years to get a really detailed survey of the sky complete.

The Sloan Digital Sky Survey

7:19Okay, so let's take a look then at Sloan. And so, when did Sloan get operating? Well, they started building the telescope back in the 1990s, and it first started taking images in 2000. And it's still, here in 2008, reinventing itself, coming out with new ways of doing things. And so then, what does it capture? I mean, obviously, it takes a picture of a chunk of space, but I know there's more information that it's helping gather, right? So the Sloan Digital Sky Survey, it's using a technique called drift scanning, where you might start off with an object on the right-hand side of the CCD.

7:54And over time, it slowly drifts from the right side to the left side. But as it's drifting, the digital camera is taking that right-hand-most column of data and shifting it one pixel to the left and shifting it one pixel to the left. And it's shifting the recording of the data one column at a time at the same rate that the object's moving across the CCD. So by the time it's read all the way across the chip, you might have many, many minutes of observations of that particular object, allowing you to get extremely deep images all across the sky.

8:34Okay, I understand. So it's like the big CCD is, you know, it's not moving, and so it's able to then just pick it up, see the same object again and again and again. It's almost like it's taking multiple photographs of the same object, because it's just grabbing such a big swath of the sky at the same time. And it's using the rotation of the Earth to move the objects in it through its field of view. Yeah, that's exactly what's happening. And what's really cool is in addition to doing this detailed imaging of the sky, the Sloan Digital Sky Survey is also going back and doing follow-up spectroscopy.

9:12In this case, they're actually taking plates and drilling holes in them and then aligning fiber optics onto the holes on the plate. And for each hole in the plate and each fiber, they get an individual spectra that allows them to get a sense of what elements are in the objects that they're looking at, what is the redshift of some of the galaxies that they're looking at. Now, these are pretty big fibers, so while they're able to capture a lot of light all at once, they also aren't so good for dealing with really crowded fields like galaxy clusters.

9:47But for isolated objects, this system is allowing us to sample a huge number of galaxies scattered all across the sky to find out where they are in redshift space. That gives us a sense of their distance and to also give us a sense of, well, what are some other things that we can learn by looking at the elements? And this starts to play more of a role when we're saying, well, does this thing have absorption lines? Active galaxies, you can start to differentiate different types of active galaxies by looking at what lines exist in a mission.

10:21This is where you have an angry supermassive black hole in the center of the galaxy that's chomping on things and heating them up. And if the alignment is just right, the heated up elements end up radiating emission lines that we can detect. And that tells us something about what's going on down in the core of the galaxy. So then how much of the sky is Sloan going to be mapping? Sloan Digital Sky Survey is looking to eventually map out about 25% of the sky.

10:51That doesn't sound like a lot, but at the level of detail that they're getting, this is actually a really powerful survey. Already they've allowed us to learn new things about the structure of our own galaxy that we never even guessed at. So galaxies are the cool part for me. But in addition to looking at all the galaxies, it's also mapping out stars in the halo of the Milky Way. And by looking at the colors of the stars, we're able to get a sense of how far away they are.

11:24So you look at the color, you look at the brightness. And basically, by taking a color magnitude diagram of, well, what would a population of stars look like at what distance? You can sample through the sky to find out where are the populations of stars that are 60 kiloparsecs away. Where are they that are 70 kiloparsecs away? And we can start to see streaks, tidal tails of shredded dwarf galaxies out in the halo of the Milky Way by probing through using the Sloan Digital Sky Survey data to find these what we call co-moving populations of stars.

12:02Well, and I guess this is where the real power is because in the olden days, you would take your telescope, you would look at a region of sky, you would make measurements of whatever you wanted to do. But in this situation, Sloan is gathering images of everything that's in that 25% of the sky. And furthermore, they're writing down numbers. They're saying there's a star here at this location. There's a galaxy there at that location. There's a quasar here at this location. And then they're figuring out what the elements are that are in those objects.

12:36So you can then do searches. And this is where it becomes a database issue. You can say, you know, find me every star that has this level of, you know, metallicity or find me all of these stars that are within this range in this region of the sky and start to do these data mining, you know? And so then you're like, whoa, look at that line. You know, I mapped out all the stars in the survey and they happen to be lined up in a very interesting line.

13:08That must be a tidal tail. So there's whole new kinds of discoveries that are being made that could never be made before because you just didn't have the raw data about the entire sky that you could then just mine and just ask questions. And I know there's some amazing work done for dark matter, quasars, as you said, cataloging active galaxies. And, you know, isn't the Galaxy Zoo using the Sloan Digital Sky Survey?

13:38Right. But before we jump on to Galaxy Zoo, I just want to make it really clear that the spectra that they're getting with the Sloan Digital Sky Survey really isn't good enough to start getting at detailed metallicities of stars. It's rough spectra, but it's enough to get us what are their velocities and to get us broad information on them. That's what's cool about the Sloan Digital Sky Survey is it gives us a broad understanding of where things are, of how they're moving, of what different types of things are out there that we can follow up on later. It's allowing us to find more white dwarfs than we ever thought we could find some other way.

