Date: December 19, 2009
Title: Lurking ULIRGS
Podcaster: Sue Ann Heatherly
Organization: NRAO — National Radio Astronomy Observatory
Description: While ULIRGs (You-lurgs) lurk in our local universe they are much more prevalent in the early universe! What’s a ULIRG, you ask? Join us as NRAO astronomer David Frayer describes his research characterizing these objects and what they tell us about galaxy evolution.
Bio: Sue Ann Heatherly is the Education Officer at the NRAO Green Bank WV site. She comes to astronomy by way of biology (BA in 1981), and science education (MA in 1985) She visited the Observatory as a teacher in 1987 and knew she’d found Camelot. She has been employed with the NRAO since 1989.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Astronomy Cast
Transcript:
SUE ANN HEATHERLY: Welcome to this addition of 365 Days of Astronomy podcasts. My name is Sue Ann Heatherly. I’ll be your host today, and this is our last podcast for the International Year of Astronomy, and we’ve enjoyed being with you.
For our December podcast, I’m joined by David Frayer, who is a newly. . .a new astronomer to NRAO. He’s been an astronomer for awhile, but he joined the staff at the Green Bank facility a few months ago. And welcome to the show. We’re happy to have you on our last podcast.
DAVID FRAYER: Happy to be here. Well, thank you.
SUE ANN HEATHERLY: All right, David. So, we’re going to talk about your research today, and, uh, we’re going to talk about this class of objects that you call ULIRGS. And, of course, I’m first going to ask you to, uh, to tell us what a ULIRG is.
DAVID FRAYER: ULIRGS are ultra luminous infrared galaxies, or they’re the most extreme examples of what we call just LRGS, which are Luminous Infrared Galaxies. They were discovered in the 1980s by the IRAS Satellite, and they’re relatively rare in the local universe. And you— They tend to show up and be the result of merging galaxies or interacting galaxies. So, if you look at our spiral galaxy, and you look at it’s infra. . .infrared, amount of infrared light that’s coming out and compare it to the stellar light that comes out, well, it’s a normal galaxy — it has a normal ratio. Uh, the infrared galaxies are infrared bright. They have more infrared light for the amount of— If you just looked at an optical picture and said “How bright is this object?” and you, you wouldn’t expect to see as much infrared emission. So, there is enhanced, um, emission coming out of the infrared.
SUE ANN HEATHERLY: So, if you were to use a regular optical telescope and look at these galaxies, uh, what would you see? Would they look like spiral galaxies? What, what do they look like?
DAVID FRAYER: Well, the lower luminosity ones you’d, you’d see maybe two spiral galaxies sort of close to each other. Um, the most luminous, the ultra luminous, or the ULIRGS, as we call them, you would, they were, they’re almost completely interacting galaxies. So, they would look like a train wreck. If the galaxies had a lot of gas to begin with, they would be infrared luminous. So, that’s the key; it’s not just fact that they’re interacting galaxies, but it’s inter, interaction between gas-rich galaxies, because then you have the gas and dust, or star formation. The star formation is heating up that dust and it’s re-radiating in the infrared, and it happens in obscured regions, meaning, you don’t, if you look at it optically, you don’t see where all the luminosity is coming out of.
SUE ANN HEATHERLY: Yeh, it’s going to—
DAVID FRAYER: It’s hidden by dust.
SUE ANN HEATHERLY: I wanted to ask you that. So, in some cases, these galaxies in the visible part of the spectrum just look like two isolated galaxies that happen to be close together.
DAVID FRAYER: They—
SUE ANN HEATHERLY: And you don’t see yet visibly the evidence for star formation. You don’t see extra star light.
DAVID FRAYER: You can see. . .they could be a little brighter, but if you look at the ratios, they would still. . .it would be under, they would be over luminous or extra emission in the infrared of what you see in the optical.
SUE ANN HEATHERLY: So, how, uh, common is it for galaxies to smash into other galaxies?
DAVID FRAYER: They’re rare locally, but a higher redshift, this is happening a lot more often. People in terms of just studying galaxy evolution in general are interested in studying this, this class of objects at higher redshifts.
