An international research team analysis rules out dark matter destruction as origin of extra radiation in galaxy center

Image caption 1: An artist’s interpretation of the Milky Way shows the “boxy” distribution of stars in the Ga-lactic Center. A research team of physicists said in a newly published study that this shape leaves very little room for excess radiation from the destruction of dark matter par-ticles. (Credit: Oscar Macias)

Image caption 2: This representation of data from the Fermi Gamma-ray Space Telescope after its launch in 2008 shows an excess of high-energy radiation in the Milky Way’s Galactic Center. Many physicists attributed this to the annihilation of weakly interacting dark matter parti-cles, but a research team study has excluded this possibility through a range of particle masses. (Credit: Oscar Macias)

The detection more than a decade ago by the Fermi Gamma-ray Space Telescope of an excess of high-energy radiation in the center of the Milky Way convinced some physicists that they were seeing evidence of the annihilation of dark matter particles, but a team led by a researcher at the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) has ruled out that interpretation.

In a paper published recently in the journal Physical Review D, the Kavli IPMU Project Researcher Oscar Macias and colleagues at other institutions report that—through an analysis of the Fermi data and an exhaustive series of modelling exercises—they were able to determine that the observed gamma rays could not have been produced by what are called weakly interacting massive particles (WIMPS), most popularly theorized as the stuff of dark matter.

“The crucial point of our recent paper is that, our approach covers the wide range of astro-physical background models that have been used to infer the existence of the Galactic Center excess, and goes beyond them. So, using any of our state-of-the-art background models, we find no need for a dark matter component to be included in our model for this sky region. This allows us to impose very stringent constraints on particle dark matter models,” said Macias.

By eliminating these particles, the destruction of which could generate energies of up to 300 giga-electron volts, the paper’s authors say, they have put the strongest constraints yet on dark matter properties.

“For 40 years or so, the leading candidate for dark matter among particle physicists was a thermal, weakly interacting and weak-scale particle, and this result for the first time rules out that candidate up to very high-mass particles,” said co-author Kevork Abazajian, pro-fessor of physics and astronomy at the University of California, Irvine (UCI).

“In many models, this particle ranges from 10 to 1,000 times the mass of a proton, with more massive particles being less attractive theoretically as a dark matter particle,” add-ed co-author Manoj Kaplinghat, also a UCI professor of physics and astronomy. “In this paper, we’re eliminating dark matter candidates over the favored range, which is a huge improvement in the constraints we put on the possibilities that these are representative of dark matter.” Abazajian said that dark matter signals could be crowded out by other astrophysical phenomena in the Galactic Center—such as star formation, cosmic ray deflection off mo-lecular gas and, most notably, neutron stars and millisecond pulsars—as sources of ex-cess gamma rays detected by the Fermi space telescope.

“We looked at all of the different modelling that goes on in the Galactic Center, including molecular gas, stellar emissions and high-energy electrons that scatter low-energy pho-tons,” said Kavli IPMU’s Macias. “We took over three years to pull all of these new, better models together and examine the emissions, finding that there is little room left for dark matter.”

Macias, who is also a postdoctoral researcher with the GRAPPA Centre at the University of Amsterdam, added that this result would not have been possible without data and software provided by the Fermi Large Area Telescope collaboration.

The group tested all classes of models used in the Galactic Center region for excess emission analyses, and its conclusions remained unchanged. “One would have to craft a diffuse emission model that leaves a big ‘hole’ in them to relax our constraints, and sci-ence doesn’t work that way,” Macias said.

Kaplinghat noted that physicists have predicted that radiation from dark matter annihila-tion would be represented in a neat spherical or elliptical shape emanating from the Galactic Center, but the gamma ray excess detected by the Fermi space telescope after its June 2008 deployment shows up as a triaxial, bar-like structure.

“If you peer at the Galactic Center, you see that the stars are distributed in a boxy way,” he said. “There’s a disk of stars, and right in the center, there’s a bulge that’s about 10 degrees on the sky, and it’s actually a very specific shape—sort of an asymmetric box—and this shape leaves very little room for additional dark matter.”

