Research, teaching and outreach in Physics at UW–Madison
Shimon Kolkowitz earns NSF CAREER award
Shimon Kolkowitz has already developed one of the most precise atomic clocks ever. Now, the UW–Madison physics professor has been awarded a Faculty Early Career Development (CAREER) award from the National Science Foundation (NSF) to use his atomic clocks to potentially answer some big questions about the physics of our universe.
The five-year, $800,000 in total award will cover research expenses, graduate student support, and outreach projects based on the research.
“I am honored and proud to receive an NSF CAREER award, which will help my research group expand our experimental efforts and build upon our recent results,” Kolkowitz says. “This award will support research into new ways to harness the remarkable precision of optical atomic clocks for exciting physics applications such as searching for dark matter and detecting gravitational waves.”
Atomic clocks are so precise because they take advantage of the natural vibration frequencies of atoms, which are identical for all atoms of a particular element. Kolkowitz and his research group have developed atomic clocks that can detect the difference in these frequencies between two clocks that would only disagree with each other by one second after 300 billion years, the tiniest detectable frequency changes to date. These clocks, then, can measure effects that shifts their frequency by only 0.00000000000000001%, opening the possibility of using them in the search for new physics.
A significant advancement in Kolkowitz’s clocks is that they are multiplexed, with six or more separate clocks in one
vacuum chamber, effectively placing each clock in the same environment. Mutliplexing means that comparisons between the clocks, and not their individual accuracy, is what matters — and allows the group to use commercially available, robust and portable lasers in their measurements. Though the clocks are not yet ready to be used to detect gravitational waves, Kolkowitz says the current setup “looks a bit like how you would eventually do that,” and will allow him to test out and demonstrate the concept.
In the spirit of the Wisconsin Idea and the NSF’s “broader impacts” to benefit society beyond scientific merit, with this award, Kolkowitz will focus efforts on quantum science outreach with pre-college students.
“We’ll be developing new demos and hands-on activities designed to introduce K-12 students to modern physics concepts,” Kolkowitz says. “We’ll use these activities to engage students at live shows and interactive events as part of The Wonders of Physics outreach program, with an emphasis on reaching rural and Native American communities in Wisconsin.”
NSF established these awards to help scientists and engineers develop simultaneously their contributions to research and education early in their careers. CAREER funds are awarded by the federal agency to junior-level faculty at colleges and universities.
Dark Energy Survey releases most precise look at the universe’s evolution
The Dark Energy Survey collaboration has created the largest ever maps of the distribution and shapes of galaxies, tracing both ordinary and dark matter in the universe out to a distance of more than 7 billion light years. The analysis, which includes the first three years of data from the survey, is consistent with predictions from the current best model of the universe, the standard cosmological model. Nevertheless, there remain hints from DES and other experiments that matter in the current universe is a few percent less clumpy than predicted.
New results from the Dark Energy Survey — a large international team that includes researchers from the University of Wisconsin–Madison — use the largest ever sample of galaxies over an enormous piece of the sky to produce the most precise measurements of the universe’s composition and growth to date. Scientists measured that the way matter is distributed throughout the universe is consistent with predictions in the standard cosmological model, the best current model of the universe.
Over the course of six years, DES surveyed 5,000 square degrees — almost one-eighth of the entire sky — in 758 nights of observation, cataloguing hundreds of millions of objects. The results, announced May 27, draw on data from the first three years — 226 million galaxies observed over 345 nights — to create the largest and most precise maps yet of the distribution of galaxies in the universe at relatively recent epochs.
Since DES studied nearby galaxies as well as those billions of light-years away, its maps provide both a snapshot of the current large-scale structure of the universe and a movie of how that structure has evolved over the course of the past 7 billion years.
“This a stringent test of the current standard cosmological paradigm, a model proposing that 95% of the universe is dark matter and dark energy that we do not yet understand,” explains UW–Madison physics professor Keith Bechtol. “By measuring the apparent positions and shapes of hundreds of millions of galaxies in our survey, we test whether the cosmic structures that have formed in the universe today match the predictions based on structures observed in the early universe.”
To test cosmologists’ current model of the universe, DES scientists compared their results with measurements from the European Space Agency’s orbiting Planck observatory. Planck used light signals known as the cosmic microwave background to peer back to the early universe, just 400,000 years after the Big Bang. The Planck data give a precise view of the universe 13 billion years ago, and the standard cosmological model predicts how the dark matter should evolve to the present. If DES’s observations don’t match this prediction, there is possibly an undiscovered aspect to the universe. While there have been persistent hints from DES and several previous galaxy surveys that the current universe is a few percent less clumpy than predicted—an intriguing find worthy of further investigation—the recently released results are consistent with the prediction.
