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Researchers find way to obtain gamma rays in small neighboring galaxy

[2 of 2] Researchers use gamma rays to detect small neighboring galaxy filled with Dark Matter
Figure 1. A little satellite galaxy (green globe on underneath 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 filled with dark matter, that is unlikely to be the reason for the observed emission. Credit: Kavli IPMU

Through giant lobes of gamma radiation, a global team of researchers have discovered a little satellite galaxy of the Milky Way filled up with dark matter, but whose emissions tend to be more likely the consequence of millisecond pulsars blasting out cosmic particles, reports a fresh study in Nature Astronomy.

The biggest market of our galaxy is blowing a set of colossal bubbles of gamma radiation (the magenta structures in figure 1) spanning 50,000 light-years across. Discovered with the Fermi Gamma-ray Space Telescope about 10 years back, the source of the hourglass-shaped phenomenon has remained unclear.

Called Fermi bubbles, these lobes of radiation are patched with several enigmatic substructures of very bright gamma-ray emission. Among the brightest spots, called the Fermi cocoon, is situated in the southern lobe (magnified inset in figure 2) and was originally regarded as because of past outbursts from the Galaxy’s supermassive black hole.

A global team of researchers co-led by former Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) Project Researcher Oscar Macias (currently GRAPPA Fellow at the University of Amsterdam) and Australian National University Associate Professor Roland Crocker, and including Kavli IPMU 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 because of emission from the Sagittarius dwarf galaxy.

This satellite galaxy of the Milky Way sometimes appears through the Fermi Bubbles from our position on the planet (figure 1). Because of its tight orbit around our galaxy and previous passages through the galactic disk, it has lost the majority of its interstellar gas and several of its stars have already been ripped from its core into elongated streams.

Considering that Sagittarius was quiescentwith no gas no stellar nurseriesthere were just a few possibilities because of its gamma-ray emission, including: i) a population of unknown or ii) dark matter annihilations.

[2 of 2] Researchers use gamma rays to detect small neighboring galaxy filled with Dark Matter
Figure 2. Gamma-ray image of the Fermi bubbles (blue) overlaid on a map of RR Lyrae stars (red) observed by the GAIA telescope. The form and orientation of the Sagittarius (Sgr) dwarf match perfectly well those of the Fermi cocoon a bright substructure of gamma-ray radiation in the southern portion of the Fermi bubbles. That is strong evidence that the Fermi cocoon is because of energetic processes occurring in Sagittarius, which from our perspective, is situated behind the Fermi bubbles. Credit: Crocker, Macias, Mackey, Krumholz, Ando, Horiuchi et al. (2022)

Millisecond pulsars are remnants of certain forms of stars, a lot more massive compared to the sun, which are in close binary systems, however now blast out due to their extreme rotational energies. The electrons fired by millisecond pulsars collide with low-energy photons of the Cosmic Microwave Background propelling them to high-energy gamma radiation.

The researchers demonstrated that the gamma-ray cocoon could possibly be explained by millisecond pulsars in the Sagittarius dwarf, therefore disfavoring the dark matter explanation.

Their discovery sheds light on millisecond pulsars as efficient accelerators of highly-energetic electrons and positrons, and in addition shows that similar physical processes could possibly be ongoing in other dwarf satellite of the Milky Way.

“That is significant because dark matter researchers have long believed an observation of gamma rays from the dwarf satellite will be a smoking gun signature for dark matter annihilation. Our study compels a reassessment of the high energy emission capabilities of quiescent stellar objects, such as for example dwarf spheroidal galaxies, and their role as prime targets for annihilation searches,” said Macias.

Information on their study were published in Nature Astronomy on September 5.



More info: Roland M. Crocker et al, Gamma-ray emission from the Sagittarius dwarf spheroidal galaxy because of millisecond pulsars, Nature Astronomy (2022). DOI: 10.1038/s41550-022-01777-x

Citation: Researchers find way to obtain gamma rays in small neighboring galaxy (2022, September 11) retrieved 12 September 2022 from https://phys.org/news/2022-09-source-gamma-rays-small-neighboring.html

This document is at the mercy of copyright. Aside from any fair dealing for the intended purpose of private study or research, no part could be reproduced minus the written permission. This content is provided for information purposes only.

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