Does this research rule out the existence of dark matter in the galaxy? “No,” Kaplinghat said. “Our study constrains the kind of particle that dark matter could be. The multiple lines of evidence for dark matter in the galaxy are robust and unaffected by our work.”

Far from considering the team’s findings to be discouraging, Abazajian said they should encourage physicists to focus on concepts other than the most popular ones.

“There are a lot of alternative dark matter candidates out there,” he said. “The search is going to be more like a fishing expedition where you don’t already know where the fish are.”

This project was made possible via funding from the World Premiere International Center Initiative (WPI), an initiative of the Ministry of Education, Culture, Sport, Science and Technology to create world-leading research centres in Japan; the National Science Foundation, and the U.S. Department of Energy Office of Science.

Paper details

Journal: Physical Review D

Title: Strong constraints on thermal relic dark matter from Fermi-LAT observations of the Galactic Center
Authors: Kevork N. Abazajian (1), Shunsaku Horiuchi (2), Manoj Kaplinghat (1), Ryan E. Keeley (1,3), and Oscar Macias (4,5)

Author affiliations:

1. Center for Cosmology, Department of Physics and Astronomy, University of California, Irvine, California 92697, USA
2. Center for Neutrino Physics, Department of Physics, Virginia Tech, Blacksburg, Virgin-ia 24061, USA

3. Korea Astronomy and Space Science Institute, Daejeon 34055, Korea

4. Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI), Uni-versity of Tokyo, Kashiwa, Chiba, 277-8583, Japan 

5. GRAPPA Institute, University of Amsterdam, 1098 XH Amsterdam, Netherlands

DOI: https://doi.org/10.1103/PhysRevD.102.043012  (Published August 20, 2020)
Paper abstract (Physical Review D)
Preprint (arXiv.org) 

Research contact:

Oscar Macias
Project Researcher
Kavli Institute for the Physics and Mathematics of the Universe
The University of Tokyo
E-mail: oscar.macias@ipmu.jp
TEL: +31 (0)20 525 6316

Media contact:

John Amari
Press officer 
Kavli Institute for the Physics and Mathematics of the Universe
The University of Tokyo
E-mail: press@ipmu.jp
TEL: 080-4056-2767

Related links:
University of California, Irvine: press release
Femi LAT—Fermi Large Area Telescope collaboration: homepage

Source: Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU)

---

Scientists “Zoom In” On Dark Matter, Revealing The Invisible Skeleton Of The Universe

"Zoomed in, computer-generated simulation of the distribution of dark matter in the universe, also referred to as the "cosmic web." The small, spherical blobs of dark matter that appeared scattered throughout the image are identified by scientists as dark matter haloes." J. Wang; S. Bose/CfA. High Resolution (jpg) - Low Resolution (jpg)

"Zoomed in, computer-generated simulation of the distribution of dark matter in the universe, also referred to as the 'cosmic web.' Top right inset: The primary magnification for the project reveals spherical blobs that appear scattered throughout the image and are identified by scientists as dark matter haloes. Bottom left inset: The highest level of magnification revealed tiny yellow blobs, or Earth-mass dark matter haloes as they would appear in the universe today. While they exist in the corresponding region of the background, they are visible only after zooming in several times. This is the first time these objects have ever been produced in a numerical simulation." J. Wang, S. Bose/CfA.High Resolution (jpg)-Low Resolution (jpg)

Cambridge, MA - Using the power of supercomputers, an international team of researchers has zoomed in on the smallest clumps of dark matter in a virtual universe. Published today in Nature, the study reveals dark matter haloes as active regions of the sky, teeming with not only galaxies, but also radiation-emitting collisions that could make it possible to find these haloes in the real sky.

Dark matter—which makes up roughly 83% of the matter in the universe—is an important player in cosmic evolution, including in the formation of galaxies, which grew as gas cooled and condensed at the center of enormous clumps of dark matter. Over time, haloes formed as some dark matter clumps pulled away from the expansion of the universe due to their own enormous gravity. The largest dark matter haloes contain huge galaxy clusters—collections of hundreds of galaxies—and while their properties can be inferred by studying those galaxies within them, the smallest dark matter haloes, which typically lack even a single star, have remained a mystery until now.