“In the area of constraining what we know about the distribution and structure of matter on large scales as driven by dark matter and dark energy, DES has obtained limits that rival and complement those from the cosmic microwave background,” said Brian Yanny, a Fermilab scientist who coordinated DES data processing and management. “It’s exciting to have precise measurements of what’s out there and a better understanding of how the universe has changed from its infancy through to today.”
Ordinary matter makes up only about 5% of the universe. Dark energy, which cosmologists hypothesize drives the accelerating expansion of the universe by counteracting the force of gravity, accounts for about 70%. The last 25% is dark matter, whose gravitational influence binds galaxies together. Both dark matter and dark energy remain invisible and mysterious, but DES seeks to illuminate their natures by studying how the competition between them shapes the large-scale structure of the universe over cosmic time.
DES photographed the night sky using the 570-megapixel Dark Energy Camera on the Victor M. Blanco 4-meter Telescope at the Cerro Tololo Inter-American Observatory in Chile, a Program of the National Science Foundation’s NOIRLab. One of the most powerful digital cameras in the world, the Dark Energy Camera was designed specifically for DES and built and tested at Fermilab. The DES data were processed at the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign.
“These analyses are truly state-of-the-art, requiring artificial intelligence and high-performance computing super-charged by the smartest young scientists around,” said Scott Dodelson, a physicist at Carnegie Mellon University who co-leads the DES Science Committee with Elisabeth Krause of the University of Arizona. “What an honor to be part of this team.”
To quantify the distribution of dark matter and the effect of dark energy, DES relied on two main phenomena. First, on large scales, galaxies are not distributed randomly throughout space but rather form a weblike structure due to the gravity of dark matter. DES measured how this cosmic web has evolved over the history of the universe. The galaxy clustering that forms the cosmic web, in turn, revealed regions with a higher density of dark matter.
Second, DES detected the signature of dark matter through weak gravitational lensing. As light from a distant galaxy travels through space, the gravity of both ordinary and dark matter can bend it, resulting in a distorted image of the galaxy as seen from Earth. By studying how the apparent shapes of distant galaxies are aligned with each other and with the positions of nearby galaxies along the line of sight, DES scientists inferred the spatial distribution (or clumpiness) of the dark matter in the universe.
Analyzing the massive amounts of data collected by DES was a formidable undertaking. The team began by analyzing just the first year of data, which was released in 2017. That process prepared the researchers to use more sophisticated techniques for analyzing the larger data set, which includes the largest sample of galaxies ever used to study weak gravitational lensing.
For example, calculating the redshift of a galaxy — the change in light’s wavelength due to the expansion of the universe — is a key step toward measuring how both galaxy clustering and weak gravitational lensing change over cosmic history. The redshift of a galaxy is related to its distance, which allows the clustering to be characterized in both space and time.
“Redshift calibration is one topic where we significantly improved upon our year-1 data analysis,” said Ross Cawthon, a UW-Madison physics postdoc who led the redshift calibration efforts for two of the main galaxy samples. “We developed new methods and refined old ones. It has been a huge effort by DES members from all over the world.”
Ten regions of the sky were chosen as “deep fields” that the Dark Energy Camera imaged repeatedly throughout the survey. Stacking those images together allowed the scientists to glimpse more distant galaxies. The team then used the redshift information from the deep fields to calibrate measurements of redshift in the rest of the survey region. This and other advancements in measurements and modeling, coupled with a threefold increase in data compared to the first year, enabled the team to pin down the density and clumpiness of the universe with unprecedented precision.
Along with the analysis of the weak-lensing signals, DES also precisely measures other probes that constrain the cosmological model in independent ways: galaxy clustering on larger scales (baryon acoustic oscillations), the frequency of massive clusters of galaxies, and high-precision measurements of the brightnesses and redshifts of Type Ia supernovae. These additional measurements will be combined with the current weak-lensing analysis to yield even more stringent constraints on the standard model.
“DES has delivered cost-effective, leading-edge science results directly related to Fermilab’s mission of pursuing the fundamental nature of matter, energy, space and time,” said Fermilab Director Nigel Lockyer. “A dedicated team of scientists, engineers and technicians from institutions around the world brought DES to fruition.”
The DES collaboration consists of over 400 scientists from 25 institutions in seven countries.
“The collaboration is remarkably young. It’s tilted strongly in the direction of postdocs and graduate students who are doing a huge amount of this work,” said DES Director and spokesperson Rich Kron, who is a Fermilab and University of Chicago scientist. “That’s really gratifying. A new generation of cosmologists are being trained using the Dark Energy Survey.”
