4 mins read 28 Sep 2021

Empty-Sky Gamma Ray Mystery Solved by ANU Scientists

ANU researchers have discovered that gamma-ray emissions coming from seemingly “empty sky” are caused by star-forming galaxies.

The Gamma-Ray Sky - the thin band in the centre is the Milky Way plane. Credit: Goddard Media Studios.

Researchers at the Australian National University (ANU) have solved one of the mysteries of gamma-ray emissions coming from seemingly “empty sky”. They found that star-forming galaxies are responsible for these gamma-ray emissions. 

For the last 60-years astronomers have been trying to find the source of diffuse background gamma-ray emissions. The two key sources proposed were active galactic nuclei (AGN) which occur when gas is accreted by supermassive black holes at the centre of galaxies and star formation in the disks of galaxies.

"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 - and the other associated with star formation in the disks of galaxies.” said lead author, Dr Roth from the ANU Research School of Astronomy and Astrophysics. 

The researchers modelled gamma-ray emissions from all the galaxies in the Universe and compared the results with the predictions for potential sources of gamma-ray emissions. They found that star-forming galaxies produce the majority of diffuse gamma-ray emissions as opposed to active galactic nuclei. 

"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 Roth.

"Gamma-rays allow us to probe the most energetic phenomena in the Universe, in essence, they are a window to the most extreme physics out there."

“Key to this work was establishing an appropriate theoretical model that could be used with limited observational data. This was important because the number of observables at high redshift (large distances) is very limited, i.e. in our case, apart from the redshift we only have access to enough spectral and photometric data to give us a star-formation rate, physical size and the stellar mass of a galaxy - which we obtained from a Hubble Space Telescope survey. These parameters helped us to create a one-zone model for the galactic disk which in turn allowed us to determine how cosmic rays propagate through the disk and how many gamma-rays they produce in the process.” said Dr Roth.

The research in this new paper was only made possible with both the telescopes available and the understanding of how cosmic rays travel. This research also has implications for future research and studies regarding galaxy formation, dark matter, radio emissions, and cosmic rays.

“A key challenge, or rather a fundamental building block, was the availability of the required telescope data to apply the model to. Without the Hubble results and the detailed analysis of the gamma-ray background as obtained using Fermi-LAT, this would not have been possible. Furthermore, the detailed understanding of how cosmic rays propagate through the interstellar medium was also only recently sufficiently understood to enable this work.” said Dr Roth.

"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." 

This discovery could help astronomers with other mysteries in the Universe such as the kind of particles that make up dark matter, which is a holy grail in astrophysics. The model they created can also be used to make predictions about radio emissions which give clues about the internal structure of galaxies. 

“The potential to look for signatures of dark matter annihilation (some of these theorised particles we expect to be producing gamma-rays when they annihilate,” said Dr Roth.

“We still do not fully understand where cosmic rays (particles like protons and electrons that move at speeds very close to the speed of light) with ultra-high energies come from - we can observe them, however the cosmic ray "factories" we see do not produce CRs up to the very highest energies. Observing the gamma-rays these ultra high energy cosmic rays produce will hopefully yield some clues as to where they originate from.”

Data was collected from NASA’s Hubble Space Telescope and the Fermi Gamma-Ray Space Telescope. Researchers analysed a number of variables for many galaxies such as their star-formation rate, total mass, physical size, and distance from Earth. 

The research team is also looking at creating maps of the gamma-ray sky which can be used to inform upcoming gamma-ray observations from next-generation telescopes such as the Cherenkov Telescope Array which Australia is involved in.

“CTA is approximately 10 times more sensitive than current instruments, which is an improvement rarely seen for new instrumentation. I expect that the biggest surprises will be discoveries we are not even thinking of at the moment.”

The paper is now available in the journal Nature.