“Amongst the things we’ve learned from our simulations is that gravity leads to dark matter particles ‘clumping’ in overly dense regions of the universe, settling into what’s known as dark matter haloes. These can essentially be thought of as big wells of gravity filled with dark matter particles,” said Sownak Bose, a postdoc at the Center for Astrophysics | Harvard & Smithsonian, and one of the lead authors on the research. “We think that every galaxy in the cosmos is surrounded by an extended distribution of dark matter, which outweighs the luminous material of the galaxy by between a factor of 10-100, depending on the type of galaxy. Because this dark matter surrounds every galaxy in all directions, we refer to it as a ‘halo.’”

Using a simulated universe, researchers were able to zoom in with the precision required to recognize a flea on the surface of the full Moon—with magnification up to 10 to the power of seven, or 10 followed by seven zeroes—and create highly detailed images of hundreds of virtual dark matter haloes, from the largest known to the smallest expected.

“Simulations are helpful because they help us quantify not just the overall distribution of dark matter in the universe, but also the detailed internal structure of these dark matter haloes,” said Bose. “Establishing the abundance and the internal structure of the entire range of dark matter haloes that can be formed in the cold dark matter model is of interest because this enables us to calculate how easy it may be to detect dark matter in the real universe.”

While studying the structure of the haloes, researchers were met with a surprise: all dark matter haloes, whether large or small, have very similar internal structures which are dense at the center and become increasingly diffuse moving outward. Without a scale-bar, it is almost impossible to tell the difference between the dark matter halo of a massive galaxy—up to 10^15 solar masses—and that of a halo with less than a solar mass—down to 10^-6 solar masses. “Several previous studies suggested that the density profiles for super-mini haloes would be quite different from their massive counterparts,” said Jie Wang, astronomer at the National Astronomical Observatories (NAOC) in Beijing, and a lead author on the research. “Our simulations show that they look similar across a huge mass range of dark haloes and that is really surprising.” Bose added that even in the smallest haloes which do not surround galaxies, “Our simulations enabled us to visualize the so-called ‘cosmic web.’ Where filaments of dark matter intersect, one sees the tiny, near spherical blobs of dark matter, which are the haloes themselves, and they are so universal in structure that I could show you a picture of a galaxy cluster with a million billion times the mass of the Sun, and an Earth-mass halo at a million times smaller than the Sun, and you would not be able to tell which is which.”

Although the images of dark matter haloes from this study are the result of simulations, the simulations themselves are informed by real observational data. For astronomers, that means the study could be replicated against the real night sky given the right technology. “The initial conditions that went into our simulation are based on actual observational data from the cosmic microwave background radiation measurements of the Planck satellite, which tells us what the composition of the Universe is and how much dark matter to put in,” said Bose.

During the study researchers tested a feature of dark matter haloes that may make them easier to find in the real night sky: particle collisions. Current theory suggests that dark matter particles that collide near the center of haloes may explode in a violent burst of high-energy gamma radiation, potentially making the dark matter haloes detectable by gamma-ray and other telescopes.

“Exactly how the radiation would be detected depends on the precise properties of the dark matter particle. In the case of weakly interacting massive particles (WIMPs), which are amongst the leading candidates in the standard cold dark matter picture, gamma radiation is typically produced in the GeV range. There have been claims of a galactic center excess of GeV-scale gamma radiation in Fermi data, which could be due to dark matter or perhaps due to pulsars,” said Bose. “Ground-based telescopes like the Very Energetic Radiation Imaging Telescope Array System (VERITAS) can be used for this purpose, too. And, pointing telescopes at galaxies other than our own could also help, as this radiation should be produced in all dark matter haloes.” Wang added, “With the knowledge from our simulation, we can evaluate many different tools to detect haloes—gamma-ray, gravitational lensing, dynamics. These methods are all promising in the work to shed light on the nature of dark matter particles.”

