Until now, it has been unclear what created gamma-rays that appear in patches of seemingly ‘empty sky.’
Roth et al. present a calculation of the contribution of star-forming galaxies to the gamma-ray background. Image credit: NASA.
“It’s a significant milestone to finally discover the origins of this gamma-ray emission, solving a mystery of the Universe astronomers have been trying to decipher since the 1960s,” said Dr. Matt Roth, an astronomer in the Research School of Astronomy and Astrophysics at the Australian National University.
“There are two obvious sources that produce large amounts of gamma-rays seen in the Universe.”
“One when gas falls into the supermassive black holes which are found at the centers of all galaxies — called an active galactic nucleus (AGN) — and the other associated with star formation in the disks of galaxies.”
In the study, Dr. Roth and colleagues modeled the gamma-ray emission from galaxies in the Universe and compared their results with the predictions for other sources.
They found that it is star-forming galaxies that produce the majority of this diffuse gamma-ray radiation and not the AGN process.
“We were able to pinpoint what created these mysterious gamma-rays after obtaining a better understanding of how cosmic rays — particles that travel at speeds very close to the speed of light — move through the gas between the stars,” they said.
They analyzed information about many galaxies such as their star-formation rates, total masses, physical size and distances from Earth.
The data from the NASA/ESA Hubble Space Telescope and NASA’s Fermi Gamma-Ray Space Telescope were a key resource used to uncover the unknown origins of the gamma-rays.
“Our model can also be used to make predictions for radio emission – the electromagnetic radiation that has a frequency similar to a car radio — from star-forming galaxies, which could help researchers understand more about the internal structure of galaxies,” Dr. Roth said.
“We are currently looking at producing maps of the gamma-ray sky that can be used to inform upcoming gamma-ray observations from next-generation telescopes. This includes the Cherenkov Telescope Array.”
“This new technology will hopefully allow us to observe many more star-forming galaxies in gamma-rays than we can detect with current gamma-ray telescopes.”
The team’s results were published in the journal Nature.
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M.A. Roth et al. 2021. The diffuse γ-ray background is dominated by star-forming galaxies. Nature 597, 341-344; doi: 10.1038/s41586-021-03802-x
