Research, teaching and outreach in Physics at UW–Madison
Astrophysics
Vandenbroucke group plays instrumental role in proving viability of innovative gamma-ray telescope
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Scientists in the Cherenkov Telescope Array (CTA) consortium have detected gamma rays from the Crab Nebula using the prototype Schwarzschild-Couder Telescope (pSCT), proving the viability of the novel telescope design for use in gamma-ray astrophysics. The announcement was made today by Justin Vandenbroucke, associate professor at the University of Wisconsin–Madison, on behalf of the CTA Consortium at the virtual 236th meeting of the American Astronomical Society (AAS).
“The Crab Nebula is the brightest steady source of TeV, or very high-energy, gamma rays in the sky, so detecting it is an excellent way of proving the pSCT technology,” says Vandenbroucke, who is also affiliated with the Wisconsin IceCube Particle Astrophysics Center (WIPAC) at UW–Madison. “Very high-energy gamma rays are the highest energy photons in the universe and can unveil the physics of extreme objects, including black holes and possibly dark matter.”
Vandenbroucke is coleader of a team made up of WIPAC scientists and other collaborators that developed and operate a critical part of the telescope: its high-speed camera. Vandenbroucke has worked on the design, construction, and integration of the camera since 2009.
Keith Bechtol, Rob Morgan win UW’s Cool Science Image contest
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Congrats to Prof. Keith Bechtol and graduate student Rob Morgan for their winning entry in the UW–Madison Cool Science Images contest! Their winning entry — one of 12 selected out of 101 entries — earns them a large-format print which initially will be displayed in a gallery at the McPherson Eye Research Institute’s gallery in the WIMR building.
This snapshot of the sky contains thousands of distant galaxies, each containing billions of stars. Bechtol and Morgan were looking for the flash of the explosion of a single star, the potential source of a sub-atomic particle called a neutrino, spotted zipping through the Earth by the IceCube Neutrino Observatory at the South Pole. The distant galaxies, swirling billions of light years away, are all the harder to see because of nearby objects, like the pictured Helix Nebula. The image was captured with a Dark Energy Camera and Victor M. Blanco telescope.
Twelve projects have been chosen for Round 6 of the UW2020: WARF Discovery Initiative, including three from faculty in the Department of Physics (Mark Eriksson, Robert McDermott, and Justin Vandenbroucke). These projects were among 92 proposals submitted from across campus. The initiative is funded by the Office of the Vice Chancellor for Research and Graduate Education and the Wisconsin Alumni Research Foundation.
The projects were reviewed by faculty across the university. The UW2020 Council, a group of 17 faculty from all divisions of the university, evaluated the merits of each project based on the reviews and their potential for making significant contributions to their field of study.
The goal of UW2020 is to stimulate and support cutting-edge, highly innovative and groundbreaking research at UW–Madison and to support acquisition of shared instruments or equipment that will foster significant advances in research.
Acquisition of a cryogen-free Physical Properties Measurement System (PPMS) for characterization of quantum materials and devices
The project addresses a barrier for UW–Madison researchers in measuring electronic, magnetic, and thermal properties of quantum materials at low temperatures, namely the increasing high costs of cryogens (liquid helium) and lack of a convenient means to perform these measurements in a shared facility. Low-temperature electronic, magnetic, and thermal properties of materials are crucial for fundamental materials discovery and for applications in quantum information, nonvolatile memory, and energy conversion devices.
This project will acquire a cryogen-free Physical Properties Measurement System (PPMS) and house it as a shared-user facility instrument within the Wisconsin Centers for Nanotechnology (CNT). This instrument would be open for all UW–Madison users.
Currently, these measurements depend on external collaborations or low-temperature setups in PI labs which either consume large amounts of cryogens or require time-consuming reconfigurations from experiment to experiment. Having a cryogen-free PPMS would allow researchers to spend less time and money in setting up experiments, potentially freeing up resources for scientific investigations that include new superconducting and topological material discoveries and characterizations of materials for advanced microelectronics and magnetic memory systems.
