Date: April 11, 2009
Title: The ALMA Deep Field Experiment
Podcaster: Richard Drumm
Description: I interview Dr. Al Wootten, project scientist for ALMA North America (and scientist at the NRAO, the National Radio Astronomy Observatory) about the ALMA Deep Field experiment that will augment the groundbreaking Hubble Deep Field experiment. The ALMA Deep Field will reach deeper in space and thus further back in time than Hubble was able to and will explore a new realm of our universe’s childhood, even into the Era of Reionization itself!
Bio: Richard Drumm is President of the Charlottesville Astronomical Society and President of 3D – Drumm Digital Design, a video production company with clients such as Kodak, Xerox and GlaxoSmithKline Pharmaceuticals. 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 Astrocamp Summer Mission of Idyllwild, California. Help introduce a child to the world of Astronomy. Learn more at www.astrocamp.org.
Transcript:
RBD: Oh and thanks to George Hrab for that wonderful introduction. Thank you George! If you don’t listen to the Geologic Podcast yet, you should.
Welcome to the 365 Days of Astronomy Podcast series, I’m Richard Drumm, President of the Charlottesville Astronomical Society, here at the headquarters of the National Radio Astronomy Observatory, the NRAO, in Charlottesville, Virginia, and I’m sitting here with Dr. Al Wootten, project scientist for ALMA, North America, the Atacama Large Millimeter/submillimeter array, uh, which is under construction in Chile. Thank you, thank you, thank you Dr. Wootten for agreeing to be uh, here with us, uh, talking with us today!
AW: Well, thanks for inviting me!
RBD: Tell me about the ALMA Deep Field that’ll be one of the first things I think you’ll probably try to do.
AW: Yeah, well what we’d like to do is be able to look in this portion of the spectrum as sensitively as possible. So this will be a low resolution experiment we’ll want maximum brightness temperature sensitivity. So we’ll observe with the compact array. We’ll want maximum sensitivity to a continuum so we’ll observe with a very broad setting of the correlator.
The correlator is always split up into a number of frequency channels, sort of like the radio in your car does. And so in some of those frequency channels we may get an emission from a molecule in another galaxy, like CO, but for the most part we’ll be looking at dust emission from these galaxies.
So we’ll have the highest sensitivity mode, basically, of ALMA, and we’ll just stare at a place like the Hubble Deep Field for some period of time. The Hubble Deep Field took a couple of weeks, I think, of integration all told. So if we just say we’re going to integrate for a couple of weeks with ALMA what will we see?
1:44
The size of the Hubble Deep Field was about 5 arcminutes squared and so you know if we want to do an equivalent area with ALMA we’ll have several different pointings of the telescope because at any frequency the beams are smaller than that. So let’s just say we observe at a one millimeter wavelength where the sky is pretty transparent, for 2 weeks over 5 arcminutes, then we’d be able to achieve a sensitivity on the order of a few tens of micro janskys.
OK, what does that mean? Ha! Well, basically, if you were to observe, look in the Hubble Deep Field as a function of redshift, you would discover that almost all of the galaxies that Hubble found were nearby, and very few were at high redshift. I think there were only a couple dozen beyond a redshift of 2 or 3.
2:30
But in the ALMA Deep Field what’s happening is the spectral energy distribution, these galaxies, which peaks at 200 microns, is being shifted into the radio region. And so what we’ll see is very few nearby galaxies. We’ll see quite a few of these more distant galaxies.
And so if you look at the ALMA Deep Field it’ll be a very complimentary view of the universe where we’ll see only a few nearby galaxies, but we’ll see on the order of hundreds in this 5 arcminute square region at a sensitivity of tens of microjanskys for instance, galaxies at a Z of greater than 2.
And so it’s very complimentary and it’s an excellent way of finding the most distant galaxies, just because of the fortunate circumstance that their brightest part of their spectrum has gotten shifted into the ALMA part of the spectrum.
3:19
RBD: What sorts of things would you be able to determine from that?
AW: Well, for uh, for any given, you know, 5 arcminutes and single setting of the correlator, the correlator maximum sensitivity at bandwidth is about 8 gigahertz, and I mentioned seeing the lines of carbon monoxide, that’s exciting because that gives you from the doppler shift to those lines you can get an idea of the mass involved in the galaxy, its recession velocity, its distance. Whereas if you just have the continuum you don’t really know either of those things.
