Research, teaching and outreach in Physics at UW–Madison
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New research helps explain why the solar wind is hotter than expected
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When a fire extinguisher is opened, the compressed carbon dioxide forms ice crystals around the nozzle, providing a visual example of the physics principle that gases and plasmas cool as they expand. When our sun expels plasma in the form of solar wind, the wind also cools as it expands through space — but not nearly as much as the laws of physics would predict.
In a study published April 14 in the Proceedings of the National Academy of Sciences, University of Wisconsin–Madison physicists provide an explanation for the discrepancy in solar wind temperature. Their findings suggest ways to study solar wind phenomena in research labs and learn about solar wind properties in other star systems.
“People have been studying the solar wind since its discovery in 1959, but there are many important properties of this plasma which are still not well understood,” says Stas Boldyrev, professor of physics and lead author of the study. “Initially, researchers thought the solar wind has to cool down very rapidly as it expands from the sun, but satellite measurements show that as it reaches the Earth, its temperature is 10 times larger than expected. So, a fundamental question is: Why doesn’t it cool down?”
Physicists to improve plasma fusion mirror devices with $5 million grant
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University of Wisconsin–Madison plasma physicists will harness the power of high-temperature superconducting magnets to design and build a more efficient plasma fusion device, thanks to a two-year, $5 million U.S. Department of Energy grant awarded April 7.
The team, led by physics Professor Cary Forest, has been conducting fusion research for over two decades and expects this new device — the Wisconsin HTS Axisymmetric Mirror (WHAM) — will serve as a prototype for the next generation of fusion reactors.
“Neutrons generated from fusion are useful for many things, from making medical isotopes to potentially being a power source in the future,” Forest says. “Our idea initially — and this was funded by a UW2020 grant — was to build a neutron source which could go several orders of magnitude beyond current medical isotope production efficiencies but also provide a key first step in the direction of advancing fusion energy.”
Just as the sun has planets and the planets have moons, our galaxy has satellite galaxies, and some of those might have smaller satellite galaxies of their own. To wit, the Large Magellanic Cloud (LMC), a relatively large satellite galaxy visible from the southern hemisphere, is thought to have brought at least six of its own satellite galaxies with it when it first approached the Milky Way, based on recent measurements from the European Space Agency’s Gaia mission.
Astrophysicists believe that dark matter is responsible for much of that structure, and now researchers with the Dark Energy Survey — including University of Wisconsin–Madison assistant professor of physics Keith Bechtol and his research group — have drawn on observations of faint galaxies around the Milky Way to place tighter constraints on the connection between the size and structure of galaxies and the dark matter halos that surround them. At the same time, they have found more evidence for the existence of LMC satellite galaxies and made a new prediction: If the scientists’ models are correct, the Milky Way should have an additional 150 or more very faint satellite galaxies awaiting discovery by next-generation projects such as the Vera C. Rubin Observatory’s Legacy Survey of Space and Time.
Prof. Keith Bechtol
Two new studies, forthcoming in the Astrophysical Journal and available as preprints (pre-print 1; pre-print 2), are part of a larger effort to understand how dark matter works on scales smaller than our galaxy.
“The ultra-faint galaxies that orbit the Milky Way are small clouds of dark matter with just enough stars to see that they exist. They are nearly invisible, but if spotted, they make excellent natural laboratories to study dark matter,” Bechtol says. “We hope to learn what dark matter is made of, how it was produced in the early Universe, and what relationship it has to the known particle species.”
Shining galaxies’ light on dark matter
Astronomers have long known the Milky Way has satellite galaxies, including the Large Magellanic Cloud, which can be seen by the naked eye from the southern hemisphere, but the number was thought to be around just a dozen or so until around the year 2000. Since then, the number of observed satellite galaxies has risen dramatically. Thanks to the Sloan Digital Sky Survey and more recent discoveries by projects including the Dark Energy Survey (DES), the number of known satellite galaxies has climbed to about 60.
Such discoveries are always exciting, but what’s perhaps most exciting is what the data could tell us about the cosmos. “For the first time, we can look for these satellite galaxies across about three-quarters of the sky, and that’s really important to several different ways of learning about dark matter and galaxy formation,” said Risa Wechsler, director of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC). Last year, for example, the DES team used data on satellite galaxies in conjunction with computer simulations to place much tighter limits on dark matter’s interactions with ordinary matter.
Now, the team is using data from a comprehensive search over most of the sky to ask different questions, including how much dark matter it takes to form a galaxy, how many satellite galaxies we should expect to find around the Milky Way and whether galaxies can bring their own satellites into orbit around our own – a key prediction of the most popular model of dark matter.
