Date: July 13, 2010
Title: Dr. Lucas Macri on the Extragalactic Distance Scale
Organization: Slacker Astronomy – http://www.slackerastronomy.org/
Podcaster: Michael Koppelman
Description: Michael Koppelman from Slacker Astronomy interviews Lucas Macri, an astronomer from Texas A&M, about his work on the cosmic distance ladder using extragalactic cepheid variable stars.
Bio: Slacker Astronomy is a light-hearted podcast that wanders the astronomical road-less-traveled. Visit us at http://www.slackerastronomy.org/.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by This episode of “365 Days of Astronomy” is sponsored by Elizabeth Fracek, and dedicated to the Chicago Academy of Sciences, the original home of my good friend, the Atwood Sphere.
Transcript:
Michael Koppelman: Hello again. Welcome to the 365 Days of Astronomy podcast hosted by Slacker Astronomy. This is Michael from Slacker Astronomy and slackerastronomy.org the amusing byways of astronomy and science podcasting by astronomers, amateurs and you.
We have an interview today with a great professional astronomer from Texas A & M University. Lucas Macri is going to talk to us about the cosmic distance scale, the distance ladder and his work on it, getting it to finer and finer precision. The whole interview is over at slackerastronomy.org. It’s really good. The part I had to cut out is good.
You should go listen to it at slackerastronomy.org. In the meantime we have something like 9 minutes and 17 seconds of the interview following and I hope you enjoy it. Here we go with me interviewing Lucas Macri about the cosmic distance scale.
Michael: Hey everybody I’, here with Lucas Macri who is a Doctor of Astronomy and you are out of Texas right now, right Lucas?
Dr. Lucas Macri: Hi Michael. Yeah I’m an assistant professor of astronomy at Texas A&M University.
Michael: Well, see you’re a Texan. How do you like that?
Lucas: It’s nice. I’ve been here for about two years in College Station. I’m part of a new astronomy group in the Physics and Astronomy department. We’re just getting started which has its challenges but also a lot of opportunities so it is cool.
Michael: You’re originally from Argentina, yes?
Lucas: Yes, that’s right.
Michael: In fact, we met in Argentina which was a blast. I had a lot of fun at that conference.
Lucas: Yeah the American Association of Variable Star Observers meeting. It was really nice. I had never been to one of those even though I lived in Boston for 11 years. I guess I was within a mile of AAVSO headquarters. Shame on me but it was really cool to meet all the people involved in Mendoza and enjoy the southern sky. It was nice.
Michael: I hear you’re an AAVSO member now?
Lucas: Yes, I remedied that travesty.
Michael: I’m sorry about your World Cup team. Were you following that?
Lucas: Yeah but you know Germany lost today so I’m feeling pretty good.
Michael: You gave a talk at the AAVSO meeting in Mendoza, Argentina about the Cosmic Distance Ladder and essentially some of the research you’ve been doing in that topic. Can you tell us a little bit about what you’re studying there?
Lucas: Sure. The Cosmic Distance Scale or the Extra-galactic Distance Scale is a generic term for the different techniques that astronomers use to measure distances between galaxies. About a hundred years ago a lady by the name of Henrietta Leavitt was working as a researcher at Harvard College Observatory. She discovered this class of variables known as cepheid variables, exhibited a very striking correlation between the period of their pulsation and their brightness.
Back then they didn’t know what the physical reason was for this but they realized that they had something good when they saw it. Miss Leavitt essentially what she discovered was what we now call the cepheid period luminosity relation.
This is a really good way of measuring distances between galaxies as long as those galaxies have this type of variable known as a cepheid. It works very well for any galaxy that has had a recent star formation. It works for spiral galaxies or the regular galaxies.
The cepheid phase is the end of the lifetime of a very massive star somewhere between four and twelve solar masses. Cepheids then are one of the many ways that astronomers have nowadays of measuring distances to galaxies. It is one of the most reliable I would argue since I work with them. I have to say that, right? [Laughter]
They’re pretty good and with the launch of the Hubble Space Telescope twenty years ago, especially after the servicing mission in 1994 when Hubble’s vision was restored to its design specifications. Astronomers have been able to discover cepheid variables further and further away from the Milky Way.
In the days of the wide-field planetary camera II which was in Hubble from 1994 until last year we could, not easily, but we could detect cepheid variables all the way out to almost 20 mega parsecs. That’s 60 million light years, give or take a few. That’s far enough that you can actually calibrate what we call secondary distance indicators.