SUE ANN HEATHERLY: So, you mean that as you look farther and farther, deeper into the universe—
DAVID FRAYER: It’s a thou—
SUE ANN HEATHERLY: —you see more examples?
DAVID FRAYER: Yeah, it’s a thousand times more common at a redshift of two than it is locally.
SUE ANN HEATHERLY: You’re an astronomer. What’s the. . .what’s your hypothesis as to why that’s the case?
DAVID FRAYER: Well, first of all galaxies had more gas in the past. And galaxies were closer together.
SUE ANN HEATHERLY: The universe was smaller, I guess.
DAVID FRAYER: The universe was smaller.
SUE ANN HEATHERLY: Yeah.
DAVID FRAYER: So, you had a lot more interactions.
SUE ANN HEATHERLY: Are these typically spiral galaxies that you see doing this?
DAVID FRAYER: Locally, what. . .yes, because, to get the, well, this is to get the extreme cases. There is a full spectrum. So, I mean, it’s sort of arbitrary where we draw the classification lines. So, we— The ULIRG is more than ten to the twelfth times a solar luminosity, but that’s just arbitrary. Uh, but, for those high luminosities, yes, it’s basically mergers between gas-rich systems, and the galaxies that are gas rich in the local universe are just spiral galaxies.
SUE ANN HEATHERLY: Uh-huh.
DAVID FRAYER: The mergers between elliptical galaxies without any gas, they would just go through each other — the stars wouldn’t care. You wouldn’t even. . .they wouldn’t really notice. I mean, they’re, the galaxies’ orbits could change if you’re looking at it from an outside reference frame, but you wouldn’t necess. . .you wouldn’t see a big burst of new star formation, because those galaxies have already consumed their gas. So, when you have merging galaxies that are gas rich, the gas sort of runs into each other, falls into the middle of the potential well, and then there is nowhere for it to go, and then, boom, you get a lot of star formation, as well as you get AGN activity, as well.
SUE ANN HEATHERLY: What, what’s an AGN?
DAVID FRAYER: Oh. Sorry. It’s a black hole that’s active; meaning, gas is falling in on it, so then it’s bright.
SUE ANN HEATHERLY: In your research, uh. . .uh, you’re part of this wave of renewed interest. . .
DAVID FRAYER: Yeah.
SUE ANN HEATHERLY: . . .in these.
DAVID FRAYER: Well, I was interested even before. So, the initial extreme examples that were found at high redshift, I was involved in studying their gas properties by searching for molecular gas using CO as a tracer.
SUE ANN HEATHERLY: Good. I’m glad you brought that up, because, we have been talking about galaxies that are bright in the infrared, but here you are at a radio astronomy observatory, rather than an infrared observatory, although you’ve—
DAVID FRAYER: Yeah.
SUE ANN HEATHERLY: —worked with infrared telescopes too.
DAVID FRAYER: That’s right.
SUE ANN HEATHERLY: So—
SUE ANN HEATHERLY: Tell us—the radio connection.
DAVID FRAYER: Well, the ra. . .it. . .for me personally, it’s going full, full circle. I started in radio, in millimeter wavelengths, looking for CO emission from whatever we could see at high redshift. And the, and the things that you could see in CO are the most luminous cases, which happen to be these ultra-luminous or even hyper-luminous, another order of magnitude, more luminous than . .than ultra-luminous is hyper-luminous galaxies. And they were uncovered by the SCUBA instrument on the JCMT. So, when
SUE ANN HEATHERLY: Which is the. . .?
DAVID FRAYER: Oh, it’s a, uh. . . submillimeter instrument that is studying dust continuum.
SUE ANN HEATHERLY: And that’s the James Clerk Maxwell Telescope.
SUE ANN HEATHERLY: I also want to define for everybody, just in case when we’re talking about “CO”, we’re talking about carbon monoxide. . .molecules.
DAVID FRAYER: That’s right.
SUE ANN HEATHERLY: Existing in these galaxies.
DAVID FRAYER: But it happens to be one of the most common molecules that you can see.
SUE ANN HEATHERLY: Okay. So, you started out looking for carbon monoxide at high redshifts, which means that the, the spectral lines from the carbon monoxide would be at low enough frequency–
DAVID FRAYER: Frequency. That we could—
SUE ANN HEATHERLY: —to see them.
