Research, teaching and outreach in Physics at UW–Madison
UW–Madison named member of new $25 million Midwest quantum science institute
As joint members of a Midwest quantum science collaboration, the University of Wisconsin–Madison, the University of Illinois at Urbana–Champaign and the University of Chicago have been named partners in a National Science Foundation Quantum Leap Challenge Institute, NSF announced Tuesday.
The five-year, $25 million NSF Quantum Leap Challenge Institute for Hybrid Quantum Architectures and Networks (HQAN) was one of three in this first round of NSF Quantum Leap funding and helps establish the region as a major hub of quantum science. HQAN’s principal investigator, Brian DeMarco, is a professor of physics at UIUC. UW–Madison professor of physics Mark Saffman and University of Chicago engineering professor Hannes Bernien are co-principal investigators.
“HQAN is very much a regional institute that will allow us to accelerate in directions in which we’ve already been headed and to start new collaborative projects between departments at UW–Madison as well as between us, the University of Illinois, and the University of Chicago.” says Saffman, who is also director of the Wisconsin Quantum Institute. “These flagship institutes are being established as part of the National Quantum Initiative Act that was funded by Congress, and it is a recognition of the strength of quantum information research at UW–Madison that we are among the first.”
Chicago Quantum Exchange, including UW–Madison, welcomes seven new partners in tech, computing and finance, to advance research and training
The Chicago Quantum Exchange, a growing intellectual hub for the research and development of quantum technology, has added to its community seven new corporate partners in computing, technology and finance that are working to bring about and primed to take advantage of the coming quantum revolution.
These new industry partners are Intel, JPMorgan Chase, Microsoft, Quantum Design, Qubitekk, Rigetti Computing, and Zurich Instruments.
The Chicago Quantum Exchange and its corporate partners advance the science and engineering necessary to build and scale quantum technologies and develop practical applications. The results of their work – precision data from quantum sensors, advanced quantum computers and their algorithms, and securely transmitted information – will transform today’s leading industries. The addition of these partners brings a total of 13 companies into the Chicago Quantum Exchange to work with scientists and engineers at universities and the national laboratories in the region.
“These new corporate partners join a robust collaboration of private and public universities, national laboratories, companies, and non-profit organizations. Together, their efforts — with federal and state support —will enhance the nation’s leading center for quantum information and engineering here in Chicago,” said University of Chicago Provost Ka Yee C. Lee.
“Developing a new technology at nature’s smallest scales requires strong partnerships with complementary expertise and significant resources. The Chicago Quantum Exchange enables us to engage leading experts, facilities and industries from around the world to advance quantum science and engineering,” said David Awschalom, the Liew Family Professor in Molecular Engineering at the University of Chicago, senior scientist at Argonne, and director of the Chicago Quantum Exchange. “Our collaborations with these companies will be crucial to speed discovery, develop quantum applications and prepare a skilled quantum workforce.”
Chicago Quantum Exchange member institutions engage with corporate partners in collaborative research efforts, joint workshops to develop new research directions, and opportunities to train future quantum engineers. The CQE has existing partnerships with Boeing, IBM, Applied Materials, Inc., Cold Quanta, HRL Laboratories, LLC, and Quantum Opus, LLC.
The CQE’s newest corporate partners include a broader set of companies ranging in interest and expertise from quantum communication hardware to quantum computing systems and controls to finance and cryptography applications.
Intel is advancing a systems-level approach to quantum research that demonstrates quantum practicality and a path to commercially viable quantum computing systems. Its research efforts – in partnership with QuTech, the quantum institute of TU Delft and TNO—include technology advancements in silicon spin qubits, control and interconnect systems for large-scale quantum systems, and quantum algorithms.
JPMorgan Chase is a leader in the field of quantum algorithms and applications for financial use cases, such as portfolio optimization, option pricing and reinforcement learning, as well as general foundational algorithms with cross-domain applicability, such as quantum search. The firm has made a significant investment in quantum computing, collaborating with multiple quantum providers and forums. Its research team is also actively working in the area of post-quantum cryptography.
Microsoft has driven advances in scalable quantum technology for nearly two decades. Their global team of physicists, computer and materials scientists, engineers, developers, and enthusiasts are collaborating with a broad community to advance a full-stack quantum computing system, develop practical solutions, enable a quantum community, and accelerate quantum workforce development.
Quantum Design manufactures automated characterization systems that allow research and exploration of new materials & devices. With the partnership, Quantum Design will support research and advanced teaching at the CQE, launching a new student laboratory for quantum measurements and the study of quantum materials.