UW–Madison physics graduate student Megan Tabbutt was one of the many significant contributors to this work, developing new methods that contributed to an independent validation of the galaxy clustering analysis.
DES concluded observations of the night sky in 2019. With the experience of analyzing the first half of the data, the team is now prepared to handle the complete data set. The final DES analysis is expected to paint an even more precise picture of the dark matter and dark energy in the universe. And the methods developed by the team have paved the way for future sky surveys to probe the mysteries of the cosmos.
“This work represents a ‘big statement’ from the Dark Energy Survey. DES data combined with other observations provide world-leading constraints on the nature of dark energy,” Bechtol says. “At the same time, we are training a new generation of cosmologists, and pioneering advanced methodologies that will be essential to realize the full potential of upcoming galaxy surveys, including the Vera C. Rubin Observatory Legacy Survey of Space and Time.”
The recent DES results were presented in a scientific seminar on May 27. Twenty-nine papers are available on the arXiv online repository.
The Dark Energy Survey is a collaboration of more than 400 scientists from 25 institutions in seven countries. For more information about the survey, please visit the experiment’s website.
Yang Bai promoted to full professor
The Department of Physics is pleased to announce that Prof. Yang Bai has been promoted to the rank of full professor.
“It is my pleasure and honor as Dean to approve Prof. Yang Bai’s promotion to Full Professor. His creativity and impressive breadth in particle physics research make him a leader not only on dark matter, but also more generally on Beyond-the-Standard-Model Physics,” says Eric Wilcots, Dean of the College of Letters & Science. “He is also a valued teacher, appreciated by students especially at the graduate level. Graduate students and junior researchers in Madison are in good hands.”
Bai joined the department in 2012, and was promoted to associate professor in 2017. In addition to his robust and well-funded research program, he has trained several successful graduate students, taught all levels of departmental courses, and served on several departmental and university committees.
“Professor Yang Bai is widely recognized as one of the leading theoretical particle physicists of his generation with a broad and vigorous research program, covering both the collider-related frontiers and the cosmic frontier. His work includes significant contributions in essentially every area related to dark matter,” says Sridhara Dasu, professor and department chair. “The Physics Department very strongly endorses the promotion of Yang Bai to Full Professor.”
Congrats, Prof. Bai on this well-earned recognition!
Dark Energy Survey makes public catalog of nearly 700 million astronomical objects
The second data release from the Dark Energy Survey, or DES, is the culmination of over a half-decade of astronomical data collection and analysis with the ultimate goal of understanding the accelerating expansion of the universe and the phenomenon of dark energy, which is thought to be responsible for this accelerated expansion. It is one of the largest astronomical catalogs released to date. Keith Bechtol, assistant professor of physics at UW–Madison, has served as the DES Science Release co-coordinator since 2017, guiding the effort to assemble, scientifically validate, and document data releases for both cosmology analysis by the DES Collaboration and exploration by the broad astronomical community.
Including a catalog of nearly 700 million astronomical objects, DR2 builds on the 400 million objects cataloged with the survey’s prior data release, or DR1, and also improves on it by refining calibration techniques, which, with the deeper combined images of DR2, lead to improved estimates of the amount and distribution of matter in the universe.
Astronomical researchers around the world can access these unprecedented data and mine them to make new discoveries about the universe, complementary to the studies being carried out by the Dark Energy Survey collaboration. The full data release is online and available to the public to explore and gain their own insights as well.
“Most of the nearly 700 million objects visible in DES DR2 images had never been seen by humans before the past few years,” Bechtol says. “If you take a moment to look at even a small patch of sky in the DES images, you can see asteroids of our Solar System, stars out to the edge of the Milky Way, and distant galaxies as they were billions of years ago. We look forward to see how our colleagues use this enormous new dataset for research and education.”
DES was designed to map hundreds of millions of galaxies and to discover thousands of supernovae in order to measure the history of cosmic expansion and the growth of large-scale structure in the universe, both of which reflect the nature and amount of dark energy in the universe. DES has produced the largest and most accurate dark matter map from galaxy weak lensing to date, as well as a new map, three times larger, that will be released in the near future.
One early result relates to the construction of a catalog of a type of pulsating star known as “RR Lyrae,” which tells scientists about the region of outer space beyond the edge of our Milky Way. In this area nearly devoid of stars, the motion of the RR Lyrae hints at the presence of an enormous “halo” of invisible dark matter, which may provide clues on how our galaxy was assembled over the last 12 billion years. In another result, DES scientists used the extensive DR2 galaxy catalog, along with data from the LIGO experiment, to estimate the location of a black hole merger and, independent of other techniques, infer the value of the Hubble constant, a key cosmological parameter. Combining their data with other surveys, DES scientists have also been able to generate a complete map of the Milky Way’s dwarf satellites, giving researchers insight into how our own galaxy was assembled and how it compares with cosmologists’ predictions.
