For years, scientists have been perplexed by a peculiar gamma-ray glow emanating from the Milky Way’s galactic center. Is it a sign of something more familiar, like a mass of rapidly spinning stars known as millisecond pulsars, or of dark matter, the invisible substance that appears to hold galaxies together? A group at the Leibniz Institute for Astrophysics Potsdam, headed by Moorits Mihkel Muru, tested both theories using simulations of Milky-Way-like galaxies.
Their conclusion is that, given the general form and brightness, the likelihood of a millisecond pulsar and dark matter annihilation is roughly equal, with a slight edge in favor of the dark matter explanation. One of the greatest mysteries in space science remains this glow, which was first observed in 2009 with NASA’s Fermi Gamma-ray Space Telescope.
“Dark matter dominates the Universe and holds galaxies together. It’s extremely consequential and we’re desperately thinking all the time of ideas as to how we could detect it,” according to astrophysicist Joseph Silk. “Gamma rays, and specifically the excess light we’re observing at the center of our galaxy, could be our first clue,” he continues. They might soon find out which theory is correct thanks to upcoming instruments like the Southern Wide-field Gamma-ray Observatory and the Cherenkov Telescope Array.
What the glow might be
The extra gamma rays in the galactic center are thought to be caused by two main candidates. One is dark matter, which could manifest as WIMP (weakly interacting massive particles). High-energy light, including gamma-ray photons, can be destroyed and released when a WIMP and its anti-particle collide. A population of millisecond pulsars is the other. These are supernova remnants: extremely dense neutron stars that rotate hundreds of times per second and act as cosmic lighthouses, sweeping radiation beams of gamma, X-ray, and radio waves across space.
Astronomers hoped for years that the glow’s shape would select a winner. The glow should appear somewhat “boxy” if it were caused by a millisecond pulsar in the old star’s bulge. The pattern would look more rounded if the source were dark matter in the galactic halo. However, the new simulations indicate that the Milky Way’s gradual merger with other galaxies may have flattened the galactic halo of dark matter. That flattened halo can also give the glow a boxy appearance when viewed from our position, which is roughly 8 kiloparsecs from the center. That is to say, dark matter is not excluded by a boxy appearance.
What the simulations say
Under the direction of Moorits Mihkel Muru, the group mapped the probable dark matter structure, performed supercomputer simulations of galaxies such as the Milky Way, and contrasted it with the distribution of old stars, which are used as a proxy for millisecond pulsars. Instead of focusing on how the signal varies over time, they examined how bright the gamma rays should be today. According to their paper’s conclusion, “Both hypotheses for the GCE, that of dark matter annihilations and millisecond pulsars, are equally plausible based on morphology, spectrum, and intensity, with perhaps a slight edge for the dark matter hypothesis on the last of these attributes in view of the observed deficiency in millisecond pulsars.”
The study has one catch, though. Tiny “speckles” in the glow have been detected in some analyses of Fermi Gamma-ray Space Telescope data, suggesting that numerous point sources were shining; this would support millisecond pulsars. The signal from dark matter would probably be smoother. Instead of that fine, speckled texture, the new piece emphasizes the large-scale shape. As a result, both sources—many millisecond pulsars plus some dark matter annihilation—may still be involved. As Joseph Silk notes, “It’s possible we will see the new data and confirm one theory over the other. Or maybe we’ll find nothing, in which case it’ll be an even greater mystery to resolve.”