Qubitekk develops and manufactures a variety of key components for quantum networks. Qubitekk provides entangled photon sources in its support for researchers across the CQE working on the Argonne quantum loop.
Rigetti Computing builds and delivers integrated quantum systems and offers a distinctive hybrid cloud computing access model for practical near-term applications. The company owns and operates Fab-1, the world’s first dedicated quantum integrated circuit foundry.
Zurich Instruments develops advanced instrumentation including quantum control systems that enable reliable control and measurement of superconducting qubits and silicon spin qubits. The company will collaborate with the CQE on student opportunities and research.
Many of the new industry partners already have ongoing or recent engagements with CQE and its member institutions. In recent collaborative research, spectrally entangled photons from a Qubitekk entangled photon source were transported and successfully detected after traveling through one section of the Argonne quantum loop.
Another example of these relationships is the work that University of Chicago computer scientist Fred Chong and his students have done with both Intel and Rigetti Computing on software and hardware solutions. With Intel’s support, Chong’s team invented a range of software techniques to more efficiently execute quantum programs on a coming crop of quantum hardware. For example, they developed methods that take advantage of the hierarchical structure of important quantum circuits that are critical to the future of reliable quantum computation.
Jim Clarke, director of quantum hardware at Intel, looks forward to further collaborations with Chicago Quantum Exchange members.
“Intel remains committed to solving intractable challenges that lie on the path of achieving quantum practicality,” said Clarke. “We’re focusing our research on new qubit technologies and addressing key bottlenecks in their control and connectivity as quantum systems get larger. Our collaborations with members of the Chicago Quantum Exchange will help us harness our collective areas of expertise to contribute to meaningful advances in these areas.”
The Chicago Quantum Exchange’s partnership with JPMorgan Chase will enable the use of quantum computing algorithms and software for secure transactions and high-speed trading.
“We are excited about the transformative impact that quantum computing can have on our industry,” said Marco Pistoia, managing director, head of applied research and engineering at JPMorgan Chase. “Collaborating with the Chicago Quantum Exchange will help us to be among the first to develop cutting-edge quantum algorithms for financial use cases, and experiment with the power of quantum computers on relevant problems, such as portfolio optimization and option pricing.”
Applying quantum science and technology discoveries to areas such as finance, computing and healthcare requires a robust workforce of scientists and engineers. The Chicago Quantum Exchange integrates universities, national laboratories and leading companies to train the next generation of scientists and engineers and to equip those already in the workforce to transition to quantum careers.
“Microsoft is excited to partner with the Chicago Quantum Exchange to accelerate the advancement of quantum computing,” said Chetan Nayak, general manager of Microsoft Quantum Hardware. “It is through these academic and industry partnerships that we’ll be able to scale innovation and develop a workforce ready to harness the incredible impact of this technology.”
Particle collider experiment CMS — and UW physicists who contribute — celebrate 1000th publication
In June 2020, The Compact Muon Solenoid (CMS) collaboration announced the submission of its 1000th scientific publication since the experiment began a decade ago. With multiple University of Wisconsin–Madison physics faculty involved in CMS over the years, the physics department wanted to use this milestone to celebrate their achievements.
CMS is an international collaboration of over 4000 scientists at CERN’s Large Hadron Collider, which churns out data that have contributed immensely to our understanding of particle physics and pointing directions to moving beyond the Standard Model. Amongst its achievements, CMS announced in 2012 the discovery of the Higgs boson, along with ATLAS collaboration.
“It’s a proud moment for CMS in general and for the UW CMS group to see our work over the years culminate in this historic milestone!” says Bose, who currently serves as the Deputy U.S. CMS Software and Computing Operations program manager. “We are looking forward to more with the upcoming run and with the High-Luminosity LHC upgrade.”
Of the current UW–Madison physics faculty involved:
Sridhara Dasu currently leads the UW–Madison High Energy Physics group. On CMS, his focus is in better understanding the Higgs boson, searching for its partners, and possible new physics connections, especially to dark matter. He helped design the CMS calorimeter trigger system and continues to dabble in designing its upgrades.
Matthew Herndon is involved in the ongoing upgrade of the CSC (cathode strip chamber) forward muon system and well as detailed studies of the performance of the CSC system. He studies the physics of multiple gauge boson interactions and associated new physics phenomena involving multiple gauge bosons.
Tulika Bose previously served as the Physics Co-coordinator (PC) of the CMS Experiment during 2017-2019 and as the CMS Trigger Co-coordinator (2014-2016). In addition to her current program manager role, she is involved in physics studies that cover both precision measurements of Standard Model processes as well as direct searches for new physics including dark matter and top quark partners.
