The significant gamma-ray emission is coming from a population of millisecond pulsars within the Sagittarius dwarf spheroidal galaxy, an elliptical loop-shaped galaxy located approximately 78,300 light-years away, according to a new study led by Dr. Roland Crocker from Australian National University and the Max-Planck-Institut für Kernphysik and Dr. Oscar Macias from the Kavli Institute for the Physics and Mathematics of the Universe and the University of Amsterdam.
In their new study, Dr. Crocker, Dr. Macias and their colleagues focused on the Fermi Bubbles, clouds of gas towering about 30,000 light-years above and below the plane of our Milky Way Galaxy.
These gamma-ray-emitting structures were discovered in 2010 by the Large Area Telescope (LAT) on board NASA’s Fermi Gamma-ray Space Telescope.
Further analyses of the Fermi-LAT data identified two counterpropagating, collinear gamma-ray substructures within the bubbles: a jet in the northern Galactic hemisphere and a cocoon in the south.
Named the Fermi cocoon, the latter was originally thought to be due to past outbursts from Sagittarius A*, the 4.3-million-solar-mass black hole at the Milky Way’s center.
Using the new data from Fermi-LAT and ESA’s Gaia star-mapping mission, the astronomers found that the Fermi cocoon is actually due to emission from the Sagittarius dwarf spheroidal galaxy.
“This satellite of the Milky Way is seen through the Fermi bubbles from our position on Earth,” they explained.
“Due to its tight orbit around our Galaxy and previous passages through the Galactic disk, it has lost most of its interstellar gas and many of its stars have been ripped from its core into elongated streams.”
“Given that the Sagittarius dwarf is completely quiescent — it has no gas, and no stellar nurseries — there are only a few possibilities for its gamma-ray emission, including an unknown population of millisecond pulsars or the annihilation of dark matter.”
Millisecond pulsars are the remnants of certain types of stars, significantly more massive than the Sun, that are in close binary systems, and now blast out cosmic particles as a result of their extreme rotational energies.
The electrons fired by millisecond pulsars collide with low-energy photons of the Cosmic Microwave Background, propelling them to become high-energy gamma radiation.
The study authors demonstrated that the Fermi cocoon is explained by millisecond pulsars in the Sagittarius dwarf, and that the dark matter hypothesis is strongly disfavored.
This discovery sheds light on millisecond pulsars as efficient accelerators of highly energetic electrons and positrons.
It also suggests that similar physical processes could be ongoing in other dwarf satellite galaxies of the Milky Way.
“This is highly significant because dark matter researchers have long believed that an observation of gamma rays from a dwarf satellite would be a smoking gun signature for dark matter annihilation,” Dr. Macias said.
“Our study compels a reassessment of the high energy emission capabilities of quiescent stellar objects, such as dwarf spheroidal galaxies, and their role as prime targets for dark matter annihilation searches.”
The results appear in the journal Nature Astronomy.
R.M. Crocker et al. Gamma-ray emission from the Sagittarius dwarf spheroidal galaxy due to millisecond pulsars. Nat Astron, published online September 5, 2022; doi: 10.1038/s41550-022-01777-x
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