A Light in the Dark? Observations of Milky Way's Center Hint at Existence of Dark Matter

A view of the entire gamma-ray sky from the Fermi Space Telescope, shaded to emphasize the center of the Milky Way. The inset is a map of the galactic center with known sources removed, which reveals the gamma-ray excess (red, green and blue) found there. This excess emission is consistent with annihilations from some hypothesized forms of dark matter. Image Credit/Caption: NASA/DOE/Fermi LAT Collaboration and T. Linden (Univ. of Chicago)

A view of the entire gamma-ray sky from the Fermi Space Telescope, shaded to emphasize the center of the Milky Way. The inset is a map of the galactic center with known sources removed, which reveals the gamma-ray excess (red, green, and blue) found there. This excess emission is consistent with annihilations from some hypothesized forms of dark matter.
Image Credit/Caption: NASA/DOE/Fermi LAT Collaboration and T. Linden (Univ. of Chicago)

Are we any closer to discovering the nature of the invisible, elusive substance thought to make up approximately 85 percent of all the mass in the Universe, also known as “dark matter”? According to a team of astronomers which analysed data that had been previously gathered by NASA’s Fermi Gamma-ray space telescope, we may have found some of the first direct evidence for its existence.

It has been one of the longest-standing mysteries of modern cosmology and astrophysics: The velocities of ordinary matter inside galaxies and galaxy clusters, first calculated by astronomers in the 1930s, were revealed to be too high to be accounted for by the presence of ordinary, observable matter alone. Either our understanding of gravity was wrong, or there was far more mass in the Universe than what could be directly observed. Thus, the notion of “dark matter” was born. Further observations during the following decades showed that galaxy clusters could bend the light of more distant, background cosmic objects (an effect known as gravitational lensing) more than what could be explained by the gravity of their observed matter alone, further strengthening the case for the existence of the hypothesized dark matter.

One of the leading candidates for the composition of this elusive substance are the so-called Weakly Interacting Massive Particles, or WIMPs. These are hypothesized particles that have no electromagnetic interaction with the rest of the ordinary matter in the Universe, but their presence can nevertheless be inferred by their gravitational attraction. Despite their apparent invisibility, there are several ways with which WIMPs could be detected, according to theoretical predictions: either indirectly, through their extremely rare collisions with particles of ordinary matter, or directly, by their mutual annihilation after colliding with one another. Since every WIMP is its own antimatter particle, according to theory, every time two of them would meet they would be annihilated, producing a shower of high-energy photons in the form of gamma rays and streams of other secondary particles.

Since dark matter is thought to be mostly concentrated around the centers of galaxies, astronomers have studied the center of the Milky Way persistently for years, in the hopes of detecting those annihilations between WIMPs using NASA’s Fermi Gamma-ray Space Telescope.

Launched in 2008, Fermi (formerly known as the Gamma-ray Large Area Space Telescope, or GLAST) continuously observed the whole sky in the entire high-energy part of the electromagnetic spectrum between 8 KeV and 300 GeV, from an orbit of 550 km above the ground, completing an all-sky survey every three hours. With the help of its two science instruments, the GLAST Burst Monitor, or GBM, and the Large Area Telescope, or LAT, Fermi discovered many new pulsars and detected high-energy cosmic rays coming from supernova remnants and gamma ray flares from active galactic nuclei, or blazars. It also studied our Milky Way galaxy in great detail, discovering two large gamma-ray-emitting bubbles extending 25,000 light-years north and south of the galactic center, something that hadn’t been observed before.

Video Credit: NASA/Goddard Space Flight Center

During its five-year mission, Fermi consistently recorded an unexplained excess of gamma rays coming from our galaxy’s central regions, which prompted many astronomers to announce that these observations were the first direct evidence of annihilations between WIMPs. Yet, although intriguing, the observations weren’t detailed enough for other more conventional explanations, like the existence of an unknown types of pulsars, to be ruled out completely. Subsequently, many within the scientific community cast doubt on the findings of the studies that presented dark matter as a viable explanation for the Fermi observations.