PRINCIPAL INVESTIGATOR
Jason Kawasaki, assistant professor of materials science and engineering
CO-PRINCIPAL INVESTIGATOR
Jerry Hunter, director of the Wisconsin Centers for Nanotechnology
CO-INVESTIGATOR
Paul Voyles, professor of materials science and engineering and MRSEC Director
Song Jin, professor of chemistry
Mark Eriksson, professor of physics
Thomas Kuech, professor of chemical and biological engineering
Daniel Rhodes, assistant professor of materials science and engineering
Chang-Beom Eom, professor of materials science and engineering
Paul Evans, professor of materials science and engineering
Michael Arnold, professor of materials science and engineering
Dakotah Thompson, assistant professor of mechanical engineering
Cracking the structure of ice: establishing a cryogenic electron backscatter diffraction and Raman capability at UW–Madison
The structure and physical properties of ice determine the behavior of glaciers, ice sheets, and polar ice caps (both terrestrial and extraterrestrial). Moreover, ice is of interest because of its unique light transmission properties, which are currently being harnessed by one of the world’s largest astrophysical experiments through the UW–led IceCube collaboration.
This project will develop the capability to perform scanning electron microscopy (SEM) of water and CO2 ice in the UW–Madison Geoscience Department, focusing on electron backscatter diffraction (EBSD) analysis for ice microstructure and Raman spectroscopy for ice composition. EBSD of ice is an extremely rare analytical capability worldwide.
Having this highly specialized type of analysis capability for ice will enable advances in glaciology, climate science, physics, materials science and planetary science. This technology can accelerate research on glacial sliding and ice deformation, and inform long-standing questions about the transformation of air bubbles to clathrates in glacial ice and their potential as archives of Earth’s past atmosphere. In addition, understanding the structure of ice is critical, for example, to accurate measurement of cosmic ray interactions in the IceCube Neutrino Observatory.
As the only lab in the U.S. offering combined ice EBSD analysis and ice Raman analysis, UW–Madison will establish itself as a nexus for cryosphere research, attracting many collaborations from outside UW–Madison.
PRINCIPAL INVESTIGATOR
Chloe Bonamici, assistant professor of geoscience
CO-PRINCIPAL INVESTIGATORS
Lucas Zoet, assistant professor of geoscience
Shaun Marcott, associate professor of geoscience
Justin Vandenbroucke, associate professor of physics/WIPAC
John Fournelle, senior scientist of geoscience
CO-INVESTIGATORS
Pavana Prabhakar, assistant professor of civil and environmental engineering
Richard Hartel, professor of food engineering
Hiroki Sone, assistant professor of geological engineering
Interdisciplinary engineering of quantum information systems
This project represents a synergistic effort toward engineering practical quantum information systems (QIS). The research unites the experimental superconducting and semiconducting qubit teams on campus with advanced materials characterization and microwave engineering expertise to uncover the underlying sources of decoherence that limit qubit performance and develop next-generation quantum devices for scalable quantum computing and quantum sensing. This effort will build new interdisciplinary connections that nourish the quantum ecosystem at UW–Madison, cutting across departmental and disciplinary lines.
The potential of QIS has been recognized recently by the $1.4 billion federal National Quantum Initiative, and the newly formed Wisconsin Quantum Institute at UW is home to world-leading efforts in the physics of QIS. This project is a next step in expanding these directions to incorporate the engineering effort necessary to develop practical systems capable of solving real-world problems.
PRINCIPAL INVESTIGATOR
Robert McDermott, professor of physics
CO-PRINCIPAL INVESTIGATORS
Mark Eriksson, professor of physics
Susan Hagness, professor of electrical and computer engineering
Paul Voyles, professor of materials science and engineering
Kangwook Lee, professor of electrical and computer engineering
The Milky Way’s satellites help reveal link between dark matter halos and galaxy formation
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.