Once you have a secondary distance to a whole bunch of galaxies then you can try to understand or you can try to calibrate something else that is brighter and you can actually take much further out. One of the best secondary distance indicators we have are what we call a temporary supernovae which are the explosions that occur when a white dwarf goes over 1.4 solar masses, probably due to the accretion of material from a neighboring star although we don’t know exactly why temporary supernovae blow up.
We do know that they explode and when they do, they outshine every other star in the galaxy where they are. They can be seen basically to the edge of the universe if you have a large enough telescope or if you expose your telescope long enough. With temporary supernovae you can actually reach much larger distances. You can go all the way out into what we call the Hubble flow.
Those are distances far enough away from the Milky Way that the galaxies that you are looking at are essentially receding from us due to the expansion of the universe and due to the Big Bang. So by doing this then we can use cepheid variables and temporary supernovae and measure distances to very distant galaxies.
By looking at the correlation between the distances to those galaxies and the recession velocities that they exhibit that’s the famous plot that was first made in the 1920s by Edwin Hubble by showing that more distant galaxies appear to recede faster from us. He realized that was evidence for the Big Bang. But also the slope of that correlation tells you essentially the age of the universe times a correction factor that has to do with the amount of stuff in the universe whether it is matter, or dark energy.
That sort of changes the number that you want to multiply that slope by. But in general it tells you the sort of the age of the universe. The better that we can measure distances to galaxies by using cepheids the better then we can calibrate the luminosity of temporary supernovae, the better we can measure or estimate the Hubble constant and therefore estimate the age of the universe.
A couple years ago Adam Reese, myself, and a whole bunch of other collaborators finished a project where we used the Advanced Camera for Surveys on Hubble and the NICMOS camera to do optical observations of near infrared observations of cepheid variables in six galaxies that had previously hosted the temporary supernovae explosion. We used that to calibrate the entire Cosmic Distance Ladder and measure the Hubble Constant with an accuracy and precision of about five percent. This actually means that we measured the universe to about five percent.
Michael: That’s awesome. So, you’re actually doing what many amateur astronomers do in that you’re getting light curves of these stars except just using space-borne really cool cameras.
Lucas: Yeah, I’m pretty lucky in that sense. Today I was working with some really cool new images from the Wide Field Camera 3, which is the latest optical in near-infrared camera was installed by the astronauts during the last servicing mission to Hubble. It’s a very sweet instrument.
The finest samples of a resolution that Hubble can deliver by the angular resolution of Hubble, the big saucer only one twenty-fifth of an arc second across, like .04 arc seconds per pixel. It makes all the previous cameras on Hubble, especially with big 2 which was really cool to have back in 1994, but by last year it was showing its age. It has been there 15 years in space – a lot of radiation damage and for Hubble, fairly large pixels, about a tenth of an arc second.
Now you have two and a half times further refinement in the resolution of the camera and it is pretty fast in the sense that for the same amount of exposure time you can go a lot deeper. That allows us to discover cepheids much further away than we were able to do before, not only in the optical but also more importantly in the near-infrared where the improvements they were likened to NICMOS which was the previously infrared camera is quite dramatic. Now we can do in two hours what before would have taken us 30 hours. It’s also a great way of saving Hubble time for other important projects.
Michael: That’s awesome. Is there some record like the farthest away galaxy we’ve ever measured a cepheid in?
Lucas: Well for cepheids another team of astronomers led by Kem Cook at Lawrence Livermore National Lab – I was a member of that team – attempted to measure cepheid variables in the ——– 11:17 cluster. That is a hundred mega parsecs away. That’s really difficult even for Hubble.
We had to do a couple of hours worth of integration just at one time to try to reach a percentage which would essentially be about around a 30th magnitude. We got the time and were all set and were starting the project, unfortunately this was at a time when the Advanced Camera for Surveys which is another great camera installed on Hubble in the late nineties, by the time we were doing our observations, 45 days into our sequence the camera had a short circuit. It didn’t recover for a long time…
Michael: Oh yeah, you broke ACS.
Lucas: Well fortunately it wasn’t [laughter] exposing on our object when it short-circuited. That would have probably resulted in our termination from future Hubble observations. No, it wasn’t anyone’s fault it was just radiation damage or just bad luck.
Whatever happened, the ACS died and eventually astronauts replaced one of the circuit boards and now it is back up and running on at least two of the channels I believe. We were actually able to get some pretty tantalizing evidence that there are things that are varying like you would expect cepheids to vary, but our observation only spanned 45 days.