The results of the study provide a pathway both for current and future researchers to better understand what’s out there, whether we can see it or not. “Understanding the nature of dark matter is one of the Holy Grails of cosmology. While we know that it dominates the gravity of the universe, we know very little about its fundamental properties: how heavy an individual particle is, what sorts of interactions, if any, it has with ordinary matter, etcetera,” said Bose. “Through computer simulations we have come to learn about its fundamental role in the formation of the structure in our universe. In particular, we have come to realize that without dark matter, our universe would look nothing like the way it does now. There would be no galaxies, no stars, no planets, and therefore, no life. This is because dark matter acts as the invisible skeletal structure that holds up the visible universe around us.”

DOI: 10.1038/s41586-020-2642-9

Headquartered in Cambridge, Mass., the Center for Astrophysics | Harvard & Smithsonian (CfA) is a collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

For more information, contact:

Center for Astrophysics | Harvard & Smithsonian
Fred Lawrence Whipple Observatory
Amy Oliver, Public Affairs
amy.oliver@cfa.harvard.edu
520-879-4406

 Source:  Harvard-Smithsonian Center for Astrophysics (CfA)

---

News Center Beyond the WIMP: Unique Crystals Could Expand the Search for Dark Matter

A computerized simulation of the large-scale distribution of dark matter in the universe. An overlay graph (in white) shows how a crystal sample intensely scintillates, or glows, when exposed to X-rays during a lab test. This and other properties could make it a good material for a dark matter detector. Credit: Millennium Simulation, Berkeley Lab.   Hi-res image

A new particle detector design proposed at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) could greatly broaden the search for dark matter – which makes up 85 percent of the total mass of the universe yet we don’t know what it’s made of – into an unexplored realm.

While several large physics experiments have been targeting theorized dark matter particles called WIMPs, or weakly interacting massive particles, the new detector design could scan for dark matter signals at energies thousands of times lower than those measurable by more conventional WIMP detectors.

The ultrasensitive detector technology incorporates crystals of gallium arsenide that also include the elements silicon and boron. This combination of elements causes the crystals to scintillate, or light up, in particle interactions that knock away electrons.

This scintillation property of gallium arsenide has been largely unexplored, said Stephen Derenzo, a senior physicist in the Molecular Biophysics and Integrated Bioimaging Division at Berkeley Lab and lead author of a study published March 20 in the Journal of Applied Physics that details the material’s properties.

“It’s hard to imagine a better material for searching in this particular mass range,” Derenzo said, which is measured in MeV, or millions of electron volts. “It ticks all of the boxes. We are always worried about a ‘Gotcha!’ or showstopper. But I have tried to think of some way this detector material can fail and I can’t.”

The breakthrough came from Edith Bourret, a senior staff scientist in Berkeley Lab’s Materials Sciences Division who decades earlier had researched gallium arsenide’s potential use in circuitry. She gave him a sample of gallium arsenide from this previous work that featured added concentrations, or “dopants,” of silicon and boron.

Derenzo had previously measured some lackluster performance in a sample of commercial-grade gallium arsenide. But the sample that Bourret handed him exhibited a scintillation luminosity that was five times brighter than in the commercial material, owing to the silicon and boron that imbued the material with new and enhanced properties. This enhanced scintillation meant it was far more sensitive to electronic excitations.

“If she hadn’t handed me this sample from more than 20 years ago, I don’t think I would have pursued it,” Derenzo said. “When this material is doped with silicon and boron, this turns out to be very important and, accidentally, a very good choice of dopants.”

Derenzo noted that he has had a longstanding interest in scintillators that are also semiconductors, as this class of materials can produce ultrafast scintillation useful for medical imaging applications such as PET (positron emission tomography) and CT (computed tomography) scans, for example, as well as for high-energy physics experiments and radiation detection.

The doped gallium arsenide crystals he studied appear well-suited for high-sensitivity particle detectors because extremely pure crystals can be grown commercially in large sizes, the crystals exhibit a high luminosity in response to electrons booted away from atoms in the crystals’ atomic structure, and they don’t appear to be hindered by typical unwanted effects such as signal afterglow and dark current signals.

