Date: April 20, 2010
Title: Megamasers
Podcaster: Richard Drumm
Link: Richard’s blog is at http://theastronomybum.blogspot.com/
Description: Dr. Braatz talks about his work on the MCP, the Megamaser Cosmology Project, where water megamasers are beginning to be used to augment the current rungs of the cosmic distance scale ladder. He describes for us what exactly a megamaser is and how it operates. Dr. Braatz explains how a direct measurement of the Hubble constant H0 will help define the nature of Dark Energy, the biggest mystery in the universe and the cause of the universe’s accelerating expansion.
Bio: Richard Drumm is President of the Charlottesville Astronomical Society in Charlottesville, Virginia.
Richard is the owner of 3D – Drumm Digital Design, an award-winning video production company. He was an observer with the UVa Parallax Program at McCormick Observatory in 1981 & 1982. He’s found that his greatest passion in life is public outreach astronomy and he pursues it at every opportunity.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Steve Layman, member of the Charlottesville Virginia Astronomical Society and is dedicated to his wife Alice and daughters Laura and Martha for supporting his interest in astronomy.
Additional sponsorship for this episode of “365 Days of Astronomy” has been provided by Tom Foster.
Transcript:
And thanks go out once again to George Hrab for that rousing intro! You can buy his book at lulu.com and his music can be purchased at CD Baby and iTunes. While you’re over at the iTunes Store, you should subscribe to the Geologic Podcast, which, though it has little to do with geology, has a lot to do with Geo and logic.
Hello, I’m Richard Drumm The Astronomy Bum in Charlottesville, Virginia, home of the University of Virginia, which has a great astronomy department, and also the NRAO, the National Radio Astronomy Observatory. It was at the NRAO last week where I met Jim Braatz. And I’m an astronomer at the National Radio Astronomy Observatory in Charlottesville, Virginia. And we talked about something near and dear to his heart, megamasers!
0:46
RBD:
Tell me about the Megamaser Cosmology Project.
JB:
OK, well the Megamaser Cosmology Project is a project that we have with a group of colleagues at the National Radio Astronomy Observatory and Harvard and some colleagues in Germany as well as some others. And it’s a, what we call a NRAO large project, meaning that it’s a project that requires a significant ammount of telescope time and has some lofty science goals.
So the megamaser part of the project refers to the fact that we observe masers in galaxies. Now a maser is essentially the same thing as a laser that you’re used to seeing in visible light, but a maser works in radio waves. So we observe masers that come from the centers of active galaxies and the cosmology part comes from the fact of what we’re trying to do with these masers. Now we use them to try to observe disks in the centers of active galaxies and by observing the disks and studying the dynamics we try to put the pieces together and measure in the end a distance to these galaxies.
And the ultimate goal of the project is to measure distances to several galaxies using this technique, and putting them all together we would be able to measure the expansion rate of the universe, the Hubble constant, by making a Hubble diagram, much the same way that Hubble did himself. Now the difference is, Hubble made measurements using Cepheid variables…
2:13
RBD:
Right
JB:
… and Cepheid variables have a number of complications. The biggest complication is that they’re based on luminosity, distances to the sources that they’re looking at. So when you’re looking and trying to determine the distance based on luminosity you have to worry about things like how bright intrinsically is the object you’re looking at, and is it buried in a dusty cloud and are all the objects the same intrinsically bright as the next that you’re trying to measure.
And they’ve got most of these things worked out but there’s a simpler method, and that is to try to measure what we call angular diameter distances. And this is instead of using the luminosity, what we do is we try to measure the linear size of the disk in the center of these galaxies, in other words how big is this disk in parsecs. And then by comparing the linear size to the angular size, which we can measure by imaging the disk we get a distance just through trigonometry, it’s fairly straightforward. Now I’ve oversimplified things a little bit. Measuring this linear size has some components to it, and it requires a significant ammount of observing time with two different types of telescopes. One type is with a telescope, the Green Bank Telescope, which is in Green Bank, West Virginia…
3:35
RBD:
One large dish, largest steerable dish in the world.
JB:
… largest steerable dish in the world, it’s one hundred meters in diameter, and very sensitive as you might imagine. And we use the Green Bank Telescope to take a spectrum every month of the galaxies that we’re interested in, we take a spectrum of the maser emission. And we use that spectrum to look for changes in the doppler velocities of the individual maser lines. And when we look at these changes in velocities they relate to a change, sorry, the change in frequency relates to a change in velocity…
4:06
RBD:
Right, doppler shift.
