Date: July 17, 2011
Title: The Galactic Star Thief
Podcaster: Rob Sparks and Knut Olson
Organization: NOAO, the National Optical Astronomy Observatory
Links: NOAO, NOAO’s Twitter feed
Description: People who live in the southern hemisphere can see the Large Magellanic Cloud(LMC) and the Small Magellanic Cloud (SMC), two nearby companion galaxies to the Milky Way. These galaxies are currently undergoing complex gravitational interactions resulting in stars being transferred between the galaxies. Knut Olson discusses some of his recent results showing the LMC has a population of stars that appear to have originated in the SMC.
Bio: Rob Sparks is a science education specialist in the EPO group at NOAO and works on the Galileoscope project (www.galileoscope.org), providing design, dissemination and professional development. He also blogs at halfastro.wordpress.com.
Knut Olson is an astronomer at the National Optical Astronomy Observatory in Tucson, Arizona.
Sponsor: This episode of the 365 Days of Astronomy podcast is sponsored by the National Optical Astronomy Observatory. NOAO is a US national research and development center for ground-based nighttime astronomy. We provide astronomers access to world-class observing facilities on a peer-reviewed basis. Our mission is to engage in programs to develop the next generation of telescopes, instruments, and software tools necessary to enable exploration and investigation through the observable Universe. For information on observing proposals or our public programs, please visit www.noao.edu for more information.
Transcript:
Rob: Hi, this is Rob Sparks from the Education and Public Outreach Group at the National Optical Astronomy Observatory in Tucson and I would like to welcome you to this episode of the 365 Days of Astronomy podcast. I am here today with Knut Olsen of the National Optical Astronomy Observatory. Hi Knut.
Knut: How are you doing?
Rob: Good, how about you?
Knut: Good, thanks.
Rob: First, I’d like you to tell us a little bit about yourself and your role here at NOAO.
Knut: I am an astronomer here at NOAO, born in Norway, lived in various places in the U.S. Last place I lived before Tucson was Chile and I was there for eight years working at the Cerro Tololo Inter-American Observatory. I have been in Tucson since 2007. I work as part of the NOAO systems science center. We take care of users who are using telescopes like Gemini and Keck and Magellan where they apply for time through NOAO and we help them with their observing runs and we try to make sure that the telescopes are serving the needs of those users and that sort of takes half my time and the rest of my time I am doing research in stellar populations of nearby galaxies.
Rob: Okay, and that’s exactly what we will be talking about today. You were measuring velocities of stars in the Large Magellanic Cloud, a companion of the Milky Way. How do you make these measurements of stellar velocities?
Knut: Yes, the basic principle is using the Doppler Effect where you look at the spectrum of the star and the stars have features in their spectrum, absorption lines that are due to various elements in the atmospheres of the stars and those give you some sort of sharp markers that you can measure in the stars and then you look to see whether those features are at the same wavelength as you would expect them to be if they were moving at the same velocity as you were. If you find them to be redder compared to some standard star that you use that has a known velocity, that means that star is moving away from you. If it’s bluer, than it’s moving toward you. That’s the basic measurement. We want to know what the velocities of the stars in the LMC are doing but we also have to worry about the velocity of the Earth around the Sun, the Sun of course moves around the galaxy so that imparts an additional velocity. What we are interested in is the velocity of stars compared to the Sun so taking out the motion of the Earth.
Rob: Yeah there are a lot of corrections you have to make in there I am aware of. So what telescope did you use and what instrument did you use to make these observations?
Knut: We used a telescope that I am pretty familiar with from working at Cerro Tololo Inter American Observatory (CTIO). It’s the four meter Blanco Telescope at CTIO and the spectrograph was the Hydra CTIO positioner and spectrograph and this is a fiber fed spectrograph where there is a robot that puts the fibers anyehwere you want them to be on the plate that is roughly a meter in diameter which covers forty arc minutes on the sky so that’s almost a degree, a little bit larger than the full Moon appears on the sky and can take 140 spectra at a time. Those fibers run down into what is the actual spectrograph. You pass the light, bounce it off of the grating which splits it up into its component wavelengths and then you image the spectra on a detector and then you extract the spectra on a computer and make the measurements.
Rob: That sure beats doing it one at a time!
Knut: Yeah, it sure does.
Rob: It speeds things up. So after analyzing the data, you found a distinct population of stars that was not rotating around the galaxy as expected. What were these stars doing that made them stand out?
Knut: Yes, there was a fair bit that went into identifying these particular stars. The first thing is that the LMC is a galaxy as you said, that is orbiting around our own. It has a fairly large motion and depending on what part of the galaxy you are looking at the amount that you see of that motion is different and so we basically had to remove, make a correction for that. But after having done that, we then looked at, tried to understand the gravitational potential that would produce the motion we saw. For 95% of the stars we found they sort of behaved like we expect stars to behave in galaxies. They were all rotating around the galaxy on what were roughly circular orbits. We can’t know for sure for any individual star whether its orbit is circular but on average they are circular. It’s rotation speed rises for a while and then flattens out. That flattening out is a sign in most galaxies that there is unseen matter that is making this stuff spin. But 5% of the stars actually, if they were in the same plane of the LMC as the rest of the stars are going around the wrong way. This is sort of like looking out into our solar system and finding a planet that is going around the wrong way. Another possibility, since we don’t know the geometry of this unusual population is that it is not spinning around in the opposite sense, but at a highly inclined disc so that it looks like it is going around the wrong way but is just the geometry of the population is very different from what we naively think it would be.
