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How a hidden population of pulsars may leave the Milky Way aglow

Most people think dark matter is the culprit in this mysterious warm light — but the real answer may be much simpler.
Fermi_8_year
When viewed in gamma rays, the center of the Milky Way is clearly illuminated as an area of high-energy emission.
NASA/DOE/Fermi LAT Collaboration

Searches for dark matter aren’t limited to facilities hundreds of feet underground. In the sky, astronomers continually seek observational evidence of the influence of dark matter on galactic scales. A recent study performed by an international team of astronomers, however, has proposed that the gamma ray glow coming from the Milky Way’s center, previously attributed to dark matter, may not arise from so exotic a source. Instead, the study says, the gamma rays could be produced by pulsars.


The study, which has been submitted to The Astrophysical Journal, says that pulsars — the rapidly spinning cores left behind by massive stars after they die — are responsible for the gamma rays seen in the center of our galaxy. Using data from the Large Area Telescope on NASA’s Fermi Gamma-ray Space Telescope, the researchers examined the central portions of the galaxy to determine the origin of the gamma-ray glow that has long been observed there. In a press release, Mattia Di Mauro of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) said, “Our study shows that we don’t need dark matter to understand the gamma-ray emissions of our galaxy. Instead, we have identified a population of pulsars in the region around the galactic center, which sheds new light on the formation history of the Milky Way.”


Why was this glow previously thought to be a signal of dark matter? Although dark matter doesn’t interact with normal matter directly, dark matter particles can decay or annihilate each other. Seth Digel, head of KIPAC’s Fermi group, explained: “Widely studied theories predict that these processes would produce gamma rays.” Thus, observers have searched for unexplained gamma rays in areas where dark matter is thought to accumulate, such as the centers of galaxies. And, indeed, the Milky Way’s center is brighter in gamma-ray light than expected. Thus, one explanation for the excess radiation is reactions powered by dark matter.


But the galactic center is a challenging place to observe. Not only is it shrouded in dust, it’s also densely packed with stars and the home of energetic processes that could also explain the gamma-ray excess observed there. A significant portion of the glow is produced when cosmic rays resulting from supernovae hit the molecules in interstellar gas clouds, causing them to give off light. But pulsars can also inject energy into these gas clouds, causing them to glow as well.


And with the addition of this new data, Eric Charles of KIPAC explained, “the gamma-ray excess at the galactic center is speckled, not smooth as we would expect for a dark matter signal.” The “speckles” may be individual sources — such as pulsars, which are small and hard to see, especially in such a crowded region — in the galactic center. By contrast, a signal from dark matter should be smooth, following the general distribution of dark matter particles expected in the region.


Approximately 70 percent of the Milky Way’s point sources are pulsars, Di Mauro said. And “Pulsars have very distinct spectra – that is, their emissions vary in a specific way with the energy of the gamma rays they emit.” By modeling the gamma-ray glow expected from the specific emissions of pulsars, the group found that their expectations matched the observations, indicating that pulsars, not dark matter, is responsible.


The study is in agreement with some other findings, which show that gamma-ray signals attributable to dark matter in the centers of other galaxies, particularly dwarf galaxies, are not seen. While our neighbor, the Andromeda Galaxy, also shows a gamma-ray excess in its center, the group argues that it might be due to pulsars as well.


But the complexity of the centers of galaxies continues to make pinpointing the exact source of these gamma rays difficult, and the study can’t completely rule out the possibility of dark matter as a contributor to the gamma-rays observed in the Milky Way’s center. More direct evidence will be needed; the team is already planning to observe the area with radio telescopes to identify individual pulsars in an attempt to better characterize the origin of gamma rays in the Milky Way’s bulge.


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