Mystery Of “Empty Sky” Gamma-Ray Origins Has Been Solved

The gamma-ray sky seen in the first 60 months of Fermi. The bright line in the middle is the Milky Way. Image credit: NASA/DOE/Fermi LAT Collaboration

Gamma-rays are one of the most energetic forms of light in the universe. They are emitted by certain radioactive processes and extreme cosmic events like supernovas, pulsars, and black hole accretion disks. However, there are some that don't have a clear origin; these events appear in patches of empty sky. Now, researchers have finally worked out their sources: star-forming galaxies.

As reported in the journal Nature, researchers modeled the possible origins of the diffuse gamma-ray background, which is particularly evident in the data from NASA’s Fermi Gamma-Ray Space Telescope. According to the researchers, the emission must be coming from galaxies that are undergoing major bursts of star formation.

In particular, it is cosmic rays – particles moving close to the speed of light – that could be playing a major role in the emission. When these particles hit interstellar gas, they release gamma-rays, something seen very strongly in star-forming galaxies.

“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,” Dr Matt Roth from the Australian National University (ANU) said in a statement.

“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 centres of all galaxies – called an active galactic nucleus (AGN) – and the other associated with star formation in the disks of galaxies," Roth explained. "We modelled the gamma-ray emission from all the galaxies in the universe and compared our results with the predictions for other sources and found that it is star-forming galaxies that produce the majority of this diffuse gamma-ray radiation and not the AGN process.”  

The modeling required a better understanding of how cosmic rays move through the interstellar gas of star-forming galaxies as well as better knowledge of those galaxies. That’s where the power of NASA’s Hubble Space Telescope comes in, allowing us to estimate the masses of galaxies as well as their distances, physical sizes, and star formation rates.

“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 added.

“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, which Australia is involved in. 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 future for gamma-ray astronomy remains extremely bright.

 
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