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EI2GYB > ASTRO 10.11.25 14:05l 42 Lines 5152 Bytes #999 (0) @ WW
BID : 47447_EI2GYB
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Subj: Why the Milky Way's Dark Heart Might Be Shaped Like a Box
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Why the Milky Way's Dark Heart Might Be Shaped Like a Box
Back in 2009, astronomers using the Fermi Gamma-ray Space Telescope noticed that there was a lot more gamma-ray light coming from the center of the Milky Way than might otherwise be expected given the objects there. Since then, two theories have appeared to explain this Galactic Center Excess (GCE) as it's become known. One theory posits that the extra gamma rays are created by thousands of unseen milli-second pulsars (MSPs) in the Galactic center, while the other suggests that dark matter annihilating itself could also be the source. A new paper from Moortis Muru and hisco-authors at the Leibniz Institute for Astrophysics Potsdam (AIP) hasn't necessarily solved the conundrum, but does level the playing field between the two theories again.
MSPs are a type of neutron star that rotate thousands of times a second, spraying radiation along their path as they do so. They're useful for everything from astrophysical testing to space navigation, and in many cases are very easy to see. However, the "bulge" at the center of our galaxy is luminous enough to obscure their visible light, making them effectively invisible in the chaos of the galactic center. Theory would say that their gamma rays would make it out, though, and that, if there were sufficient numbers of MSPs in the center, they could be the source of the GCE.
Dark matter, on the other hand, is notoriously invisible no matter where it is. In the theory that explains the GCE, it is comprised of Weakly Interacting Massive Particles (WIMPs), which are drawn together in a cluster by the gravitational pull of the galactic center and then annihilate each other, creating the excess gamma-rays and other standard model particles in the process. It might seem surprising that dark matter can interact with itself, since it so rarely does so with all other types of matter, but our current understanding of it suggests that it can, and relatively frequently as well.
A key piece of data collected by Fermi during its original observational run seemed to point towards the MSPs rather than dark matter as the GCE source. It seemed the gamma rays weren't coming from a perfect sphere, as might be expected by dark matter being gravitationally drawn to a spherical center of the galaxy. Rather, it has a "boxy" look to it, with some sides more flat than spherical. This matched up well with models of where the MSPs were expected to be, but not so much with dark matter models.
Enter the new paper - the authors used a simulation suite called the High-Resolution Environment Simulations of The Immediate Area (HESTIA) to model what happened during the formation of the Milky Way. They created a "digital twin" of the galaxy, and watched it form by interacting with, and sometimes absorbing, other nearby galaxies. As a result, their modeled Milky Way doesn't have dark matter collected in its center in a spherical shape - it looks "boxy", just like the pattern expected of the GCE. When they modeled what the gamma rays emitted from annihilation events with that shape, they found it matched the data about as well as the models using MSPs did.
What this means, of course, is that more data is needed. The paper can't definitively say which model is more accurate - it was describing a simulation that simply shows it is possible to match the GCE data with gamma rays from annihilating dark matter. But to truly differentiate between the two theories, astrophysicists will need more data. They hope to receive it when the Cherenkov Telescope Array (CTA) comes fully online in 2028. Models are only as good as the data they're based on, and the scientists will just have to wait for that better data - for now at least.
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