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Fermi Cocoon Occurs Due to Emission from the Sagittarius Dwarf Galaxy

According to a recent study published in Nature Astronomy, an international team of researchers has discovered a small satellite galaxy of the Milky Way loaded with dark matter. However, the emissions are most likely the product of millisecond pulsars shooting forth cosmic particles.

Emission from the Sagittarius Dwarf Galaxy Causes Fermi Cocoon
A small satellite galaxy (green globe on the bottom left) of the Milky Way—called Sagittarius—has been observed from Earth through giant lobes of gamma radiation (aka the Fermi bubbles, purple areas below and above the galaxy). Although Sagittarius is stuffed with dark matter, this is unlikely to be the cause of the observed emission. Image Credit: Kavli Institute for the Physics and Mathematics of the Universe.

A pair of enormous gamma radiation bubbles the size of the galaxy’s center is being blown, each with a diameter of 50,000 light-years. This hourglass-shaped anomaly was spotted using the Fermi Gamma-ray Space Telescope roughly a decade ago, but its origin has remained a mystery.

These radiation lobes are known as Fermi bubbles, and they are patched with a few mysterious substructures that emit very intense gamma rays. The Fermi cocoon, one of the brightest regions in the southern lobe, was formerly believed to be the result of previous explosions from the galaxy’s supermassive black hole.

The Fermi cocoon is actually due to form soon, according to an international team of researchers headed 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.

The study also included Kavli IPMU Visiting Scientists Shunsaku Horiuchi and Shin'ichiro Ando.

From Earth, viewers may observe this Milky Way satellite galaxy through the Fermi Bubbles. It has lost the majority of its interstellar gas and several of its stars have been ripped from its core into elongated streams as a result of its close orbit around the galaxy and past trips through the galactic disk.

There were only a few potential explanations for Sagittarius’ gamma-ray emission because it was quiet, lacking both star nurseries and gas. These explanations included: A population of unidentified millisecond pulsars or dark matter annihilations, respectively.

Because of their incredibly high rotational energy, millisecond pulsars are the remains of specific stars in near-binary systems and were far more massive than the Sun. The Cosmic Microwave Background’s low-energy photons were hit with the electrons produced by millisecond pulsars, accelerating them to high-energy gamma radiation.

The scientists disproved the theory of dark matter by demonstrating that the gamma-ray cocoon could be described by millisecond pulsars in the Sagittarius dwarf.

Their discovery clarifies millisecond pulsars as effective accelerators of highly energetic electrons and positrons and raises the possibility that related physical processes are taking place in additional Milky Way dwarf satellite galaxies.

This is 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.

Oscar Macias, GRAPPA Fellow, University of Amsterdam

Macias adds, “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.”

Journal Reference:

Crocker, R. M., et al. (2022) Gamma-ray emission from the Sagittarius dwarf spheroidal galaxy due to millisecond pulsars. Nature Astronomy.

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