Kevin Black joined CMS when he joined the UW–Madison physics department in 2018, after 13 years on the CMS companion experiment ATLAS. Since then, he has been involved in the forward muon upgrade project — which will install GEM (Gas Electron Multiplier) detectors — as manager of the U.S. component of the electronic readout project and as deputy run coordinator of the GEM system. His group is focusing on the data-acquisition development for that system.
“I am especially proud of our eighteen PhD graduates who have contributed about two papers each to this set of thousand; one on a search for new physics channel and another on a carefully made measurement,” Dasu says.
Adds Herndon, “It’s an amazing milestone and a testament to the scientific productivity of the CMS experiment! UW personnel, especially our students, have been a major part of that achievement contributing to nearly 100 of those papers.”
In collaboration with the Physical Sciences Laboratory, the UW Physics team helped design the steel structures and other mechanical systems of the CMS experiment, especially leading the installation, commissioning and operations of the endcap muon system. The UW Physics team has also helped design, build, install and operate the electronics and data acquisition systems, in particular the calorimeter trigger system, and began collecting data from day one of LHC operations. They also collaborated with the HT Condor group of the Department of Computer Science to design and build the Worldwide LHC Computing Grid (WLCG), hosting one of the productive Tier-2 computing centers in Chamberlin.
The UW–Madison group was a key player in the discovery of Higgs boson in 4-lepton decay mode and establishing its coupling to fermions. The group has also searched for new physics especially looking for evidence of beyond the standard model in the form of heavy Higgs bosons that decay to tau-pairs. The group also upgraded the calorimeter trigger system and completed the endcap muon chamber system for the second higher energy run of the LHC. Searches continue for new Higgs partners, rare decays of the SM-like Higgs boson, and searches for new particles. They have added to our repertoire a series of searches for anomalous production of single high energy objects that are indicative of dark matter production in the LHC collisions.
The abundant production of papers proclaiming discoveries or the best measurements to date were possible in large part because of numerous UW–Madison electronics and computing personnel.
“The publication of the 1000th paper of the CMS collaboration is a significant milestone capping the achievement of thousands of physicists worldwide on a wide range of topics that can only be made at this unique instrument and facility,” Black says.
Three undergraduate students awarded Hilldale Fellowships
Congratulations to the three physics undergraduate research students who earned Hilldale fellowships for 2020-21! They are:
Owen Rafferty, in Robert McDermott’s group
Yanlin Wu, in Peter Timbie’s group
Yan Qian, in Sau Lan Wu’s group
The Hilldale Undergraduate/Faculty Research Fellowship provides research training and support to undergraduates at UW–Madison. Students have the opportunity to undertake their own research project in collaboration with UW–Madison faculty or research/instructional academic staff. Approximately 97 – 100 Hilldale awards are available each year.
Vandenbroucke group plays instrumental role in proving viability of innovative gamma-ray telescope
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
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.
Mark Eriksson has been named the John Bardeen Professor of Physics, through the Wisconsin Alumni Research Foundation (WARF) named professorship program.
The WARF named professorship program provides recognition for distinguished research contributions of the UW–Madison faculty. The awards are intended to honor those faculty who have made major contributions to the advancement of knowledge, primarily through their research endeavors, but also as a result of their teaching and service activities.
Eriksson joined the UW–Madison physics faculty in 1999. His research has focused on quantum computing, semiconductor quantum dots, and nanoscience. He currently leads a multi-university team focused on the development of spin qubits in gate-defined silicon quantum dots. A goal of this work is to enable quantum computers, which manipulate information coherently, to be built using many of the materials and fabrication methods that are the foundation of modern, classical integrated circuits.
“If you look back at my work here over the last, it’ll be 21 years in August, it’s almost all been collaborative, and I’ve really enjoyed the people I’ve worked with,” Eriksson says. “Going into the future, those collaborations are going to continue, of course. We have a real opportunity to see what semiconductor fabrication technology can do for qubits and quantum computing — how can we make really high-quality, silicon qubits in a way that leverages and makes use of the same technology that people use to make classical computer chips?”
Eriksson’s past and present UW–Madison collaborators include, in addition to many students and postdocs, physics professors Victor Brar, Sue Coppersmith, Bob Joynt, Shimon Kolkowitz, and Robert McDermott; physics senior scientist Mark Friesen; and materials science and engineering professor Max Lagally and scientist Don Savage.
The WARF program asks recipients to choose the name of their professorship. Eriksson, who graduated with a B.S. in physics and mathematics from UW–Madison in 1992, chose fellow alum John Bardeen — a scientist who has the unique honor of being the only person to receive the Nobel Prize in Physics twice.
