The diffuse photon background that fills the universe does not limit itself to the attention-hogging cosmic microwave background, but spans a wide spectrum extending up to TeV energies. The origin of the photon emission at X-ray and gamma-ray wavelengths, first discovered in the 1970s, remains poorly understood. Many possible sources have been proposed, ranging from active galactic nuclei to dark-matter annihilation. Thanks to many years of gamma-ray data from the Fermi Large Area Telescope (Fermi-LAT), a group from Australia and Italy has now produced a model that links part of the diffuse emission to star-forming galaxies (SFGs).
As their name implies, SFGs are galaxies in which stars are formed, and therefore also die through supernova events. Such sources, which include our own Milky Way, have gained interest from gamma-ray astronomers during the past decade because several resolvable SFGs have been shown to emit in the 100 MeV to 1 TeV energy range. Given their preponderance, SFGs are thus a prime-suspect source of the diffuse gamma-ray background.
The source of gamma rays within SFGs is very likely the interaction between cosmic rays and the interstellar medium (ISM). The cosmic rays, in turn, are thought to be accelerated within the shockwaves of supernova remnants, after which they interact with the ISM to produce a hadronic cascade. The cascade includes neutral pions, which decay into gamma rays. This connection between supernova remnants and gamma rays is strengthened by a clear correlation between the star-formation rate in a galaxy and the gamma-ray flux they emit. Additionally, such sources are theorised to be responsible for the neutrino emission detected by the IceCube observatory over the past few years, which also appears to be highly isotropic.
Based on additional SFG gamma-ray sources found by Fermi–LAT, which could be used for validation, the Australian/Italian group developed a physical model to study the contribution of SFGs to the cosmic diffuse gamma-ray background. The model used to predict the gamma-ray emission from galaxies starts with the spectra of charged cosmic-rays produced in the numerous supernovae remnants within a galaxy, and greatly benefits from data collected from several such remnants present in the Milky Way. Subsequently the production and energies of gamma rays through their interaction of cosmic rays with the ISM is modelled, followed by the gamma-ray transport to Earth, which includes losses due to interactions with low-energy photons leading to pair production.
The main uncertainty in previous models was the efficiency of a galaxy to transform the energy from cosmic rays into gamma rays, since it is not possible to use our own galaxy to measure it. The big breakthrough in the new work is a more thorough theoretical modelling of this efficiency, which was first tested extensively using data from resolved SFG sources. After such tests proved successful, the model could be applied to predict the gamma-ray emission properties of galaxies spanning the history of the universe. These predictions indicate that the low-energy part of the spectrum can be largely attributed to galaxies from the so-called cosmic noon: the period when star formation in large galaxies was at its peak, about 10 billion years ago. Nearby galaxies, on the other hand, explain the high-energy part of the spectrum, which, for old and distant sources, is absorbed in the intergalactic medium by low-energy photons undergoing pair production with TeV emission. Overall, the model predicts not only the spectral shape but also the overall flux (see “Good fit” figure), negating the need for other possible sources such as active galactic nuclei or dark matter.
These new results once again indicate the importance of star-forming regions for astrophysics, after also recently being proposed as a possible source of PeV cosmic rays by LHAASO (CERN Courier July/August 2021 p11). Furthermore, it shows the potential for an expansion to other astrophysical messengers, with the authors stating their ambition to apply the same model to radio-emission and high-energy neutrinos.
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