Tracing the Hidden Hand of Magnetism in the Galaxy
Susan Clark has long been drawn to the interstellar medium, the gas and dust that magnetic fields sculpt into stars.
Rachel Bujalski for Quanta Magazine
Introduction
Amid the roilings of the Milky Way, immense pockets of gas coalesce into clouds where stars are born. In this process, there is a hidden hand at play: magnetism.
“There’s this fantastic quote: You can measure a man’s ignorance by the strength of his magnetic field,” said Susan Clark, an astrophysicist at Stanford University. “In other words, when we have a piece of a problem historically in astrophysics that we don’t fully understand, just blame the magnetic field. Just wave our hands and say, ‘Ah! The magnetic field!’”
Exactly how this fundamental force helps sculpt our galaxy has long eluded scientists, largely because measuring the galactic magnetic field is a considerable challenge. Unable to detect it directly, astronomers tease out clues by studying dust that has been aligned by the magnetic field and the light that passes through this dust.
While much remains unknown, new tools and methods are bringing us closer to perceiving the influence of magnetism on the evolution of stars and galaxies, and Clark is one of the scientists spearheading this effort. As the leader of the Cosmic Magnetism and Interstellar Physics group at Stanford, she uses a combination of novel observational techniques, simulations and theory to unravel the puzzles of galactic magnetism. This year, she was awarded the Sloan Research Fellowship for “outstanding early-career faculty who have the potential to revolutionize their fields of study.”
“We are trying to not have it be a measure of our ignorance, but really understand the detailed physics of how the magnetic field affects things,” Clark said.
Quanta recently caught up with Clark at a conference in Santa Barbara, California, to learn more about her progress in probing the unseen forces of the galaxy. The interview has been condensed and edited for clarity.
Clark at her desk at Stanford University.
Rachel Bujalski for Quanta Magazine
What are you trying to figure out about the galaxy’s magnetic field?
There’s so much we don’t know. We really want to understand what role the magnetic field is playing in all the different physical processes that shape the diffuse gas between stars, called the interstellar medium, or ISM. We know that the magnetic field is playing some sort of still unclear role in the evolution of the gas to form very dense, cold clumps of material called molecular clouds that are the birthplaces of the stars. And then it plays a role in how that molecular cloud fragments and forms stars. We really don’t understand the details yet at all. But you see these long tendrils or filaments of gas in areas of the ISM with low density that are very well aligned with the magnetic field. We want to understand what that means for the transition of gas between phases in the ISM, and for turbulence in the ISM.
So the gas in the interstellar medium goes through phases?
The gas in the galaxy spans a mind-bending range of physical states. You have these very dense, cold molecular clouds. And then you have, at the other extreme end, very hot plasma. And you have a range of states in between, and we know that the story of star formation and the evolution of galaxies in general involves gas flowing between these different physical states, and we want to understand this whole life cycle of how gas gets converted into stars and spewed back out into the interstellar medium to seed future generations of star formation.
One thing we’ve discovered in recent years is that as you move to denser structures in the interstellar medium, the filamentary molecular clouds actually prefer to orient orthogonally with respect to the local magnetic field. This is very tantalizing for the idea that the structure of the magnetic field might be important for where and how you sculpt these long filamentary knots of molecular material that are eventually the things that fragment and form stars. There are so many open questions about that, and what we want to do ultimately is put together that whole evolutionary picture of how gas and the magnetic field interact to regulate this process of star formation.
The Antennae galaxies are merging, as seen in these images from the Hubble Space Telescope. Clark and her colleagues mapped the magnetic field orientations on top of the image at left.
SALSA V: Lopez-Rodriguez et al. 2022 (left); ESA/Hubble
Given that the magnetic field itself is invisible, how do you map its structure?
The ISM has a lot of dust in it, chunks of material that are micron-size or smaller, and these little dust grains have some amorphous shape; they’re not perfect spheres. And they align with a preferred orientation with respect to the local magnetic field that they’re sitting in. One consequence is that if you look at optical or near-infrared light coming off of a background star, the light is filtering through all of these magnetically aligned dust grains, and they absorb the light that’s polarized parallel to their long axis. So you can measure the polarization of the remaining starlight, and what you see is an imprint of the magnetic field in the dust in between your telescope and the star — isn’t that cool?
And then those dust grains are also radiating light, and that radiation has a polarization angle set by the orientation of the local magnetic field. So those are just two of the tools that we use.
Rachel Bujalski for Quanta Magazine
What do we know about how the galaxy’s magnetic fields are generated?
This is another big open question. We want to understand the ultimate origin of magnetism in the universe. But the really pertinent tricky question is: How do we have these large-scale, coherent, organized magnetic fields? How is it that, probably in the process of forming the galaxy, the motions of the gas were able to amplify and distribute the magnetic field to have the structure that we observe today? And one of the things that has been really helpful for trying to answer this question is to have observations of the magnetic field structure in other galaxies as well.
Do you see different magnetic field structures with different types of galaxies?
Yes, and we are still trying to understand exactly what the whole picture is here. One of the very cool observations we published recently is of the polarization of far-infrared light from the Antennae galaxies. These are a pair of merging galaxies, and we observed that in this interaction region between them there was a very coherent magnetic field structure inferred from dust polarization, which is fascinating for all sorts of reasons. We are really at the beginning of understanding these things.
Clark in her office at Stanford.
Rachel Bujalski for Quanta Magazine
Can studying the galactic magnetic field tell us anything about how it originated?
We do want to know what’s responsible for the “seed” magnetic field that got amplified during the formation of galaxies. It’s possible that that field was primordial, meaning it originated during the universe’s birth. But it seems to me that our measurements of the modern-day magnetic fields in galaxies on their own are not going to tell us where the initial seed field came from. It tells you there was some weak seed field, but that’s all.
Why did you become interested in galactic magnetic fields? It seems like a very specific problem.
I love problems where there are big, exciting, open questions. And what I also love about my field is that we get to tackle these problems where it’s not just that we don’t know the answer, we sometimes don’t know the best way to ask the question. Often we are asking: How do we approach this? Where is the information in these astrophysical tracers that lets us even ask the question that we want to ask? So we get to be a bit creative.
Rachel Bujalski for Quanta Magazine
When were you first drawn to learning about it?
I don’t think that was from some deep-seated, lifelong need to study magnetism, but it grabbed me in grad school as an area of astrophysics that is not well understood and is avoided for its complexity.
For astrophysics in general, I did a National Science Foundation research experience for undergraduates at Arecibo in Puerto Rico the summer before my senior year, and it was incredible. That’s when I realized I wanted to work on the ISM, when I really appreciated what the ISM was. It was my first experience with full-time research, and it was at this incredible facility — both because the telescope is incredible and because you live there on-site in these little cabins. The cabin that Jodie Foster was in, in the movie Contact, that’s where my bunk bed was.
Was there an earlier moment when you realized you wanted to be a scientist?
The honest truth is that I did not always want to be a scientist. At the point of entering college, I was like, maybe I will double major in biology and English. I loved biology in particular, and I’ve always loved writing, so I thought maybe I’d be a writer.
I have always been very interested in everything. It’s a common refrain for astronomers to say, “Oh, ever since I was a little kid, I absolutely loved space, and I knew that’s exactly what I wanted to do when I grew up.” And I definitely loved space as a little kid, but I also loved rocks, and dinosaurs, and lizards. Salamanders in particular. If anything, it all started with looking under rocks for salamanders with my sisters in the backyard in Virginia. It’s just a curiosity about nature and a love of learning, and that’s what you get to do as a scientist.