14:13It's allowing us to find quasars. It's, in fact, allowing us to determine the distribution of galaxies of different shapes and sizes and orientations on the sky.

Galaxy Zoo discoveries

14:24And this is where Galaxy Zoo comes in. And people are finding asteroids and Kuiper Belt objects that are sort of in the pictures. And they're starting to find really rare objects. One of the really cool things that came out of the Galaxy Zoo One project, this is a project that was originated by folks over at Oxford. My collaborator, Chris Lindhott, Kevin, who's now at Yale, a whole bunch of different folks were sitting around. And Kevin was given the task of go look at 50,000 galaxies and tell us what their distribution is in terms of are they spirals, are they ellipticals, how are they oriented.

15:06And after doing 50,000 objects, he really didn't want to do anymore. And so in a pub, the idea was originated, well, let's get the public who like looking at galaxies to look at almost a million objects from the Sloan Digital Sky Survey and catalog how they're oriented and what their shape. And basically, are they clockwise, counterclockwise, or edge-on spirals, or are they ellipticals, or are they mergers? And they did this. And the results of the public looking at all these objects were just as accurate as getting a small number of professionals to look at a much smaller sample of objects.

15:45Because professionals really can't make us look at too many of them before we go crazy. You ask 170,000 people to look at things, and there's 170,000 people can look at a lot more objects than five or six professionals. Right, and you're saying that they turned up some amazing things. So one of the really cool things that they were able to do was build up a catalog of galaxies that are overlapping on the sky. These aren't galaxies that are merging, but rather two galaxies that are superimposed on one another's line of sight, we say.

16:17One is nearby, one is further away. Gravitationally, they really don't care about each other very much. But the background galaxy can act like a spotlight going through the dust lanes of the foreground galaxy, allowing us to make out the details in the structure that we might not otherwise be able to see. And I guess it's almost impossible for a computer to seek out those kinds of objects, because it can barely tell, you know, that there's a galaxy, you know, it can just barely tell there's a galaxy there at all.

16:52So this is something that a human being is great at. Computers are generally pretty good about going, this is a star, this is not a star. But beyond going not a star, computers tend to get kind of confused. At a certain level, you can start to program them to look for things that are S-shaped or Z-shaped. So we have clockwise and counterclockwise-shaped galaxies. We can program them to look for things that have a radial profile, things that are basically fuzzy blobs like elliptical galaxies. But you can't program a computer to go, this thing looks like nothing anyone's ever seen before.

17:25They'd be showing that up every time an asteroid happens to pass in front of a galaxy. Human beings, you can train them, this is what an asteroid looks like when it's passing through an image. This is what a satellite looks like when it's passing through an image. This is a really weird nebula. This is reflected light inside the telescope. So we can take human beings and train them to do things that we can't train a computer to do. And human beings will naturally note, ooh, this is cool and unusual, and bring it to the attention of other people.

17:59And that's one of the wonderful things that came out of the original Galaxy Zoo project. And what's even cooler is there's now a second generation, Galaxy Zoo 2, that's been launched. So if you go to galaxyzoo.org, there's now a link off of it to Galaxy Zoo 2. But you're going to have to take a survey if you participated in Zoo 1 so that some of the folks working with Galaxy Zoo, and in this case it includes me, so please take the survey, can find out a little bit about why is it that you love using Galaxy Zoo? I know why I love using it.

18:30I want to know why you love using it.

Radio and other waveband surveys

18:33All right.

Radio and other waveband surveys

18:34So what are some other surveys that are happening? Well, surveys aren't restricted to just being optical telescopes. There's lots of other surveys out there. The Very Large Array Out in New Mexico is working on a survey called FIRST. And it's looking to do both the northern and southern galactic poles. It's a survey that's looking at 21-centimeter continuum radiation. This is the type of radiation that you get, the type of light you get from disks of galaxies, from blobs of gas.

19:06It allows you to see star-forming regions. It allows you to see jets off of radio galaxies. So the first survey, it has about five arc-second resolution, which is what you get on a really bad day with an optical telescope.

19:22It's about five times what you'd get from a reasonable sight with an amateur telescope. It's going pretty faint. And it's looking to collect data that will allow us to figure out where are all the galaxies that are actively forming stars? Where are the most distant radio galaxies? Galaxies that have actively feeding black holes in their center and are thus giving off radio emission. So this is an ongoing survey that's constantly working to increase its area, and it's working in radio.

19:56So it's just another way of looking at the universe.

20:01And so same deal, right? Catalog, gather as much data, raw data as you can, catalog it as best you can, and then make that information available to the scientific community and, I mean, the general public. I mean, it's all on the Internet, to look for whatever they want in the data as it stands, which is just amazing. And we're working to try and cover as much of the sky as we can in as many different colors as we can. There's two masks out there working in micron radiation. It used two 1.3-meter telescopes in Arizona and Chile to look at the sky.

20:32There have been infrared satellites. There have been X-ray satellites that have also worked to cover the sky. And we also look at data that was perhaps taken for other purposes as another source of perhaps serendipitous observations. There's a telescope called the Near-Earth Asteroid Telescope, NEET. It's primarily out there trying to make sure nothing hits the planet Earth. It's a good goal. But as it's out there surveying for this very specific purpose, it's also picking up supernova.