DAVID FRAYER: We could see them.
SUE ANN HEATHERLY: Okay.
DAVID FRAYER: And then, they had to be bright enough so we could detect them, because our instrumentation was pretty poor back then. So, we did all the easy ones that we could see, because our instrumentation was somewhat limited. And then, after, it sort of hit a stopping point, and then I moved more towards doing infrared work, because then, then I joined the Spitzer Space Telescope, which does infrared research.
SUE ANN HEATHERLY: So, what are the questions that you’re trying to answer. . .answer now?
DAVID FRAYER: Now, I’ve mentioned the fact you have merging galaxies — so one of the hot topics is what fraction of the amount of stars that we see today come from one of these events versus more what we call quiescence, or, you know, more common star formation just percolating in the disks. And then, just understanding the evolution of different types of galaxies at high redshift and what they evolve into with low redshift.
SUE ANN HEATHERLY: So, is there any evidence that the Milky Way has merged, or—
DAVID FRAYER: Not. . .probably not a super major merger, because the common paradigm is that you, once you have two big spiral galaxies merging, if it’s sort of equal masses, they form something that’s more elliptical like and most of the gas is consumed. But certainly little dwarf galaxies and stuff have come through, and that has, uh, enhanced the star formation in episodes in the past, and there is evidence for that.
SUE ANN HEATHERLY: So, at some point in the future, we’re always told that we’re going to run into Andromeda or—
DAVID FRAYER: Yeah.
SUE ANN HEATHERLY: Andromeda, the Andromeda Galaxy will run into us. And what should we expect to see, if we’re around to see something—
DAVID FRAYER: That would be—
SUE ANN HEATHERLY: —during that merger?
DAVID FRAYER: That would be a very intense ultra luminous infrared galaxy phase, so. . .if we’re in the middle of our galaxy, where the action was happening, then I would guess a lot of the sky would be pretty bright, day and night.
SUE ANN HEATHERLY: Oh!
DAVID FRAYER: Potentially.
SUE ANN HEATHERLY: Yeah.
DAVID FRAYER: We’re out farther away, so what you would see would be just. . .you would see a big fuzzy thing in the sky. It’s something pretty bright. I would, I would guess something. . .you know, you would be able to, I would guess, a naked eye.
SUE ANN HEATHERLY: What do you hope to do with the, uh, with the telescope here?
DAVID FRAYER: But that’s not going to happen for—
SUE ANN HEATHERLY: I know. That’s— We don’t have to worry about it. I just always wonder how would our skies change.
DAVID FRAYER: –billions, a billion years or something. And the sun will survive just happily, probably. It will just keep on going around. It might not be going around the center of our galaxy, it will be going around the center of the two dynamic centers of the galaxies that merge together.
So, going– You are getting ready– You were– You were asking me, before I jumped in there again, on how, what I’m doing now. So, I’ve done the infrared, so I’m going back and, uh, we’re now at a radio observatory, and Herschel and Spitzer, the infrared telescopes, we found tons of these high redshift objects. So, now we get to follow them up in radio wavelengths, HI observations for the neutral gas, CO observations for molecular gas. There is other tracers to study the dense molecular gas. So, radio astronomy should have a, an active area of research in the next ten years doing more traditional, what I call traditional studies, evolution of galaxies, not from their stellar light or optical emission, but from the radio emission studying the ISM, the gas and dust from which the stars form.
SUE ANN HEATHERLY: Well, that sounds like fun.
DAVID FRAYER: It will be.
SUE ANN HEATHERLY: Yeah. Well, uh, thank you so much for joining us today. And thank you all out there for tuning into 365 Days of Astronomy. I’m Sue Ann Heatherly and I’m here today with David Frayer.
DAVID FRAYER: Nice talking to you.
SUE ANN HEATHERLY: Okay!
End of podcast:
365 Days of Astronomy
=====================
The 365 Days of Astronomy Podcast is produced by the New Media Working Group of the International Year of Astronomy 2009. Audio post-production by Preston Gibson. Bandwidth donated by libsyn.com and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at 365DaysOfAstronomy.org or email us at info@365DaysOfAstronomy.org. Until tomorrow…goodbye.