Some of the larger WIMP-hunting detectors – such as that of the Berkeley Lab-led LUX-ZEPLIN project now under construction in South Dakota, and its predecessor, the LUX experiment – incorporate a liquid scintillation detector. A large tank of liquid xenon is surrounded by sensors to measure any light and electrical signals expected from a dark matter particle’s interaction with the nucleus of a xenon atom. That type of interaction is known as a nuclear recoil.

A crystal of gallium arsenide

Credit: Wikimedia Commons

In contrast, the crystal-based gallium arsenide detector is designed to be sensitive to the slighter energies associated with electron recoils – electrons ejected from atoms by their interaction with dark matter particles. As with LUX and LUX-ZEPLIN, the gallium arsenide detector would need to be placed deep underground to shield it from the typical bath of particles raining down on Earth.

It would also need to be coupled to light sensors that could detect the very few infrared photons (particles of light) expected from a low-mass dark matter particle interaction, and the detector would need to be chilled to cryogenic temperatures. The silicon and boron dopants could also possibly be optimized to improve the overall sensitivity and performance of the detectors.

Because dark matter’s makeup is still a mystery – it could be composed of one or many particles of different masses, for example, or may not be composed of particles at all – Derenzo noted that gallium arsenide detectors provide just one window into dark matter particles’ possible hiding places.

While WIMPs were originally thought to inhabit a mass range measured in billions of electron volts, or GeV, the gallium arsenide detector technology is well-suited to detecting particles in the mass range measured in millions of electron volts, or MeV.

Berkeley Lab physicists are also proposing other types of detectors to expand the dark matter search, including a setup that uses an exotic state of chilled helium known as superfluid helium to directly detect low-mass dark matter particles.

“Superfluid helium is scientifically complementary to gallium arsenide since helium is more sensitive to dark matter interactions with atomic nuclei, while gallium arsenide is sensitive to dark matter interacting with electrons,” said Dan McKinsey, a faculty senior scientist at Berkeley Lab and physics professor at UC Berkeley who is a part of the LZ Collaboration and is conducting R&D on dark matter detection using superfluid helium.

“We don’t know whether dark matter interacts more strongly with nuclei or electrons – this depends on the specific nature of the dark matter, which is so far unknown,” he said.

Another effort would employ gallium arsenide crystals in a different approach to the light dark matter search based on vibrations in the atomic structure of the crystals, known as optical phonons. This setup could target “light dark photons,” which are theorized low-mass particles that would serve as the carrier of a force between dark matter particles – analogous to the conventional photon that carries the electromagnetic force.

Still another next-gen experiment, known as the Super Cryogenic Dark Matter Search experiment, or SuperCDMS SNOLAB, will use silicon and germanium crystals to hunt for low-mass WIMPs.

“These would be complementary experiments,” Derenzo said of the many approaches. “We need to look at all of the possible mass ranges. You don’t want to be fooled. You can’t exclude a mass range if you don’t look there.”

Stephen Hanrahan, a staff scientist in Berkeley Lab’s Molecular Biophysics and Integrated Bioimaging Division; and Gregory Bizarri, a senior lecturer in manufacturing at Cranfield University in the U.K., also participated in this study. The work was supported by Advanced Crystal Technologies Inc.

###

Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel Prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more, visit www.lbl.gov.

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.

Source: Berkeley Lab/News Center

NASA's Fermi Mission Expands its Search for Dark Matter

Top: Gamma rays (magenta lines) coming from a bright source like NGC 1275 in the Perseus galaxy cluster should form a particular type of spectrum (right). Bottom: Gamma rays convert into hypothetical axion-like particles (green dashes) and back again when they encounter magnetic fields (gray curves). The resulting gamma-ray spectrum ((lower curve at right) would show unusual steps and gaps not seen in Fermi data, which means a range of these particles cannot make up a portion of dark matter. Credits: SLAC National Accelerator Laboratory/Chris Smith. Download additional visuals at NASA's Scientific Visualization Studio

Dark matter, the mysterious substance that constitutes most of the material universe, remains as elusive as ever. Although experiments on the ground and in space have yet to find a trace of dark matter, the results are helping scientists rule out some of the many theoretical possibilities. Three studies published earlier this year, using six or more years of data from NASA's Fermi Gamma-ray Space Telescope, have broadened the mission's dark matter hunt using some novel approaches.

