The international Dark Energy Survey maps hundreds of millions of galaxies to deepen our understanding of the structure of the cosmos. For six years, the Dark Energy Survey imaged these celestial objects, producing a mountain of data that researchers will mine for decades to come. Below is an article from SLAC National Accelerator Laboratory on how researchers from the Dark Energy Survey have discovered an important connection between the tiny galaxies surrounding the Milky Way and the dark matter halos that they inhabit.
Over the past five years, the Dark Energy Survey, a DOE-funded project led by Fermilab, has revolutionized our view of small satellite galaxies. DES discovered a large number of tiny galaxies close to the Milky Way’s largest satellites, the Magellanic Clouds, suggesting that multiple galaxies may have been captured by the Milky Way at the same time. In this recent analysis (resulting in two articles published in Astrophysical Journal), the DES group has combined observations from DES with those from the Pan-STARRS survey to cover 75% of the sky and confirm this hypothesis. In order to understand the sensitivity of these searches, the Fermilab group tested the algorithms ability to detect simulated galaxies injected into the survey data. The results of this search were then interpreted in the context of cosmological simulations to understand the connection between dark matter halos and the galaxies that reside in them. Analysis of the DES and Pan-STARRS data was led by Wilson Fellow Alex Drlica-Wagner and involved former Lederman Fellow Ting Li, Fermilab Scientist Brian Yanny, and many other collaborators from DES.
“By studying the smallest galaxies, we can better understand the fundamental physics that governs dark matter,” Drlica-Wagner said.
Image: Ralf Kaehler/SLAC National Accelerator Laboratory
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 at the Department of Energy’s SLAC National Accelerator Laboratory and the Dark Energy Survey 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.
The new study, forthcoming in the Astrophysical Journal and available as a preprint here, is part of a larger effort to understand how dark matter works on scales smaller than our galaxy, said Ethan Nadler, the study’s first author and a graduate student at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) and Stanford University.
“We know some things about dark matter very well – how much dark matter is there, how does it cluster – but all of these statements are qualified by saying, yes, that is how it behaves on scales larger than the size of our local group of galaxies,” Nadler said. “And then the question is, does that work on the smallest scales we can measure?”
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 KIPAC. Last year, for example, Wechsler, Nadler and colleagues used data on satellite galaxies in conjunction with computer simulations to place much tighter limits on dark matter’s interactions with ordinary matter.
Now, Wechsler, Nadler and the DES team are 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. Those observations are particularly interesting, Nadler said, in light of the Gaia measurements, which indicated that six of these satellite galaxies fell into the Milky Way with the LMC.
To study the LMC’s satellites more thoroughly, Nadler and team analyzed computer simulations of millions of possible universes. Those simulations, originally run by Yao-Yuan Mao, a former graduate student of Wechsler’s who is now at Rutgers University, 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.
An effort led by the others in the DES team, including former KIPAC students Alex Drlica-Wagner, a Wilson Fellow at Fermilab and an assistant professor of astronomy and astrophysics at the University of Chicago, and Keith Bechtol, an assistant professor of physics at the University of Wisconsin-Madison, and their collaborators produced the crucial final step: 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.
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, Nadler said, 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, Nadler said, there are around 100 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, he said, and likely place tighter constraints on the nature of dark matter itself.
The research was a collaborative effort within the Dark Energy Survey, led by the Milky Way Working Group, with substantial contributions from junior members including Sidney Mau, an undergraduate at the University of Chicago, and Mitch McNanna, a graduate student at UW-Madison. The research was supported by a National Science Foundation Graduate Fellowship, by the Department of Energy’s Office of Science through SLAC, and by Stanford University.
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SLAC is a vibrant multiprogram laboratory that explores how the universe works at the biggest, smallest and fastest scales and invents powerful tools used by scientists around the globe. With research spanning particle physics, astrophysics and cosmology, materials, chemistry, bio- and energy sciences and scientific computing, we help solve real-world problems and advance the interests of the nation.
SLAC is operated by Stanford University for the U.S. Department of Energy’s Office of Science. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time.
DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.
Missing March Madness? Let Fermilab fill a small part of the void created in these times of social distancing and sheltering-in-place. Participate in our sendup of the NCAA tournament: March Magnets.
Particle physics fans know that magnets are major players in the instruments scientists use to examine the universe’s smallest constituents. Less appreciated is their sheer variety: diverse purposes, sizes, shapes and materials.
Below are eight distinct magnet types used in particle physics, each with an example from a project or experiment in which the U.S. Department of Energy’s Fermilab is a player. We start at Elite Eight stage of the playoffs. So add these eight magnets to your repertoire of particle physics knowledge.
