Galactic Center's Gamma Rays Are From Stars, Not Dark Matter

An excess of gamma rays has been found to follow the stellar distribution, not that of dark matter. ANU

The center of our galaxy is awash with gamma rays, which astronomers have explained using multiple competing models. Detailed mapping of the distribution of sources has ruled out dark matter as the source, with pulsars now appearing to be the most likely candidates, although further study is needed to confirm this.

The rate at which galaxies spin reveals their total mass far exceeds that of the stars and gas that make them up, indicating the existence of a missing component, dubbed dark matter. Decades after this realization, we have only a faint grasp of dark matter's composition

Consequently, when the Fermi Large Area Telescope discovered more gamma rays coming from the center of the galaxy than known sources could explain, astronomers wondered if dark matter might be responsible. Dark matter concentrates at the center of galaxies (spin rates would be different otherwise), so it seems possible component particles will collide there often, releasing gamma rays in the process.

However, the galactic center also hosts a concentration of stars, including pulsars. The distributions of these stars and dark matter differ as a product of the way spiral galaxies evolve. Dr Roland Crocker of the Australian National University is part of a team that mapped the gamma radiation in unprecedented detail. In Nature Astronomy they show it follows the stellar distribution, not that of dark matter.

Several previously proposed theories for the radiation's source all predicted this distribution, but Crocker regards emissions from millisecond pulsars as the most likely. These are supernova remnants that have been “spun up” to rotate in fractions of seconds.

“At the distance to the center of our galaxy, the emission from many thousands of these whirling dense stars could be blending together to imitate the smoothly distributed signal we expect from dark matter,” Crocker said in a statement. Nevertheless, Crocker told IFLScience they plan to investigate further to see if they can confirm whether millisecond pulsars are responsible, rather than another type of star with a similar spatial distribution.

Crocker told IFLScience that attempts to explain how pulsars could produce the radiation we see are so far rough, but suggest the team is on the right path. Insights into pulsar behavior are likely to emerge as modeling improves. The pulsars we are familiar with, those in our local neighborhood, are a mix of ages. Those near the galactic center are the remnants of stars born at least 8, and usually 10, billion years ago, and therefore may differ.

Meanwhile, the biggest implications could be for dark matter. Crocker told IFLScience; “We definitely need dark matter to explain the universe,” but the lack of radiation indicates the particles thought to form it interact less frequently than previously proposed.

Stars are not common in the area filled by the dark matter halo, instead being part of a flat disk with a bulge around the galactic center. ANU



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