Now, a team of astronomers, led by Tansu Daylan, a graduate student at the Harvard University’s Department of Physics, has made the most detailed analysis to date of the excess gamma-ray light coming from the Milky Way center, as captured in the all-sky maps produced by Fermi, in an attempt to see if more conventional explanations could account for the observations. “Past studies have identified a spatially extended excess of 1-3 GeV gamma rays from the region surrounding the Galactic Center, consistent with the emission expected from annihilating dark matter,” writes the team in their pre-print paper, published online at arXiv, also scheduled for publication at the Physical Review D journal. “In this study, we have revisited and scrutinized the gamma-ray emission from the central regions of the Milky Way, as measured by the Fermi Gamma-Ray Space Telescope. In doing so, we have confirmed a robust and highly statistically significant excess, with a spectrum and angular distribution that is in excellent agreement with that expected from annihilating dark matter.”

Daylan’s team subtracted all the known sources that could account for the gamma-ray levels seen around the Milky Way center, like pulsars, binary star systems, and other background, diffuse gamma-ray emissions. In addition, they used very strict selection criteria in their analysis to shift through only the gamma-ray photons whose directions pointed to the Milky Way center as a point of origin, with a very high accuracy. Their results showed that even after this subtraction process, a level of excess gamma radiation still remained in the signal, spread out evenly inside a sphere extending 5,000 light-years from the galactic center. This excess radiation also appeared to be stronger in energies between 1 and 3 GeV, leading the researchers to conclude that the most probable explanation for its existence was the annihilation of WIMPs with a mass between 30 to 40 GeV. This explanation was in perfect agreement with several theoretical models that describe the properties of these elusive particles.

At left is a map of gamma rays with energies between 1 and 3.16 GeV detected in the galactic center by Fermi's LAT instrument; red indicates the greatest number. Prominent pulsars are labeled. Removing all known gamma-ray sources (right) reveals excess emission that may arise from dark matter annihilations. Image Credit/Caption: T. Linden (Univ. of Chicago)

At left is a map of gamma rays with energies between 1 and 3.16 GeV detected in the galactic center by Fermi’s LAT instrument; red indicates the greatest number. Prominent pulsars are labeled. Removing all known gamma-ray sources (right) reveals excess emission that may arise from dark matter annihilations. Image Credit/Caption: T. Linden (Univ. of Chicago)

“The new maps allow us to analyze the excess and test whether more conventional explanations, such as the presence of undiscovered pulsars or cosmic-ray collisions on gas clouds, can account for it,” says Dan Hooper, an associate professor at the University of Chicago’s Department of Astronomy and Astrophysics and co-author of the study. “The signal we find cannot be explained by currently proposed alternatives and is in close agreement with the predictions of very simple dark matter models.”

“Our case is very much a process-of-elimination argument,” adds Douglas Finkbeiner, Professor of Astronomy and of Physics at the Department of Astronomy at Harvard and a member of Daylan’s team. “We made a list, scratched off things that didn’t work, and ended up with dark matter.”

Although the results look promising, the team’s study hasn’t been peer-reviewed yet, and the researchers caution that additional observations are required before more conclusive results could be derived. “This is not the first time that an observational anomaly has been attributed to dark matter,” conclude the authors in their study. “Most, if not all of these signals, have nothing to do with dark matter, but instead result from some combination of astrophysical, environmental, and instrumental backgrounds. Given the frequency of such false alarms, we would be wise to apply a very high standard before concluding that any new signal is, in fact, the result of annihilating dark matter. There are significant reasons to conclude, however, that the gamma-ray signal described in this paper is far more likely to be a detection of dark matter than [other] anomalies. Anticipated measurements of the cosmic-ray antiproton-to-proton ratio by AMS [the Alpha Magnetic Spectrometer experiment module mounted on the International Space Station], may also be sensitive to annihilating dark matter with the characteristics implied by our analysis.”

The Fermi Space Telescope has been granted a five-year extension of its mission until 2018, thus allowing for a full decade of detailed observations at the high-energy Universe. Astronomers also plan to use Fermi for additional observations of the small satellite dwarf galaxies that are distributed around the Milky Way, in the hopes of detecting similar excess levels of gamma rays, something that would strengthen the case in favor of dark matter considerably. The data that the telescope will help to collect during this time, complemented by observations from other ground and space-based instruments that are planned to go online in the near future, might give us our first conclusive answers concerning this long-standing enigma of modern science, finally solving one of the biggest questions about the nature of the Universe.

Video Credit: NASA/Goddard Space Flight Center

 

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