Then on Monday, March 30, head over to the Fermilab Twitter feed to participate in our March Magnets playoffs. On Monday and Tuesday, March 30 and 31, you can vote on which four of our eight magnets get to advance to the next stage. Vote for your Final Four on Friday, April 3. And vote for the champion on Monday, April 6.
The champion will be announced on Fermilab’s Twitter feed.
Want to understand how magnets work in the field of particle physics? Read a Physics 101 primer of magnets’ roles in accelerators.
Have fun with our March Magnets tournament!
1. Quadrupole magnet for linear particle accelerator
PIP-II quadrupole magnet. Photo: Reidar Hahn, Fermilab
What is it?
Physicists use sequences of quadrupole magnets to keep particle beams focused as they travel through a particle accelerator. At the exact center of these magnets, the magnetic field is zero, and particles feel no force. But the farther a particle deviates from the ideal beam trajectory that goes through the center of these magnets, the stronger the magnetic field. It is these fields that push the charged particles back to the center of the magnet and keep the beam on track.
Example
PIP-II linear accelerator quadrupole magnet at Fermilab
Why it’s cool
The 26 quadrupole magnets of the new PIP-II linear accelerator at Fermilab, built as in-kind international contributions by the Bhabha Atomic Research Center in India, each have four coils made of copper wire, arranged in alternating orientation (north-south-north-south). The challenge is that a single quadrupole magnet only can create a focusing force in one direction perpendicular to the beam (x axis), and it defocuses the beam in the other direction perpendicular to the beam (y axis). The solution is the installation of a second magnet with switched polarization (south-north-south-north) to create a focusing force along the y-axis direction. The installation of a carefully calculated and placed series of quads with different polarizations, often designed as doublet and triplet magnets, produces a net focusing force and helps ensure that the protons will stay on track.
Specifications for PIP-II quadrupole magnet
Size: 0.1 meters long and 0.33 meters high and wide
Weight: 57 kilograms
Electric current: 10 amps DC peak
Strength: 1.5 teslas integrated peak field
Polarity/magnetic field: quadrupole
Permanent, superconducting, normal-conducting: normal-conducting electromagnet
Material: copper, magnetic steel, stainless steel
2. Superconducting focusing magnet for particle collider
HL-LHC focusing magnet. Photo: Dan Cheng, Lawrence Berkeley National Laboratory
What is it?
A focusing magnet squeezes a charged-particle beam, making it as tight and compact as experiments require. In particle colliders, the stronger the magnets that focus the opposing beams before they reach the collision point, the more collisions the machine produces.
Example
Focusing magnet for the High-Luminosity LHC at CERN
Why it’s cool
At present, the Large Hadron Collider at CERN has superconducting focusing magnets built with niobium-titanium wire. Now Berkeley Lab, Brookhaven Lab, CERN and Fermilab are working on replacing these magnets as part of the HL-LHC upgrade. The new magnets feature niobium-tin wire, and the first production magnet achieved the required field strength of 11.5 teslas in a test at Brookhaven Lab, a triumph of painstaking and innovative engineering by the U.S. team. It is the result of years of R&D development and understanding how to take advantage of this superior but fragile superconducting material. When installed in the HL-LHC in a few years, it will be the first time a focusing magnet made from niobium-tin will operate in a particle accelerator anywhere in the world.
Specifications for High-Luminosity LHC focusing magnet (Q1 and Q3)
Size: 4.7 meters long, 60 centimeters in diameter
Weight: 6.8 metric tons
Electric current: 16,500 amps for 7-TeV beams
Magnetic field strength: 11.5 teslas
Polarity/magnetic field: quadrupole
Permanent, normal-conducting or superconducting: superconducting electromagnet
Material: niobium-tin, iron, aluminum
3. Bending magnet for circular accelerator
Main Injector bending magnet. Photo: Reidar Hahn, Fermilab
What is it?
A bending magnet bends the path of a charged-particle beam. Scientists use them to keep a beam on its track in a ring-shaped accelerator. As the machine propels the beam to higher energy, operators keep the particles on their orbit by increasing the electric current in the steering magnets, which increases the strength of the magnet field.
