Researchers find a gamma ray source in a small neighboring galaxy

[2 of 2]    Researchers use gamma rays to reveal a small nearby galaxy filled with dark matter

Figure 1. A small satellite galaxy (green globe at bottom left) of the Milky Way – called Sagittarius – seen from Earth by giant lobes of gamma rays (also known as Fermi bubbles, which are 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. Credit: Kavli IPMU

Through giant lobes of gamma radiation, an international team of researchers has found a small satellite galaxy of the Milky Way filled with dark matter, but whose emissions are likely from millisecond pulsars exploding cosmic particles, a new study reports in natural astronomy.

The center of our galaxy is blowing out a pair of massive bubbles of gamma rays (purple structures in Figure 1) spanning 50,000 light-years across. It was discovered by the Fermi Gamma Ray Space Telescope about 10 years ago, and the source of this hourglass-shaped phenomenon remains unclear.

Called Fermi bubbles, they are lobes of radiation patched with an opaque substructure from the emission of extremely bright gamma rays. One of the brightest spots, called the Fermi cocoon, is found in the southern lobe (magnified inset in Figure 2) and was originally thought to be caused by past eruptions from the galaxy’s supermassive black hole.

An international team of researchers co-led by former Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) project researcher Oscar Macias (currently a GRAPPA Fellow at the University of Amsterdam) and Australian National University Associate Professor Roland Crocker, among them Kavli ITU Visiting Scientists Shunsaku Horiuchi and Shin’ichiro Ando, Analyzed data from GAIA and Fermi space telescopes to reveal that the Fermi cocoon is in fact the result of an emission from the Sagittarius dwarf galaxy.

This satellite galaxy of the Milky Way is seen through the Fermi bubbles from our location on Earth (Fig. 1). Due to its narrow orbit around our galaxy and past passes through the galactic disk, it lost most of its interstellar gas and many of its stars were torn from its core into long streams.

Given that Sagittarius was quiescent – with no gas and no stellar nurseries – there were only a few possibilities for gamma ray emission, including: 1) an unknown group Millisecond pulsars or ii) the annihilation of dark matter.

[2 of 2]    Researchers use gamma rays to reveal a small nearby galaxy filled with dark matter

Figure 2. Gamma-ray image of Fermi bubbles (blue) overlaid on a map of the stars RR Lyrae (red) observed by the GAIA telescope. The shape and orientation of the arc dwarf (Sgr) exactly coincides with the Fermi cocoon – a bright substructure of gamma rays in the southern part of the Fermi bubbles. This is strong evidence that the Fermi cocoon is caused by active processes occurring in the arc, which, in our view, lies behind the Fermi bubbles. Credit: Crocker, Macias, Maki, Krumholz, Ando, ​​Horiuchi et al. (2022)

Millisecond pulsars are the remnants of certain types of stars, much larger than the Sun, that are in nearby binary systems, but are now exploding cosmic particles As a result of their intense rotational energies. Electrons released by millisecond pulsars collide with low-energy cosmic microwave background photons, forcing them into high-energy gamma radiation.

The researchers showed that the gamma-ray cocoon can be explained by millisecond pulsars in the Sagittarius dwarf, thus resenting the dark matter interpretation.

Their discovery highlights millisecond pulsars as efficient accelerators of high-energy electrons and positrons, and also suggests that similar physical processes could be ongoing in another dwarf dwarf. galaxies from the Milky Way.

“This is important because dark matter researchers have long believed that observing gamma rays from a dwarf moon would serve as a smokescreen for dark matter annihilation. 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 for extermination,” Macias said.

Details of their studies have been published in natural astronomy On September 5th.

Gamma rays from a dwarf galaxy solve an astronomical mystery

more information:
Roland M. Crocker et al, Gamma ray emission from the Sagittarius dwarf globular galaxy due to millisecond pulsars, natural astronomy (2022). DOI: 10.1038 / s41550-022-01777-x

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