Covering 5,000 square degrees of the southern sky (one-eighth of the entire sky) and spanning billions of light-years, the survey data enables many other investigations in addition to those targeting dark energy, covering a vast range of cosmic distances — from discovering new nearby solar system objects to investigating the nature of the first star-forming galaxies in the early universe.
“This is a momentous milestone. For six years, the Dark Energy Survey collaboration took pictures of distant celestial objects in the night sky. Now, after carefully checking the quality and calibration of the images captured by the Dark Energy Camera, we are releasing this second batch of data to the public,” said DES Director Rich Kron of Fermilab and the University of Chicago. “We invite professional and amateur scientists alike to dig into what we consider a rich mine of gems waiting to be discovered.”
Once captured, these images (and the large amount of data surrounding them) are transferred to the National Center for Supercomputing Applications for processing via the DES Data Management project. Using the Blue Waters supercomputer at NCSA, the Illinois Campus Cluster and computing systems at Fermilab, NCSA prepares calibrated data products for public and research consumption. It takes approximately four months to process one year’s worth of data into a searchable, usable catalog.
The detailed precision cosmology constraints based on the full six-year DES data set will come out over the next two years.
NCSA, NOIRLab and the LIneA Science Server collectively provide the tools and interfaces that enable access to DR2.
“Because astronomical data sets today are so vast, the cost to handle them is prohibitive for individual researchers or most organizations. CSDC provides open access to big astronomical data sets like DES DR2 and the necessary tools to explore and exploit them — then all it takes is someone from the community with a clever idea to discover new and exciting science,” said Robert Nikutta, project scientist for Astro Data Lab at CSDC.
“With information on the positions, shapes, sizes, colors and brightnesses of over 690 million stars, galaxies and quasars, the release promises to be a valuable source for astronomers and scientists worldwide to continue their explorations of the universe, including studies of matter (light and dark) surrounding our home Milky Way galaxy, as well as pushing further to examine groups and clusters of distant galaxies, which hold precise evidence about how the size of the expanding universe changes over time,” said Dark Energy Survey Data Management Project Scientist Brian Yanny of Fermilab.
This work is supported in part by the U.S. Department of Energy Office of Science.
The Dark Energy Survey is a collaboration of more than 400 scientists from 26 institutions in seven countries. Funding for the DES Projects has been provided by the U.S. Department of Energy, the U.S. National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, Funding Authority for Studies and Projects in Brazil, Carlos Chagas Filho Foundation for Research Support of the State of Rio de Janeiro, Brazilian National Council for Scientific and Technological Development and the Ministry of Science, Technology and Innovation, the German Research Foundation and the collaborating institutions in the Dark Energy Survey, the list of which can be found at www.darkenergysurvey.org/collaboration.
Researchers awarded Department of Energy Quantum Information Science Grant
Three UW–Madison physics professors and their colleagues have been awarded a U.S. Department of Energy (DOE) High Energy Physics Quantum Information Science award for an interdisciplinary collaboration between theoretical and experimental physicists and experts on quantum algorithms.
The grant, entitled “Detection of dark matter and neutrinos enhanced through quantum information,” will bring a total of $2.3 million directly to UW-Madison. Physics faculty include principal investigator Baha Balantekin as well as Mark Saffman, and Sue Coppersmith. Collaborators on the grant include Kim Palladino at the University of Oxford, Peter Love at Tufts University, and Calvin Johnson at San Diego State University.
With the funding, the researchers plan to use a quantum simulator to calculate the detector response to dark matter particles and neutrinos. The simulator to be used is an array of 121 neutral atom qubits currently being developed by Saffman’s group. Much of the research plan is to understand and mitigate the behavior of the neutral atom array so that high accuracy and precision calculations can be performed. The primary goal of this project is to apply lessons from the quantum information theory in high energy physics, while a secondary goal is to contribute to the development of quantum information theory itself.
NSF Physics Frontier Center for neutron star modeling to include UW–Madison
A group of universities, including the University of Wisconsin–Madison, has been named the newest Physics Frontier Center, the National Science Foundation announced Aug. 17. The center expands the reach and depth of existing capabilities in modeling some of the most violent events known in the universe: the mergers of neutron stars and their explosive aftermath.