Rob: So how did these stars get in this strange orbit?
Knut: Our interpretation rested on a bunch of things. One was that the stars have these strange orbits, but that they have the same sort of velocity signatures as these streamers of gas we see connecting the LMC and the SMC. The Small Magellanic Cloud (SMC) is another companion to the Milky Way and a closer companion to the LMC. That connection led us to think that there is gas and stars falling off the SMC and hitting the LMC. The gas stops because basically gas collides and it comes to a halt and conserves momentum I should say, but the stars don’t feel the impact of stars in other galaxies because there is so much space in between them so they just keep going. So we think these stars have been accreted by the Large Magellanic Cloud.
Rob: So a form of galactic cannibalism if you will.
Knut: Yeah, that’s right.
Rob: I know sometimes you can determine a star’s origin by its composition, especially what we call its metallicity. Were you able to make these types of measurements and what did you find?
Knut: This was another key piece of evidence in this interpretation. What we used, is there are some lines of calcium which are very strong in many stars and the measurements of the strengths of these lines have been calibrated against the bulk abundance of iron in some astronomical objects so we can use the strength of the calcium lines as a proxy for the iron abundance. We did this and we found that most of the stars in the LMC that we looked at had one sort of very common, distinct iron distribution whereas the stars that had very strange kinematics or very strange velocities were very much more poor in their iron abundance compared to these other stars. In fact they match very well the metallicity distribution we see in the SMC. That was a key piece of evidence that led us to think they had been accreted from the SMC.
Rob: So what are the next steps in figuring out this complex system?
Knut: Yeah, so there’s a lot of things we can do. One thing we would like to do is that emanating from the LMC and SMC binary system is this trail of hydrogen gas that extends about 200 degrees across the sky called the Magellanic Stream and no one has ever found stars associated with that stream. We think that if there has been an accretion of stars by the large cloud from the small cloud, there have to be stars in that stream. That stream was formed from the interaction. That was a recent model by Kartina Besla and her collaborators. She is at the Center for Astrophycis at Harvard. Her model shows that an interaction between those two galaxies about 1.2 formed the Magellanic stream, one of the features that we see due to this interaction. If those stars have been accreted from the SMC there have to be stars in the stream, so that’s one of the things we want to look for. A second thing is we want to be able to trace this population of stars away from the LMC toward what we think is their origin in the SMC. This could be very interesting because we are pretty sure they should be there and if we see them displaced either in velocity or in space compared to the gas that connects the small and the large clouds then we could perhaps make a measurement of the density of the gas in the galactic halo. We think that there is hot gas in the galactic halo and hot gas exerts a ram pressure. It’s just like the gas from the SMC stops essentially when it hits the LMC. This hot halo gas would push on the gas but it can’t push on the stars so we can actually make a measure of the hot halo gas density in our own galaxy which would be important for understanding how the galaxy is put together.
Rob: So one last question for you. In the big picture of stellar evolution or galactic interactions do you hope to learn about from this system?
Knut: Yeah, so in the big picture what we would like to know is for one thing, the LMC is a laboratory for all sorts of astrophysical processes. It happens to contain a star forming region, 30 Dorades, which is more active and larger than any we see in our galaxy.
Rob: That’s the Tarantula Nebula?
Knut: That’s the Tarantula Nebula, right. The interesting thing is that these streamers of gas that we see coming from the SMC and hitting the LMC hit the LMC at exactly the location of the Tarantula Nebula. So we think that the very large gas pressure that you get is what is allowing 30 Dorades or the Tarantula Nebula to form because when you form that many hot stars in one place they exert a tremendous pressure and you need some way to contain that pressure from the stellar winds and the supernova. So if this high pressure from the outside wasn’t there, the Tarantula Nebula should basically shred itself apart. So that’s one thing, the star formation. Another thing you can learn is what are the effects of galaxy interaction on the individual galaxies themselves. We know that in our model of the universe galaxies are built up by eating smaller bits and turning them into larger galaxies. This is very important in how we think galaxies grew to their current size. In our backyard we have the beginning, sort of a laboratory case of this kind of interaction where one galaxy is beginning to eat another.
Rob: And it’s nice and nearby and easy to study;
Knut: That’s right. Most of the merging galaxies that you see are many hundred of millions of light years away. And these are a mere 160,000 light years away.
Rob: Thank you, Knut. I have never been to the southern hemisphere and seen these objects but I hope to get there sometime and see them with my own eyes.
Knut: Yeah, you should definitely go. I did my whole thesis on the LMC before having actually seen it but it was great to see it with my own eyes.
Rob: Thank you for joining me today.
Knut: All right. Thank you
Rob: This is Rob Sparks from the National Optical Astronomy Observatory. Thanks for listening to this episode of the 365 Days of Astronomy podcast.
End of podcast:
365 Days of Astronomy
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