“We've looked for the usual suspects in the usual places and found no solid signals, so we've started searching in some creative new ways," said Julie McEnery, Fermi project scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "With these results, Fermi has excluded more candidates, has shown that dark matter can contribute to only a small part of the gamma-ray background beyond our galaxy, the Milky Way, and has produced strong limits for dark matter particles in the second-largest galaxy orbiting it."

Dark matter neither emits nor absorbs light, primarily interacts with the rest of the universe through gravity, yet accounts for about 80 percent of the matter in the universe. Astronomers see its effects throughout the cosmos -- in the rotation of galaxies, in the distortion of light passing through galaxy clusters, and in simulations of the early universe, which require the presence of dark matter to form galaxies at all.

The leading candidates for dark matter are different classes of hypothetical particles. Scientists think gamma rays, the highest-energy form of light, can help reveal the presence of some of types of proposed dark matter particles. Previously, Fermi has searched for tell-tale gamma-ray signals associated with dark matter in the center of our galaxy and in small dwarf galaxies orbiting our own.

Although no convincing signals were found, these results eliminated candidates within a specific range of masses and interaction rates, further limiting the possible characteristics of dark matter particles.

Among the new studies, the most exotic scenario investigated was the possibility that dark matter might consist of hypothetical particles called axions or other particles with similar properties. An intriguing aspect of axion-like particles is their ability to convert into gamma rays and back again when they interact with strong magnetic fields. These conversions would leave behind characteristic traces, like gaps or steps, in the spectrum of a bright gamma-ray source.

Manuel Meyer at Stockholm University led a study to search for these effects in the gamma rays from NGC 1275, the central galaxy of the Perseus galaxy cluster, located about 240 million light-years away. High-energy emissions from NGC 1275 are thought to be associated with a supermassive black hole at its center. Like all galaxy clusters, the Perseus cluster is filled with hot gas threaded with magnetic fields, which would enable the switch between gamma rays and axion-like particles. This means some of the gamma rays coming from NGC 1275 could convert into axions -- and potentially back again -- as they make their way to us.

"While we don't yet know what dark matter is, our results show we can probe axion-like models and provide the strongest constraints to date for certain masses," Meyer said. "Remarkably, we reached a sensitivity we thought would only be possible in a dedicated laboratory experiment, which is quite a testament to Fermi."

Another broad class of dark matter candidates are called Weakly Interacting Massive Particles (WIMPs). In some versions, colliding WIMPs either mutually annihilate or produce an intermediate, quickly decaying particle. Both scenarios result in gamma rays that can be detected by the LAT.

Regina Caputo at the University of California, Santa Cruz, sought these signals from the Small Magellanic Cloud (SMC), which is located about 200,000 light-years away and is the second-largest of the small satellite galaxies orbiting the Milky Way. Part of the SMC's appeal for a dark matter search is that it lies comparatively close to us and its gamma-ray emission from conventional sources, like star formation and pulsars, is well understood. Most importantly, astronomers have high-precision measurements of the SMC's rotation curve, which shows how its rotational speed changes with distance from its center and indicates how much dark matter is present. In a paper published in Physical Review D on March 22, Caputo and her colleagues modeled the dark matter content of the SMC, showing it possessed enough to produce detectable signals for two WIMP types.

The Small Magellanic Cloud (SMC), at center, is the second-largest satellite galaxy orbiting our own. This image superimposes a photograph of the SMC with one half of a model of its dark matter (right of center). Lighter colors indicate greater density and show a strong concentration toward the galaxy's center. Ninety-five percent of the dark matter is contained within a circle tracing the outer edge of the model shown. In six years of data, Fermi finds no indication of gamma rays from the SMC's dark matter. Credits: Dark matter, R. Caputo et al. 2016; background, Axel Mellinger, Central Michigan University

"The LAT definitely sees gamma rays from the SMC, but we can explain them all through conventional sources," Caputo said. "No signal from dark matter annihilation was found to be statistically significant."

