For more than a decade, scientists have been perplexed by a weird light at the center of our galaxy that eerily shines in gamma-rays for no discernable reason. A scientist has now proposed that this gamma glow at the heart of the Milky Way might have a counterintuitive source: dark matter, a mysterious substance that is primarily known for not emitting light.
This mind-bending explanation is described in a recent study published in Physical Review D authored by Mattia di Mauro, a researcher at Italy’s National Institute for Nuclear Physics (INFN) in Turin.
Di Mauro is not the first to suggest that dark matter might account for the unexplained gamma rays, which are collectively known as the Galactic Center GeV Excess (GCE). But by poring over new data collected by various observatories, di Mauro bolsters the case for a dark matter origin of the GCE, which he calls “one of the most intriguing mysteries in astroparticle physics” in the study.
“Although several proposed interpretations have appeared in the literature, there are no firm conclusions as to its origin,” di Mauro noted in the study, referring to the GCE. “The main difficulty in solving this puzzle lies in modeling a region of such complexity and thus, precisely measuring the characteristics of the GCE.”
The core of the Milky Way is an extremely radiant and busy region that is home to an enormous abundance of stars, gas, and dust clustered around a central supermassive black hole. So, it’s not surprising that this region is brilliant across a wide variety of wavelengths, which is why it is such a popular target for astrophotographers looking for a perfect night sky shot.
But in 2009, scientists combing through observations made by NASA’s Fermi space telescope noticed that the galactic center was much brighter in high-energy gamma ray radiation than any models had predicted. Since then, researchers have debated what might account for the strange glow: Could it be an unseen population of pulsars, which are bright fast-spinning remnants of stars? Could it be the high density of stars in the so-called galactic bulge around the core? Or maybe the excess is the result of cosmic ray bursts that cumulatively emit a gamma-ray glow?
Nobody knows, but one particular origin story has frequently been floated: dark matter, which is only known by its gravitational influence on normal matter, might be the unexpected candidate for the GCE.
Though nobody has seen light emitted by this enigmatic material, scientists have spent many years searching for luminous by-products of so-called “dark matter annihilations,” events that could finally unmask the true nature of dark matter. These annihilations could occur if dark matter is made up of a hypothetical entity known as a weakly interacting massive particle (WIMP), which is only one of many possible candidates to explain this strange substance.
When a WIMP and its antimatter counterpart collide, they obliterate each other in an interaction that spews out goodies like gamma rays, neutrinos, and cosmic rays (or so models suggest). These annihilations could be the source of the GCE, though this explanation is far from confirmed.
To constrain the dark matter interpretation for GCE, di Mauro pored over 11 years of Fermi data, along with observations collected by the now-defunct Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics and by the Alpha Magnetic Spectrometer (PAMELA) orbiter and the Alpha Magnetic Spectrometer (AMS) aboard the International Space Station.
This wide-ranging dataset enabled di Mauro to shed new light on the spatial distribution of the GCE and its relative energy levels. The results revealed that the gamma-ray energy levels across the excess are fairly similar, which seems more consistent with a dark matter origin than other explanations that would cause a more lopsided distribution of energy.
Di Mauro co-authored another forthcoming study in the same journal, which is now available on the preprint server arXiv, that models the possible interactions that might be occurring inside GCE, assuming it is made of self-annihilating dark matter particles.
Neither study is a clear indication that dark matter is driving this glowing excess in the galactic center, but both suggest that this exotic explanation for the GCE is still permissible. However, the sheer complexity of the galactic center, combined with the frustratingly elusive nature of dark matter, should temper our expectations that this puzzle will be solved anytime soon.
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