Typically if you want to claim that you discovered a cepheid you want to see at least one whole period of pulsation. So for the things that we were targeting in COMA 13:05 we were hoping to discover things in 50 days and one hundred days. We didn’t quite make it.
Michael: How far away did you say that is?
Lucas: Comaclast13:16 is about 100 mega parsecs away. That is about 325 million light years. The reason we were targeting that was we were hoping then to have a cepheid distance directly to a galaxy that’s receding from us purely due to the expansion of the universe instead of having to jump from cepheids to type one13:39 supernovae. But that didn’t work out.
With the Wide Field Camera 3, Adam, Reese, myself, and a number of other collaborators were now targeting galaxies that are sort of 35 mega parsecs away from us. Whereas the COMA project was really stretching Hubble and using a lot of orbits to achieve our goal, here with WFC3 we can routinely discover hundreds of cepheids actually in galaxies that are much further away than what we could do 18 years ago. So, that’s pretty good.
Michael: That’s awesome.
Lucas: The new camera has essentially doubled or even tripled the volume that we can access with Hubble. That really helps a lot for trying to calibrate the luminosity of type one supernovae to greater accuracy and precision.
Michael: Where are you going next when you envision your career? What do you want to dig into in the long term?
Lucas: In the short to medium term we’re hoping in the next agency cycle to get time to do a few more type 1a supernovae calibrators so that we can actually measure the Hubble Constant to about two percent.
After that we probably need to wait for a European mission called Gaia that is going to go up hopefully in 2012 that is going to measure parallaxes. It is a very venerable Asian technique but the one that works the best for measuring distances to stars within the Milky Way.
We hope that Gaia will actually measure parallaxes to hundreds of cepheids within our own galaxy. That will allow us to calibrate the cepheid period also in relation to an accuracy well below, much better than one percent. Eventually, a decade from now we might actually have a one percent measurement of the Hubble Constant.
This is important not only for knowing the age of the universe to that level of accuracy and precision but also because the better you measure the Hubble Constant the better you can constrain the properties of dark energy.
We know dark energy is this mysterious component of the universe which appears to be dominating its current expansion rate and driving that expansion to an accelerated régime. We want to know what exactly dark energy is and one of the ways that we can try to understand better is by measuring what we call its equational state.
To measure that more precisely you need to know the age of the universe as well as possible. That’s what is driving us in the short to medium term.
Michael: Got it.
Lucas: In the long term Texas A&M University is one of the founding partners of the giant Magellan telescope which will be a 25 meter telescope located in northern Chile. It will be ten times bigger in diameter, larger than Hubble. It’s going to deliver, we hope, diffraction limited images meaning that it will actually give you images ten times sharper than Hubble at least in the near infrared.
Because it is ten times larger in diameter means it has a hundred times the collecting area. You’ll be able to go very deep with excellent image quality. That will be a very nice toy to have. We hope it will be ready by 2017-2018 at a cost of about a billion dollars – give or take a few hundred million. That’s really going to be great.
It is going to be located just a short distance away from the Large Synoptic Survey Telescope which is another really great project coming up in the next decade. It is going to survey the entire sky every two or three days. It will essentially make the first movie of the sky for stellar variability.
As some of your listeners out there who are variable star observers, which is going to have thousands of new supernovae exploding and detected every night. All the variable stars you can dream of across five different bands from the UV to the near infrared and that telescope is going to discover so many things. It has some very clearly stated primary science goals like measuring dark energy better, discovering all the asteroids in the solar system that could be harmful to mankind.
I think what’s going to be really sweet is all the things that LSST is going to discover that we have no idea are out there. This is the first time that we’re going to be staring deep into the universe with a cadence of minutes and hours and days and years.
Who knows what we’ll find? To have a 25 meter class telescope available for follow-up especially spectroscopic follow-up will be crucial. That’s sort of a decade away but it’s a really, really promising future.
Michael: How awesome. That’s amazing. Thank you very much. Dr. Lucas Macri from Texas A&M University and an all around good guy and fellow fan of Dulce Deleche, like that’s a part of my life now.
Lucas: Very good.
Michael: Which I’ve talked about on this podcast. Thanks a lot Lucas, I appreciate it.
Lucas: Thank you Michael back to you soon.
Michael: Remember there’s more over at slackerastronomy.org so check it out.
This transcript is not an exact match to the audio file. It has been edited for clarity. Transcription and editing by Cindy Leonard.
End of podcast:
365 Days of Astronomy
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