3:51
So it’s really interesting if you could see the CO line. So for about a third of the galaxies in the ALMA Deep Field, probably a couple hundred, we should accidentally have a CO line, they’re separated by 115 gigahertz, but as you go out to higher redshifts the spectrum gets compressed by 1 + Z. So actually those lines which are spaced every 115 gigahertz become spaced by 115 gigahertz over 1 + Z as you go further out. So by the time you get Z of 5 or 6 you probably have a CO line in any given 8 gigahertz portion of the spectrum. And you may be lucky enough to have 2.
4:29
So if you integrate at, you know, in this Deep Field for 2 weeks and then go to a different frequency and integrate for 2 weeks and compare the signals that you have, you’ll probably find that you’ve got multiple CO lines. And then not only can you determine, you know, the mass and the redshift but you can start to do some physics, and by looking at the relative intensity of these different carbon monoxide lines, (you’ll) be able to discover something about the physical conditions, the density and temperature in that region, in that galaxy that’s giving rise to them.
4:57
So you get a tremendous amount of information back. It is a long and difficult experiment but I think it’s definitely one people will try. If you, if you think about, instead of a Deep Field, a bigger region, you have just, you know, if I just point ALMA in some direction, what will I see? Well you’ll almost certainly see a galaxy per 30 arcsecond field of view in an hour or so at 1 millimeter no matter where you point it, as there are just that many of them around. Probably be a high redshift galaxy for the reasons we just discussed. So they’re going to be, you know, like grains of sand on the beach.
5:30
RBD: How close to the EOR, the Epoch of Reionization will ALMA reach? Is it that far back?
AW: Well the Epoch of Reionization is when the first stars lit up and so, you know, that’s depending on whose cosmology you believe, something like Z of 10 or 15 or something like that. And so, you know, ALMA doesn’t see the stars, but how do the stars form? The stars presumably form from clouds of gas, there isn’t any dust because we have nothing but hydrogen and helium in that part of the universe. Molecular hydrogen in our region of the universe is formed on dust grains, there are no dust grains back in the very earliest days, but if you get the hydrogen dense enough then the hydrogens colliding with each other can begin to make hydrogen molecules.
6:18
In particular if you get 3 hydrogens that all hit at once and one of them can carry off the excess energy, leaving the other 2 bound, and so you can get a bound hydrogen molecule. You need densities on the order of 10^8 for that to occur. That’s fairly dense, but it’s not as dense as a star, and presumably to make a star you have to get stuff dense in the first place, so gravity presumably takes over, we make molecular hydrogen, eventually you make a star, and the first stars are born. Ah, well, the molecular hydrogen lines are fairly energetic ’cause it’s a light molecule, so the lowest rotational line is at 28 microns, about 500 degrees above the ground state. Surely you’ll have temperatures that high when you’re forming the first stars, and so you’ll excite this 28 micron line. And at Z of 10 that falls in the ALMA highest redshift band, highest frequency band at 900 gigahertz or so.
7:12
So if there’s enough molecular hydrogen, if enough stars are being born, in some region of space, and it appears everything happened pretty quickly there at the beginning, you know, we may be able to observe those first stars at that period of time, so that is basically the Epoch of Reionization observed in a different way from the way you might observe it in the neutral hydrogen lines from the ground.
7:38
Spitzer has seen tremendous molecular hydrogen emission in these lines in various places in the sky. Some clusters of galaxies there’re shocks that have, are emitting 10^42 ergs per second of energy. That’s a large number. And so undoubtedly these sorts of conditions also happened as the first stars were being born. It’s all ripe for exploration, I’m just speculating completely now, nobody’s ever written any of this down that I know of.
8:05
Another exciting thing about ALMA is optically you may be able to see these but the isotopic differences in molecules are extremely difficult to, uh, separate optically because you need high resolution spectroscopy. High resolution is a little easier to achieve in the radio regime and so CO and 13 carbon, oxygen are separated by about 5 gigahertz, which is a huge amount in the radio region.
8:34
And so ALMA is also very well situated to be able to look at the isotopic composition of those first elements in the universe. So I think there’s a whole slew of interesting things we could do at and around the Epoch of Reionization when the first stars and galaxies formed in the universe.
8:53
RBD: You must be like a kid waiting for Christmas on Christmas Eve, just…
AW: Yeah, well, I went to my first meeting on this about twenty, a little over 25 years ago, so [laughs] it’s been a very prolonged Christmas! [laughs]
RBD: Well, super! Thank you again!
AW: Yeah, yeah, thank you. Appreciate it Richard.
9:10
End of podcast:
365 Days of Astronomy
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