The Network for Neutrinos, Nuclear Astrophysics, and Symmetries (N3AS) is already an established hub of eight institutions, including UW–Madison, that uses the most extreme environments found in astrophysics — the Big Bang, supernovae, and neutron star and black hole mergers — as laboratories for testing fundamental physics under conditions beyond the reach of Earth-based labs. The upgrade to a Physics Frontier Center adds five institutions, provides $10.9 million in funding for postdoctoral fellowships and allows members to cover an expanded scope of research.
“For 20 years, we’ve expected that the growing precision of astrophysical and cosmological measurements would make this ﬁeld an increasingly important part of fundamental physics. Indeed, four monumental discoveries — neutrino masses, dark matter, the accelerating universe, and gravitational waves — have conﬁrmed this prediction,” says A. Baha Balantekin, a professor of physics at UW–Madison and one of the principal investigators for N3AS.
In particular, the new results constrain the minimum mass of the dark matter particles, as well as the strength of interactions between dark matter and normal matter.
According to these new results, a dark matter particle must be heavier than a zeptoelectronvolt, which is 10-21 electronvolts. That’s one trillionth of a trillionth of the mass of an electron. This study also shows that dark matter’s interactions with normal matter must be roughly 1,000 times weaker than the weak nuclear force. Of the known forces, only gravity is weaker.
These novel measurements used data from the Dark Energy Survey, a cosmological survey designed to study dark energy, the mysterious force driving the accelerated expansion of the universe. In contrast, dark matter is gravitationally attractive, resisting the expansion of the universe and gravitationally binding astronomical systems such as galaxies. The smallest “dwarf” galaxies can have hundreds to thousands of times more dark matter than normal matter. Over the past five years, the Dark Energy Survey has combined with other surveys to more than double the known population of these tiny galaxies. The current total is now over 50.
“The large number of dwarf galaxies that we found orbiting the Milky Way is consistent with expectations from the simplest picture of dark matter — that is, comprising slow-moving particles that interact only through gravity,” Bechtol explained. “In this new paper, we rule out several alternative possibilities for the nature of dark matter.”
Dark matter makes up 85% of the matter in the universe, but we have yet to detect it directly in the laboratory. The gravitational effects of dark matter are clearly visible in the motions of stars in galaxies, the clumpy distribution of galaxies in the universe, and even in the amount of lightweight elements. The robust astronomical evidence for the existence of dark matter has motivated many experimental searches here on Earth, using instruments ranging from cryogenic detectors buried deep underground to energetic particle colliders.
“The faintest galaxies are among the most valuable tools we have to learn about dark matter because they are sensitive to several of its fundamental properties all at once,” said Ethan Nadler, the study’s lead author and graduate student at Stanford University and SLAC.
In these multi-year, multi-telescope sky surveys, the raw data comes in the form of tens of thousands high-resolution digital images. But identifying these ultrafaint galaxies, as their description implies, is not as simple as looking at an image and seeing a faint smudge of light. Bechtol and his group, including physics grad student Mitch McNanna, designed the search algorithms needed to identify, with some statistical assurance, which individual stars are part of a dwarf galaxy.
“We worked closely with experts in galaxy formation and particle physics theory to compare the Dark Energy Survey observations with predictions,” Bechtol said. “Part of our job was to determine the sensitivity of our search — how far away from the Earth could we spot a galaxy with only a few hundred stars?”
By combining the observed census of dwarf galaxies with advanced cosmological simulations of the distribution of dark matter around the Milky Way, scientists were able to predict how the physical properties of dark matter would affect the number of small galaxies. Small galaxies form in regions where the dark matter density in the early universe is very slightly above average. Physical processes that smooth out these regions of higher density (if dark matter moves too quickly or gains energy due to interactions with normal matter) or prevent density variations from collapsing to form galaxies (thanks to quantum interference effects) would reduce the number of galaxies observed by the Dark Energy Survey.
“Astrophysical observations provide unique information about the fundamental nature of dark matter, and are complementary to searches for dark matter particles in terrestrial experiments.” Bechtol said. “With the Dark Energy Survey, we continue to learn about the deep connection between particle physics and the growth of cosmic structure, ranging from the vast network of galaxies in the cosmic web, down to smallest individual galaxies.”
The Dark Energy Survey is a collaboration of more than 300 scientists from 25 institutions in six countries. For more information about the survey, please visit the experiment’s website.
Funding for the DES Projects has been provided by the U.S. Department of Energy, the U.S. National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of ChicagoFunding Authority for Funding and Projects in Brazil, Carlos Chagas Filho Foundation for Research Support of the State of Rio de Janeiro, Brazilian National Council for Scientific and Technological Development and the Ministry of Science and Technology, the German Research Foundation and the collaborating institutions in the Dark Energy Survey.
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.