Example
Main Injector bending magnet at Fermilab
Why it’s cool
The Main Injector bending magnet is, in a word, elegant. And the lab built lots of them, with help from collaborators. Three hundred forty-four bending magnets bend the beam around the Main Injector’s three-kilometer ring. The magnet’s design addresses nearly all of the engineering problems that bedeviled its progenitors. For example, rather than being immovably fixed inside its steel housing, the main magnet component is attached to a sliding mechanism so that, when it expands, it is also free to move, avoiding the stress a fixed component would experience. The copper-wire magnets have performed flawlessly for more than 25 years. Another magnet under development may soon rival it for best magnet: A new Fermilab-designed bending magnet based on superconducting niobium-tin wire is in the development phase for a future very-high-energy particle collider. A demonstrator magnet built by Fermilab set a world record when it achieved a peak magnetic field of 14.1 teslas in 2019.
Main Injector bending magnet
Size: two sizes: 4 meters and 6 meters long; roughly 1 meter high and wide
Weight: about 20 tons
Electric current: 9,400 amps DC peak
Strength: 1.7 teslas peak field
Polarity/magnetic field: dipole
Permanent, normal-conducting or superconducting: normal-conducting electromagnet
Material: copper, steel
4. Undulator for light source
LCLS-II undulator. Photo: Lawrence Berkeley National Laboratory
What is it?
Want to make a particle beam shimmy? Use an undulator, a series of magnets in an electron accelerator. Because the direction of the magnetic field changes from magnet to magnet, the undulator forces a beam into a fast-moving zigzag, rapidly moving left and right. With each zig and zag, the electron beam radiates particles of light, or photons. Scientists use this light to study microscopic details of nature, such as the molecular structure of proteins, how medicine affects cells or the components of air pollution. The spectrum of the light depends on the energy of the particle beam and typically ranges from infrared to ultraviolet and X-rays.
Example
Undulator for the LCLS-II electron accelerator at SLAC
Why it’s cool
Unlike most light sources, the LCLS-II under construction at SLAC National Accelerator Laboratory features a linear electron accelerator. It propels more electrons to higher energy than typical light sources. Lawrence Berkeley National Laboratory manages the production of the LCLS-II undulators. LCLS-II’s 37 accelerator cryomodules, which power the electron beam before it enters the undulator line, were built by Fermilab and Jefferson Lab.
The light produced by the undulators of the LCLS-II takes the form of X-rays. Twenty-one undulators produce soft (lower-energy) X-rays; 32 produce hard (higher-energy) X-rays.
LCLS-II’s soft X-ray undulators are arranged in two rows, which can be adjusted to within millionths of an inch to tune the properties of the X-ray light. They produce up to 1 million soft X-ray pulses per second. The undulators will provide the worldwide brightest X-ray pulses in a wide energy range, from 200 to 25,000 electronvolts, and the photon power will range between several hundred and 1,000 watts.
Specifications for LCLS-II accelerator undulator
Size: 3.4 meters long, 2 meters high
Weight: about 6.5 tons
Strength: 1.5-tesla peak field
Polarity/magnetic field: alternating dipole polarity
Permanent, normal-conducting or superconducting: hybrid permanent magnets
Material: vanadium permendur energized by a permanent magnet
5. Solenoid for particle detector
CMS magnet. Photo: CERN
What is it?
A solenoid is a cylindrical electromagnet made of many loops of current-carrying cable, which produces a constant magnetic field along its length. Inside a particle detector, a solenoid is responsible for bending the trajectories of particles that fly through it: Positively charged particles bend one way, and negatively charged particles bend the other. A detector’s solenoid also reveals the particle’s momentum: the faster a particle, the less bending of its path. By analyzing the trajectories, scientists can determine the energy and momentum of each particle.
Example
CMS at CERN
Why it’s cool
Despite its size, the CMS detector at the Large Hadron Collider is relatively compact for all the material and devices it contains, much smaller than its cousin, the ATLAS detector. Still, its solenoid is a giant — the largest superconducting magnet ever made. Multiple institutions and companies contributed to its construction. It contains almost twice as much iron as the Eiffel Tower, and it stores enough energy to melt 18 tons of gold. The winding of the solenoid cable took five years. Its size and design are optimized for detecting and measuring particles known as muons very accurately.
Specifications for Compact Muon Solenoid (CMS)
Size: 13 meters long by 6 meters high
Weight: about 12,000 tons
Electric current: 19,500 amps (nominal current)
Strength: 3.8 teslas
Polarity/magnetic field: axial field
Permanent, normal-conducting or superconducting: superconducting electromagnet
Material: iron, aluminum, niobium-titanium
6. Kicker magnet for particle accelerator
Booster kicker magnet. Photo: Salah Chaurize, Fermilab
What is it?
Kicker magnets are used in particle accelerators to deflect or transfer a particle beam from its main path, sending particles out of the accelerator and into a beamline that guides the beam to its final destination.