Hints of galactic hierarchy
The answer to that last question appears to be a resounding “yes.”
The possibility of detecting a hierarchy of satellite galaxies first arose some years back when DES detected more satellite galaxies in the vicinity of the Large Magellanic Cloud than they would have expected if those satellites were randomly distributed throughout the sky. More data was needed to conclusively attribute this excess to galaxies that arrived at the Milky Way with the Large Magellanic Cloud.
The two studies probed, analyzed and characterized data from years of observation in two sky surveys.
In the first published study, the DES group combined observations from DES with those from the Pan-STARRS survey, together covering 75% of the sky, to test this hypothesis. The DES data represents nearly 40,000 images from a 500-million-pixel camera collected over three years from a telescope in Chile.
The raw DES data was run through a series of data compressions, including a final step led by Bechtol’s group, to identify and catalog individual stars, including their color, which infers temperature, and how far away they are.
“We throw the star catalog into our search algorithms, which are responsible for identifying small groups of stars that are clustered in space and have similar colors and brightness. There’s a particular distribution for what we expect the stars to look like in ultrafaint galaxies,” says UW-Madison physics graduate student Mitch McNanna. “Even then we’re not 100 percent sure that we’ve found a real galaxy, so we also collect spectroscopic observations to measure the doppler motion of the stars. Hopefully we see the group of stars is moving in a way that’s different from the rest of the stars in the Milky Way halo.”
The team, including Alex Drlica-Wagner at Fermilab, produced a model of which satellite galaxies are most likely to be seen by current surveys, given where they are in the sky as well as their brightness, size and distance.
In the second study, led by others in the DES team including Ethan Nadler at Stanford University and collaborators, the team took the findings of the latest satellite census and analyzed computer simulations of millions of possible universes. Those simulations model the formation of dark matter structure that permeates the Milky Way, including details such as smaller dark matter clumps within the Milky Way that are expected to host satellite galaxies. To connect dark matter to galaxy formation, the researchers used a flexible model that allows them to account for uncertainties in the current understanding of galaxy formation, including the relationship between galaxies’ brightness and the mass of dark matter clumps within which they form.
Those components in hand, the team ran their model with a wide range of parameters and searched for simulations in which LMC-like objects fell into the gravitational pull of a Milky Way-like galaxy. By comparing those cases with galactic observations, they could infer a range of astrophysical parameters, including how many satellite galaxies should have tagged along with the LMC. The results were consistent with Gaia observations: Six satellite galaxies should currently be detected in the vicinity of the LMC, moving with roughly the right velocities and in roughly the same places as astronomers had previously observed. The simulations also suggested that the LMC first approached the Milky Way about 2.2 billion years ago, consistent with high-precision measurements of the motion of the LMC from the Hubble Space Telescope.
Galaxies yet unseen
In addition to the LMC findings, the team also put limits on the connection between dark matter halos and galaxy structure. For example, in simulations that most closely matched the history of the Milky Way and the LMC, the smallest galaxies astronomers could currently observe should have stars with a combined mass of around a hundred suns, and about a million times as much dark matter. According to an extrapolation of the model, the faintest galaxies that could ever be observed could form in halos up to a hundred times less massive than that.
And there could be more discoveries to come: If the simulations are correct, there are around 150 more satellite galaxies – more than double the number already discovered – hovering around the Milky Way. The discovery of those galaxies would help confirm the researchers’ model of the links between dark matter and galaxy formation, and likely place tighter constraints on the nature of dark matter itself.
Sarah McCarthy earns fellowship through National Science Foundation’s Graduate Research Fellowship Program
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Congrats to first-year grad student Sarah McCarthy on being named a 2020 NSF GRFP recipient!
The Physics Learning Center goes online for remote learning
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Like course instruction, The Physics Learning Center and other tutoring and advising groups on campus have gone online to support their students’ learning. PLC was featured in an article about how these Centers are adapting to instructional changes during the pandemic.
Two students earn Quantum Information Science and Engineering Network (QISE-NET) honors
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Congrats to grad students Xiaoyu Jiang and Abigail Shearrow on earning QISE-NET awards! Jiang will work with Argonne National Labs and Shearrow will work with Google for their respective projects.
Stanislav Boldyrev, DOE look back at his 10 years of Early Career award research
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Prof. Stanislav Boldyrev earned a DOE Early Career Award in 2010. Ten years later, the DOE checked in to see what his research accomplishments in plasma turburlence have been during that time.
