(Photo: Kavli IPMU)
A small satellite galaxy (green globe at bottom left) of the Milky Way – called Sagittarius – has been observed from Earth by giant lobes of gamma rays (also known as Fermi bubbles, purple regions below and above the galaxy). Although the arc is stuffed with dark matter, this is unlikely to be the cause of the observed emission.
An international team of astronomers has discovered a small Milky Way galaxy laden with dark matter, but its emission is likely the result of millisecond pulsating stars spewing out cosmic particles, using massive plumes of gamma rays.
Gamma rays solve cosmic anomalies
In case you didn’t know, the Milky Way’s core is constantly releasing a pair of massive gamma-ray bubbles that span an area of 50,000 light-years. This was discovered using the Fermi Gamma Ray Space Telescope about ten years ago. Phys.org Reports indicate that the origins of this hourglass-shaped anomaly remain unknown.
In detail, these radiation lobes, known as Fermi bubbles, are filled with a bewildering infrastructure of strong gamma-ray emission. The Fermi cocoon, one of the brightest regions in the southern lobe, is hypothesized to result from earlier explosions from a supermassive black hole in the galaxy.
However, that soon changed when an international team of researchers analyzed data from the GAIA and Fermi space telescopes to discover that the Fermi cocoon came from the emission from our neighboring galaxy, the Sagittarius dwarf galaxy.
study called Gamma-ray emission from the Sagittarius dwarf globular galaxy due to millisecond pulsars The Kavli IPMU was led by former researcher on the project Oscar Macias (now a GRAPPA Fellow at the University of Amsterdam) and Australian National University Associate Professor Roland Crocker, as well as Kavli IPMU visiting scientists Shunsaku Horiuchi and Shinichiro Ando.
We can glimpse this satellite of the Milky Way from Earth through the Fermi bubbles. Because its orbit is close to our galaxy and its predecessor is traveling through the galactic plane, it has exhausted most of its interstellar gas. Many of her stars were torn from her heart and spilled into streams. Given that Sagittarius was dormant with no gas and no stellar nurseries, the gamma-ray emission may have come from one of two sources. The first is through a cluster of undetected millisecond pulsars or dark matter annihilation.
For a bit of background, millisecond pulsars are remnants of certain types of stars that are much larger than the Sun and are in tight binary systems. But now it emits cosmic particles due to its enormous rotating energy. The pulsars’ millisecond electrons collided with low-energy photons from the cosmic microwave background, accelerating them into high-energy gamma radiation.
What the researchers say
Researchers demonstrated that the cocoon of gamma rays could be explained by millisecond pulsars in the Sagittarius dwarf, thus ruling out dark matter. Their discovery provides insights into millisecond pulsars as excellent accelerators of extremely dense electrons and positrons. It also shows that similar physical processes may occur in other neighboring Milky Way dwarf galaxies.
“Our study forces a reassessment of the high-energy emission capabilities of quiet stellar objects, such as dwarf spherical galaxies, and their role as prime targets for dark matter searches,” Macias said.
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By Thea Felicity
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