Example
Booster kicker magnet at Fermilab
Why it’s cool
In the circular Booster accelerator at Fermilab, there are five extraction kicker magnets positioned around the ring. As the beam’s energy ramps up, the particles approach the speed of light as they circle through the Booster. Once the beam reaches its extraction energy, an electrical pulse is transmitted to the five magnets in unison, kicking the beam out of the Booster ring. Because the particle beam passes through the Booster and its kicker magnets up to an astounding 20,000 times a second, the magnets have to activate at unimaginably fast speeds — within 35 nanoseconds — to kick the beam out at exactly the right moment.
Specifications for Booster kicker magnet
Size: 1 or 0.5 meters long (two types), 12 centimeters wide
Weight: 64 kilograms for 1-meter-long magnet
Electric current: 1,200 amps DC peak
Strength: 0.0072 teslas
Polarity/magnetic field: dipole
Permanent, superconducting or normal-conducting: normal-conducting electromagnet
Material: copper, aluminum, ferrite ceramic, RTV (silicone rubber)
7. Storage ring magnet
Muon g-2 storage ring and magnet. Photo: Reidar Hahn, Fermilab
What is it?
Lots of interesting physics experiments need to build up and store bunches of particles. Storage rings are designed to circulate particles from a few seconds to hundreds of hours. Electron storage rings, for example, allow scientists to study the synchrotron radiation the particles emit. Alternatively, scientists can extract the stored particles from the ring and smash them into a fixed target. A collider typically features two intersecting storage rings on top of each other to create head-on collisions of particles.
Example
Muon g-2 storage ring magnet at Fermilab
Why is it cool?
Most storage rings circulate electrons or protons using a series of magnets. In contrast, the Muon g-2 storage ring at Fermilab circulates muons using one giant magnet: 50 feet in diameter. Built in the 1990s at Brookhaven Lab for its Muon g-2 experiment, the magnet made its journey by boat and truck from New York to Illinois in 2013. The Muon g-2 magnetic field is incredibly uniform for such a large magnet, with a magnetic field that’s identical around the ring at the parts-per-billion level. It has to be so precise because scientists are measuring the “wobble” of the muons traveling through the ring — so if the field varied too much, the muon would behave differently in those areas. The expected announcement of the first results from the Muon g-2 experiment at Fermilab are among the most anticipated physics results of 2020 and, if they confirm the tantalizing hints observed at Brookhaven Lab, could upend the current Standard Model of particle physics.
Specifications for Muon g-2 magnet
Size: 15.3 meters in diameter, 1.5 meters tall
Weight: 700 tons
Electrical current: 5,200 amps
Strength: 1.45 tesla
Polarity/magnetic field: C-shaped dipole magnets and electrostatic quadrupole plates
Permanent, superconducting or normal-conducting: superconducting electromagnet
Material: iron and superconducting wire (pure aluminum stabilizer and niobium-titanium superconductor in a copper matrix)
8. Magnetic horn for neutrino beam
Neutrino horn. Photo: Reidar Hahn, Fermilab
What is it?
Powered by extreme pulses of electricity, magnetic horns turn a broad spray of particles into a focused beam, making for better experiments. Horns typically focus electrically charged particles called pions and kaons, which then decay into various types of particles, including no-charge neutrinos that can no longer be steered by magnets. Because the charged particles are steered by the magnets, the neutrinos they give birth to also continue along well-defined paths. Also called “focusing horns,” these devices live in harsh conditions and make accelerator-based neutrino experiments possible.
Example
NuMI focusing horn at Fermilab
Why is it cool?
The focusing horn for the Neutrinos at the Main Injector facility receives its beam from Fermilab’s most powerful particle accelerator, the two-mile-circumference Main Injector. The magnetic horn turns on and off rapidly, with every pulse clocking in around 200,000 amps. (Your toaster runs at around 10 amps). Horns must survive extreme thermal and magnetic stress over their lifetimes – the equivalent of being hit with a hammer 10 million times a year. Without the horn, an experiment would lose 95% of the neutrinos in its beam.
Specifications for NuMI focusing horn:
Size: 3 meters long
Weight: about 1 ton
Electrical current: 200,000 amps
Strength: 1 tesla
Polarity/magnetic field: toroidal magnetic field
Permanent, superconducting or normal-conducting: normal-conducting electromagnet
Material: nickel-plated aluminum and anodized aluminum
Illustrations by Jerald Pinson.
Accelerator magnet research and development at Fermilab is supported by the DOE Office of Science.
Fermilab is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.