In the third study, researchers led by Marco Ajello at Clemson University in South Carolina and Mattia Di Mauro at SLAC National Accelerator Laboratory in California took the search in a different direction. Instead of looking at specific astronomical targets, the team used more than 6.5 years of LAT data to analyze the background glow of gamma rays seen all over the sky.

The nature of this light, called the extragalactic gamma-ray background (EGB) has been debated since it was first measured by NASA's Small Astronomy Satellite 2 in the early 1970s. Fermi has shown that much of this light arises from unresolved gamma-ray sources, particularly galaxies called blazars, which are powered by material falling toward gigantic black holes. Blazars constitute more than half of the total gamma-ray sources seen by Fermi, and they make up an even greater share in a new LAT catalog of the highest-energy gamma rays.

This animation switches between two images of the gamma-ray sky as seen by Fermi's Large Area Telescope (LAT), one using the first three months of LAT data, the other showing a cumulative exposure of seven years. The blue color, representing the fewest gamma rays, includes the extragalactic gamma-ray background. Blazars make up most of the bright sources shown (colored red to white). With increasing exposure, Fermi reveals more of them. A new study shows blazars are almost completely responsible for the background glow.Credits: NASA/DOE/Fermi LAT Collaboration

Some models predict that EGB gamma rays could arise from distant interactions of dark matter particles, such as the annihilation or decay of WIMPs. In a detailed analysis of high-energy EGB gamma rays, published April 14 in Physical Review Letters, Ajello and his team show that blazars and other discrete sources can account for nearly all of this emission.

"There is very little room left for signals from exotic sources in the extragalactic gamma-ray background, which in turn means that any contribution from these sources must be quite small," Ajello said. "This information may help us place limits on how often WIMP particles collide or decay."

Although these latest studies have come up empty-handed, the quest to find dark matter continues both in space and in ground-based experiments. Fermi is joined in its search by NASA's Alpha Magnetic Spectrometer, a particle detector on the International Space Station.

NASA's Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership, developed in collaboration with the U.S. Department of Energy and with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.

For more information about NASA's Fermi Gamma-ray Space Telescope, visit:  www.nasa.gov/fermi

By Francis Reddy
NASA's Goddard Space Flight Center, Greenbelt, Md.

Editor: Ashley Morrow

 Source: NASA/Dark Energy/Matter

NASA Simulation Suggests Black Holes May Make Ideal Dark Matter Labs

A new NASA computer simulation shows that dark matter particles colliding in the extreme gravity of a black hole can produce strong, potentially observable gamma-ray light. Detecting this emission would provide astronomers with a new tool for understanding both black holes and the nature of dark matter, an elusive substance accounting for most of the mass of the universe that neither reflects, absorbs nor emits light.

"While we don't yet know what dark matter is, we do know it interacts with the rest of the universe through gravity, which means it must accumulate around supermassive black holes," said Jeremy Schnittman, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "A black hole not only naturally concentrates dark matter particles, its gravitational force amplifies the energy and number of collisions that may produce gamma rays."

A new computer simulation explores the connection between two of the most elusive phenomena in the universe, black holes and dark matter. Download this video in HD formats from NASA Goddard's Scientific Visualization Studio http://svs.gsfc.nasa.gov/goto?11894 .Credits: NASA's Goddard Space Flight Center

In a study published in The Astrophysical Journal on June 23, Schnittman describes the results of a computer simulation he developed to follow the orbits of hundreds of millions of dark matter particles, as well as the gamma rays produced when they collide, in the vicinity of a black hole. He found that some gamma rays escaped with energies far exceeding what had been previously regarded as theoretical limits.

In the simulation, dark matter takes the form of Weakly Interacting Massive Particles, or WIMPS, now widely regarded as the leading candidate of what dark matter could be. In this model, WIMPs that crash into other WIMPs mutually annihilate and convert into gamma rays, the most energetic form of light. But these collisions are extremely rare under normal circumstances.