21:06It's also picking up variable stars. It's picking up lots of other stuff that just happened to be in the background of the same fields that it's looking for asteroids in. So we can use that data as well. There are very specific surveys looking at very small regions of the sky. But because they're building up data over years, we're able to learn interesting things. There were two projects, the Macho Project and the Oogle Project, that looked at the Magellanic Clouds, specifically looking for gravitational lensing events.

21:38This is where a nearby object, in this case nearby being on the outskirts of the Milky Way galaxy, passes in front of a background object, something in one of the Magellanic Clouds, and causes it to brighten through gravitational microlensing. In the process of looking for these events, they've also done things like find light echoes from supernova moving through the interstellar medium. So there's a supernova hundreds of years ago, thousands of years ago, and the flash of light from that supernova is forming an expanding shell of light.

22:12And as that light passes through the gas and dust between the stars, it temporarily illuminates whatever section that it happens to be in. By looking in the same direction for year after year after year, we can watch these shells of light move and then track them backwards and figure out where did they originate. And this is one of the neat ways that we're starting to figure out, well, what was Kepler's supernova actually like? What was the supernova in Cassiopeia actually like?

22:45We're tracing back the light echoes to learn more about events that we weren't around to see. And this, I think, is a realm of science that has no limit. I mean, you can just imagine bigger telescopes, more telescopes gathering more of the data that are looking deeper.

Future projects and public access

23:03So are there sort of dream projects in the works to do, you know, really enormous surveys? What's really cool is they're not even dream projects. They're actual projects. There's two really cool ones coming up, PANSTARS and LSST, Large Synoptic Survey Telescope. These two different many-meter telescopes are focused on trying to find things that are going to hit the planet Earth. Protecting the planet Earth is a good way to get money to build telescopes. But along the way, while they're out there taking snapshot after snapshot after snapshot of the sky, they're also going to be turning up supernovas.

23:41They're going to be turning up variable stars. They're going to be taking image after image after image of the same place on the sky. And those images can be added together to get some of the deepest images we'll have ever achieved of distant galaxies. It's all a matter of adding up the data over time. And with these two projects, we're going to have these huge telescopes taking in pretty much everything that's visible every single night. And that data can get added up to allow deep imaging or it can get used together to get time sequence imaging to see what are all the transient events that we've been missing all these years.

24:22And I think just to be clear, all this data is available on the Internet, right? Like you just go in – if you know where to look, you can pull it down and crunch it, right? Different surveys have different proprietary periods. This is a certain amount of time where the people who invested the intellectual resources and the monetary resources to build a given instrument, telescope, or take a certain survey, they get to have all the data to themselves. But at the end of these proprietary periods, all of the data becomes publicly accessible.

24:54One of the greatest ways to go out there and access a lot of this data is through a portal called Skyview. Just open up Google and do Skyview Virtual Observatory. And it will give you a form to fill out that will allow you to get radio data, x-ray data, optical data, all of the same field on the sky. It will allow you to map different objects onto these images to see where they happen to line up. It's a great resource both for professional astronomers and for people who are just trying to get a different wavelengths perspective of the universe.

25:31So everything's out there. Sometimes you just have to wait six months to a year to get your hands on it. Right, and I think the big need is for people in the computer industry, people who understand how to make a database sing and to be able to pull in that data and help crunch and help answer some of those basic questions. But I think the reality is that anybody who wants, assuming they have the skills, can go on to those surveys, download the information, and discover brand new objects, discover asteroids that have never been seen before.

26:05You know, it's all there. There are mysteries inside that data. All you have to do is go looking. And I can't restate what you're saying with enough emphasis. It's everyday people who are going out and discovering new things in some of these surveys. I've been at American Association of Variable Star Observers meetings where amateur astronomers have stood up and given talks on how they've gone through this database or that database, pulling up variables that no one had known about before just because they knew how to do all the MySQL queries effectively.

26:40With the Galaxy Zoo project, there have been just everyday members who are participating and noticing things that are new that are making cool discoveries where go check out the forums, look up the P's in the Galaxy Zoo forums. These are a new class of galaxies that were discovered by regular people who went, wait, what are all these little green compact galaxies? There's new stuff out there just waiting to be discovered, and you might be the person to make the next cool discovery.

27:14And if you do, let us know. We'd love to hear it. All right, Pamela. Well, thanks a lot, and we'll talk to you next week. Sounds great. One last thing before we go, though. Oh. So hopefully next summer I will both have my voice back, and I will have the opportunity, perhaps with even you, to go out and see a really cool eclipse out in the Pacific Ocean. I'm going to be on the eclipseofthecentury.com tour, and there's still seats available. All right. Well, that'd be fun.

27:45I'd love to see an eclipse. I've never seen a total solar eclipse. Yeah, I've never seen one either. So it's going to be a really cool tour. All right. Well, thanks for that. We'll talk to you next week. Okay. Sounds good. Bye. Bye-bye.

28:20You are listening to the 365 Days of Astronomy Podcast.

28:31Cool.

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