Dept Chair Sridhara Dasu featured in Symmetry Magazine
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Symmetry Magazine recently ran a story about the different rungs of the academic ladder, featuring profiles of seven different reserachers at the Large Hadron Collider. UW-Madison Department of Physics Chair, Prof Sridhara Dasu, spoke to Symmetry about the added roles and responsiblities — and, often, the added benefits — of being a Chair.
Shared experiences: Conference for women in physics brings UW undergrads together
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UW–Madison sophomore Haley Stueber did not always know she wanted to study physics, but she had an inkling.
“I started taking astronomy and physics classes in high school, and what really got me into physics was the astronomy aspect,” Stueber says. “I was always of the mindset that I wanted to keep learning, and I felt like the realm where I could do that was space, because the universe is so big.”
Like most women interested in pursuing a physics major, Stueber noticed something when she started college.
Unlike most of their physics lectures, the undergrads who attend CUWiP sessions are surrounded by only women and gender minorities.
“All of my physics courses are predominantly male,” she says. “It was intimidating at first. I’ve definitely gotten more used to it, but it still just kinda sucks looking around the room and being like, ‘Alright, there’s one woman over there, one in that corner, and me.’” She notes that the male majority in her classes has not been largely problematic, but it would be nice to have more of a female presence.
Stueber’s experiences are similar to those of many women physics and physical science majors. According to Joelle Corrigan, a physics graduate student and president of GMaWiP, a UW–Madison organization for Gender Minorities and Women in Physics, only around 20 percent of undergraduate physics majors are women.
In an effort to support and retain women in physics, the American Physical Society hosts the Conference for Undergraduate Women in Physics (CUWiP). This year, 10 UW–Madison undergraduates, along with Corrigan and physics grad student Abigail Shearrow, attended the Midwest regional CUWiP, held January 17-19 and hosted by the University of Minnesota–Twin Cities.
“It is a very rare, empowering experience to be in a packed auditorium filled with women and gender minorities all excited about physics,” Corrigan says of the annual conference, which she first attended as an undergraduate. “They have many talks from amazing female scientists, sharing their work and providing role models to many students who may not have seen successful females in that role previously.”
Stueber concurs with Corrigan that it was helpful to see and meet women in the research labs they toured or heard speak during the conference — every presenter was a women scientist. Katy Jurgella, a junior astrophysics and geology major who is only now taking her first physical science course with a female professor this spring, agrees.
“Every presenter who talked about their research also gave an overview of their life story. If you see just their research, you’re like, ‘Oh, wow, this woman has a PhD in astrophysics, I’ll never get there,’” Jurgella says. “But then they mention they were born on a farm and I was like, ‘I was born on a farm, too!’ It was inspiring to me.”
Students from all over the Midwest attended the regional CUWiP, hosted by the University of Minnesota–Twin Cities.
While one focus of the conference was on research, an equal emphasis was given to professional development, including topics that often strongly apply to women.
Junior AMEP major Gabby Every says, “I went to breakout sessions this year on imposter syndrome, negotiation techniques for women specifically, well-being, work-life balance, and one on grad school. It was a catch-all of issues faced by women specifically.”
Anna Gerosolina, a junior astrophysics and chemistry major who currently has no plans to attend graduate school, says the professional development sessions were very helpful because they did not solely focus on women in academia issues.
“There was one talk about being a woman in the workforce in general, and how you need to be a little more aggressive. But it’s a hard balance because a lot of times we come across as bossy even though it comes across as great when guys are aggressive,” Gerosolina says. “That really stuck with me. It was basically, just stop apologizing for existing. And I didn’t even realize how much I did that.”
UW women physical sciences undergraduates enjoy a conference meal together.
The Midwest regional CUWiP was held January 17-19, just before the spring semester began. The students who attended have already noticed a difference in how they approach their courses, professors, and classmates.
“Even two weeks into the semester, I’ve noticed I’m better at asking questions in class because I’m less afraid,” says Jurgella. “At the conference, they stressed, ‘Don’t be embarrassed if you don’t know something, because no one knows anything!’ It’s helped me remain humble, but I’m also less embarrassed now to ask about something I don’t know.”
All the women spoke of the support they now have from their fellow attendees, such as studying together, working together on projects, and just sharing experiences as women in the physical sciences.
“The conference is a great environment because sometimes I forget how reserved I can be in a room full of dominant male voices,” says Every. “Once you’re surrounded by all these women who feel the same way and have gone through similar things, you come out of your shell and talk about things that really matter to you.”
Adds Gerosolina, “These are women I can study with and not be mansplained about how to do basic physics. We even have a Snapchat group chat now!”