JB:
And this change in velocity with time is an acceleration. So what we’re actually measuring is an acceleration of masers as they’re circling around the black hole in the center of these galaxies. This is one piece of the observation, so we have acceleration, that’s just V squared over R, from, from simple dynamics.
And the second thing that we measure is a map of the masers in this galaxy. So the masers tend to come in an edge on disk, so if you could imagine, say, a CD that you’re looking at edge on and it’s spinning around…
4:37
RBD:
Right.
JB:
… we’re actually detecting masers from the edges of the disk and from the front side of the disk, from two different locations. And, actually the edges on either side of the disk, so altogether it’s three distinct locations. And when we make a map of these masers what we see is the detailed rotation property, we can see that the masers are circling the black hole in a Keplerian orbit, the dynamics are very simple, and we can basically map out the rotation structure and compare that with the V squared over R property that we measured from the GBT and that gives us the linear size of the disk, the radius of the disk.
Now, another thing we measure from that map that I mentioned, is angular size of the disk, and that’s the other part of the puzzle. So we get a linear size, we get an angular size, that gives us a distance.
5:31
RBD:
So megamasers are MEGAmasers because they’re driven by a supermassive black hole instead of merely some star.
JB:
That’s right. The terminology came about originally, and was associated with hydroxyl masers, because it was thought that the, the intrinsic luminosity of the megamasers was about a million times brighter than what was being seen in galactic masers, those, as you said, the ones that are associated with stars. It turns out, now, that the water megamasers that we’ve been talking about may not really be intrinsically be a million times bright, but the name kind of stuck from hydroxyl to water.
And in fact it’s pretty difficult to tell exactly what the total intrinsic power is for the water megamasers just for the reason of the beaming that we talked about earlier on, because they’re beamed out into a relatively narrow angle, they’re not radiating isotropically. It’s difficult to get a handle on just how bright, how intrinsically luminous the maser is.
6:31
RBD:
So we think of a laser of course, making a beam like a pencil lead, and of course in the case of the megamasers coming out of an accretion disk of a supermassive black hole, they’re, they radiate out in a plane as opposed, as a beam, but it’s beamed in this plane and not in all directions, North, South, East, West…
JB:
That’s right.
RBD:
How does a water megamaser work? What’s, what’s the engine inside there?
JB:
So the maser is essentially taking power from the black hole, from the accretion process of the black hole and absorbing that power and then reradiating it in a single spectroscopic line, this one particular water line. So the basic mechanism is, the black hole, in the center of the galaxy, is accreting matter and as it accretes matter, the matter heats up and it rubs against each other, there’s friction, gets very hot and it produces x rays and gamma rays and …
7:32
RBD:
Everything.
JB:
… produces everything. Radio waves. As the, as the x rays impact the accretion disk, they get absorbed by dust particles and the dust particles then warm up and so you have this very warm accretion disk there. Now within the accretion disk there are also water molecules, and the water molecules are what ultimately are producing the masers.
Now in general having a water molecule and an x ray photon meet up is not a good idea, the x ray would essentially dissociate the water molecule into its constituent atoms. But fortunately we have this dust that acts as a buffer. So the dust absorbs the x rays, it can handle the x rays, and then it warms up and it reradiates in the infrared. The infrared is something that a water molecule can handle and still stay in one piece. And these infrared photons in the end are providing the energy input into the water maser.
So, so we have this big powerful energy source associated with the black hole, and then sort of this buffering mechanism that allows a fraction of that power to be reprocessed and then reradiated into this single spectroscopic line associated with the water molecule.
8:43
RBD:
So you’ve got a soup of x rays & gamma rays & infrared & visible and radio, and those water molecules that don’t get disassociated by getting x rayed will soak up some IR and reradiate in radio.
JB:
That’s right. Now in fact the actual, the pumping mechanism we call it, you know, lasers and masers have a pumping mechanism, and so this is the detailed energy source that allows the population inversion to happen. So, you know, in order to produce a laser or a maser you have to have these molecules that are kind of like springs waiting to be unsprung. And so you have lots of water molecules sitting in this state waiting to be unsprung. And what really causes the springs to compress is a collision. So it’s a collisional, we call it a collisional pumping scheme that’s producing the population inversion.
9:36
RBD:
A simple mechanical collision, yeah.
JB:
Yep.
RBD:
Well, thank you Dr. Braatz, it’s been very interesting!
JB:
Thank you for having me, it’s been my pleasure!
9:44
End of podcast:
365 Days of Astronomy
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