Over the past few years, theorists have turned to black holes as dark matter concentrators, where WIMPs can be forced together in a way that increases both the rate and energies of collisions. The concept is a variant of the Penrose process, first identified in 1969 by British astrophysicist Sir Roger Penrose as a mechanism for extracting energy from a spinning black hole. The faster it spins, the greater the potential energy gain.

A new computer simulation reveals that dark matter particles orbiting a black hole produce a strong and potentially detectable signal of high-energy gamma rays.

Left: This visualization shows dark matter particles as gray spheres attached to shaded trails representing their motion. Redder trails indicate particles more strongly affected by the black hole's gravitation and closer to its event horizon (black sphere at center, mostly hidden by trails). The ergosphere, where all matter and light must follow the black hole's spin, is shown in teal. The black hole is viewed along its equator and rotates left to right.

Right: This image shows the gamma-ray signal produced in the computer simulation by annihilations of dark matter particles. Lighter colors indicate higher energies, with the highest-energy gamma rays originating from the center of the crescent-shaped region at left, closest to the black hole's equator and event horizon. The gamma rays with the greatest chances of escape are produced on the side of the black hole that spins toward us. Such lopsided emission is typical for a rotating black hole.  Credits: NASA Goddard's Space Flight Center Scientific Visualization Studio (left) and NASA Goddard/Jeremy Schnittman


In this process, all of the action takes place outside the black hole's event horizon, the boundary beyond which nothing can escape, in a flattened region called the ergosphere. Within the ergosphere, the black hole's rotation drags space-time along with it and everything is forced to move in the same direction at nearly speed of light. This creates a natural laboratory more extreme than any possible on Earth.

The faster the black hole spins, the larger its ergosphere becomes, which allows high-energy collisions further from the event horizon. This improves the chances that any gamma rays produced will escape the black hole.
"Previous work indicated that the maximum output energy from the collisional version of the Penrose process was only about 30 percent higher than what you start with,"  Schnittman said. In addition, only a small portion of high-energy gamma rays managed to escape the ergosphere. These results suggested that clear evidence of the Penrose process might never be seen from a supermassive black hole.

But the earlier studies included simplifying assumptions about where the highest-energy collisions were most likely to occur. Moving beyond this initial work meant developing a more complete computational model, one that tracked large numbers of particles as they gathered near a spinning black hole and interacted among themselves.

Schnittman's computer simulation does just that. By tracking the positions and properties of hundreds of millions of randomly distributed particles as they collide and annihilate each other near a black hole, the new model reveals processes that produce gamma rays with much higher energies, as well as a better likelihood of escape and detection, than ever thought possible. He identified previously unrecognized paths where collisions produce gamma rays with a peak energy 14 times higher than that of the original particles.

Using the results of this new calculation, Schnittman created a simulated image of the gamma-ray glow as seen by a distant observer looking along the black hole's equator. The highest-energy light arises from the center of a crescent-shaped region on the side of the black hole spinning toward us. This is the region where gamma rays have the greatest chance of exiting the ergosphere and being detected by a telescope.

The research is the beginning of a journey Schnittman hopes will one day culminate with the incontrovertible detection of an annihilation signal from dark matter around a supermassive black hole.

"The simulation tells us there is an astrophysically interesting signal we have the potential of detecting in the not too distant future, as gamma-ray telescopes improve," Schnittman said. "The next step is to create a framework where existing and future gamma-ray observations can be used to fine-tune both the particle physics and our models of black holes."


Related links:

Download high-resolution images and video in HD formats from NASA Goddard's Scientific Visualization Studio
svs.gsfc.nasa.gov/goto?11894

Paper: The Distribution and Annihilation of Dark Matter Around Black Holes
http://iopscience.iop.org/0004-637X/806/2/264/article

Paper: Revised Upper Limit to Energy Extraction from a Kerr Black Hole
dx.doi.org/10.1103/PhysRevLett.113.261102

NASA-Led Study Explains Decades of Black Hole Observations
www.nasa.gov/topics/universe/features/black-hole-study.html

Francis Reddy
NASA's Goddard Space Flight Center, Greenbelt, Md.

 Source: NASA/Solar System and Beyond