In this series of posts, we sit down with a few of the keynote speakers of the 237th AAS meeting to learn more about them and their research. You can see a full schedule of their talks here, and read our other interviews here!
At the 237th Meeting of the AAS, Professor David Chuss of Villanova University will be telling us all about how he uses the same technique to learn about star formation in our galactic neighborhood and the oldest light in the universe, 13 billion light years away! This technique is called astronomical polarimetry — so let us dive into what that means, how it can probe such different parts of the cosmos, as well as what we can expect to hear at his plenary and why being a professor at one’s alma mater can be so rewarding.
First up: What is Astronomical Polarimetry?
Like a large part of astronomy, it all has to do with light: “As an astronomer, you have various ways of making sense of the light that happens to come from events in the past and far away,” Chuss began. He further explained that there is a rich, but ultimately limited amount you can learn from the light you observe—for instance, we can imagine getting a picture of the object through the photons it gives off, or getting the spectrum of the light in our band of interest.
However, there’s more to one of these signals than just intensity or distribution of photons: they can also be “twisted”, or polarized! “Maybe one of the most under-explored things is the fact that light is also a vector quantity,” Chuss continued. “It’s an electromagnetic field oscillation, so you can measure if it’s preferentially polarized in one direction or another, and that gives you a whole additional host of information.” (If you’re having a hard time visualizing what that looks like, I hear you and I heartily suggest the GIFs in this resource!) Measuring this polarization of the light is what we call polarimetry—and it has a number of applications at very different cosmic times. “Polarimetry probes anisotropy in some sense, whether it’s induced by magnetic fields or it’s induced by gravitational waves in the early universe in the Cosmic Microwave Background (CMB),” Chuss continued. “Anisotropy,” in this case, refers to the different polarizations of light along its axes, which is often the consequence of the physical environment where the photons originated. It tells us where the light has been, and what it’s been doing: “this is information that you can’t really get any other way.”
Anisotropies, near and far
Dr. Chuss began his journey in astronomical polarimetry in graduate school at Northwestern University, where he researched star formation. In star forming regions, dust grains will align along any present magnetic fields, which can then be mapped through detecting polarized emission from the dust itself. It was only after finishing his PhD and going to postdoc and subsequently work at NASA’s Goddard Space Flight Center that Chuss realized similar questions of light polarization were present in the CMB community. Though his colleagues were considering a much older signal with anisotropy caused by Thomson scattering in the early universe, both fields had the same technical problem: “As an instrumentalist, those are the big challenges: how to separate your tiny polarized signal from the much larger single that’s swamping your detectors.”
Today, Chuss works with two instruments, each performing polarimetry on a different target: CLASS and HAWC+. The Cosmology Large Angular Scale Surveyor (CLASS) is a dedicated microwave polarimeter (in other words: the thing that is used to do polarimetry) optimized for low frequencies (~100 GHz) and is used to measure anisotropies in the CMB from the ground. On the other hand, the High-resolution Airborne Wideband Camera Plus (HAWC+) is a far-infrared camera and imaging polarimeter located on NASA’s SOFIA, the Stratospheric Observatory for Infrared Astronomy (which is really a modified Boeing 747SP aircraft!). HAWC+ is optimized to probe anisotropies of star forming regions, and therefore measures higher frequencies of light (in the THz range) at an altitude of about 45,000 feet.
A HAWC+eye view of magnetic fields
When asked to give a preview of his AAS talk, Chuss was eager to give Astrobites a sneak peek. Hint: it features star formation, ties to astrochemistry, and even a little something about galactic centers, all courtesy of the first couple of years of HAWC+. First, Chuss explained that we have long understood magnetic fields to play an important role in star formation, but exactly what it is has not been determined. “Specifically, the star formation rate in the Milky Way is extremely inefficient—it’s way lower than you would expect from just gravitational collapse—and magnetic fields are the prime suspect in why that may be. But in addition to just star formation, we’re also kind of making progress and understanding exactly what the physics of the alignment mechanism of these grains is as well. That gives us insight into the composition of the dust grains.” This is particularly exciting since astrochemists are actively coming up with dust grain models, which HAWC+ will be able to test. Finally, the collaboration has recently been mapping magnetic fields in the galactic center, where strange physical conditions not found elsewhere prevail.
While discussing the status of magnetic fields in astronomy, which are somewhat notoriously difficult to account for, Chuss was excited about the possibilities: “We’re getting to the point where we need to consider them and we can consider them, which is the exciting part, both on the theoretical and the observational side.” He goes on to describe the wonder of HAWC+ in being able to see into the far-infrared without incurring the price tag of a space telescope. “We’re basically seeing this polarization information on finer scales with more sensitivity and larger maps and we’ve ever seen before. I would say the sensitivity/mapping speed and the angular resolution are the two things that are really kind of letting us kind of run with this.”
From Villanova to NASA and back
Recalling his first research project at Villanova University, Chuss recounts finding himself in a lab at some point with a project to build some type of instrument. “ I found that I really enjoyed that, and I wanted to combine the understanding of the universe with the ability to troubleshoot and build instruments and even think up new ones.” In graduate school, his thesis work led him to make five visits to the South Pole in the span of four years. He described immensely enjoying being able to see a small project through: from the design of the instrument through the debugging of the cryogenics and all the way to first light: “You got to see the whole the whole story arc of the astronomy getting done from conception to publication, which was that really got me kind of hooked.”
When asked about teaching at his alma mater, Professor Chuss described the initial experience as “surreal, […] familiar and brand new at the same time. A lot of people that I had as professors that are still there now, but at the same time, I just had such a different perspective. After being away for 20 years I came back as a scientist.” He went on to say that ultimately he really missed students and teaching: “I think it’s just really fun to interact with undergrads who really want to learn, and it’s fun teaching the classes, and it’s great to have casual conversations [while] kind of seeing yourself in them and remembering what it was like when you were first excited about these things.”
To learn more about Prof. Chuss’s work, check out his polarizing plenary “ The Role of Magnetic Fields: Galactic Science from HAWC+/SOFIA” at 11:00 am EST on Friday, January 15th at #AAS237!
Astrobite edited by: Haley Wahl
Featured image credit: American Astronomical Society