“Bardeen was one of the inventors of the transistor, and I work with semiconductor qubits which are very similar to transistors in many ways,” Eriksson explains. “It seemed appropriate to choose him, because he was an alum of the university, he’s a Madison native, and he was co-inventor of the transistor.”
Eriksson was one of 11 UW–Madison faculty awarded WARF named professorships this year. The honor comes with $100,000 in research funding over five years.
“Prof. Mark Eriksson is a world-leading expert in the development of quantum information systems using solid-state quantum dot qubits,” says Sridhara Dasu, physics department chair. “Recognition of his successes in research and his contribution to the training of researchers in this increasingly promising area of quantum information, through the awarding of WARF professorship, is much deserved.”
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.
Jason Kawasaki, assistant professor of materials science and engineering
Jerry Hunter, director of the Wisconsin Centers for Nanotechnology
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.
Chloe Bonamici, assistant professor of geoscience
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
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.
Robert McDermott, professor of physics
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
Manipulating the magnetic response to light in natural materials
When light moves from one material into another, it bends — like how a partially submerged object appears distorted under water when viewed from above. What if, instead of bending, a material could change the light so much that the material was no longer visible at all?
In a study published in Physical Review A, University of Wisconsin–Madison researchers have shown for the first time that a similar response can be obtained and manipulated in naturally-occurring materials. The findings have implications from the development of “perfect” lenses for improved microscopy to Harry Potter-esque invisibility cloaks.
Visible light is made of both magnetic and electric fields, and the refractive index of a material — how much it bends the light — is determined by how the material interacts with those two fields. Nearly all materials we encounter in everyday life, though, interact entirely with light’s electric field.
Researchers have spent the past two decades developing artificial materials that more strongly interact with light’s magnetic field by manipulating the refractive index. With a strong enough response, the material could eventually have a negative refractive index, leading to unique optical properties. However, the response in synthetic materials is limited by the size of their repeating units. A naturally-occurring crystal that has much smaller unit cells is likely a better choice.
“Part of producing a negative refractive index is that the material needs to have a strong response to both electric and magnetic fields, so the big challenge is getting that magnetic response in natural materials,” explains Zach Buckholtz, a graduate student in UW–Madison physics professor Deniz Yavuz’s group and lead author of the study. “A few years ago, we published a paper showing that the crystal we’re working with has a magnetic response, and in this study, we were able to manipulate the response.”
The natural material Buckholtz is working with is a silicon-based crystal, which in general is optically ordinary, except that it has been “doped” with the rare earth metal Europium. Rare earth metals are unique in that they contain an abundance of electrons in the atoms’ outer energy shells. Those electrons can then work together to create a bigger magnetic response, but only if they are all in tune with each other.
“If you have some magnetic response and a much larger electric response to light, you can connect those two responses,” Buckholtz says. “To get to a negative refractive index from there, you have to set up coherences between the energy levels, meaning you have to make sure all those energy levels are oscillating together.”
To show they can manipulate the magnetic response, Buckholtz and Yavuz did two things. First, because the crystal is a mix of ions with slightly different electron responses, they needed to set up their experimental system to select for one class of ion. This uniformity allows for a cleaner interpretation of the results.
“We send a laser into the crystal, and then measure how much of the light is transmitted. But because the crystal isn’t perfect, instead of seeing a narrow peak for the transitions, you’ll see a really broad transition,” Buckholtz explains. “So, we do this procedure known as spectral hole burning to clear out the ions we don’t want and then we’ll be left with just one transition, which is necessary to move on to experiments that involve coherence.”
Next, they wanted to show if they could increase the magnetic response. To do so, they needed to take those selected ions, put them in coherence, and then measure the response compared to ions not in coherence. In these experiments, they shined one (a probe beam) or two (probe and coupling beam) wavelengths of laser at the ions. Both lasers excite electrons in the ions to a higher-energy state, and the scientists can again measure how might light is transmitted through the crystal as a readout of the electron transitions.
“With just the probe beam, we see just the normal transition, and that’s what we did in our previous study. But with the coupling beam added in, it connects and adds another transition state in there,” Buckholtz says. “If those states are in coherence, they cancel each other out, and we see that effect as a peak in transmitted light, which means the index of refraction is going toward zero.”
Buckholtz notes that the magnetic response they see is not yet large enough to produce the materials with interesting new optical properties they are hoping for. Still, he says, this work provides a path forward to continue manipulations to improve the response, such as investigating different rare earth metals.
“We have a magnetic response, we can set up coherence, and we can manipulate the response,” Buckholtz says. “Now, we want to increase the scale of the response to with a goal of eventually making the refractive index below zero.”
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.
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.
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.