The puzzling link between star formation and radio emission in galaxies

The puzzling link between star formation and radio emission in galaxies

June 29, 2022

(Nanowerk News) To understand the formation and evolution of galaxies like our Milky Way, it is of particular importance to know the amount of newly formed stars in both nearby and distant galaxies. To this end, astronomers often use a link between galaxies’ infrared and radio radiation, which has already been discovered 50 years ago: the energetic radiation from young, massive stars formed in the densest areas of galaxies is absorbed by surrounding dust clouds and re-emitted as low-energy infrared radiation.

Eventually, when their fuel supply runs out, these massive stars explode like supernovae at the end of their lives. In this explosion, the outer star casing is thrown into the environment, which accelerates some particles of the interstellar medium to very high energies, giving rise to so-called cosmic rays. In the magnetic field of the galaxy, these fast particles, which travel at almost the speed of light, emit a lot of low-energy radio radiation with a wavelength of a few centimeters to meters.

Through this chain of processes, newly formed stars, infrared radiation and radio radiation from galaxies are closely linked. Simulation of a formation disk galaxy, where cosmic rays are accelerated by supernova remnants and then escape into the interstellar medium Simulation of a formation disk galaxy, where cosmic rays are accelerated by supernova remnants and then escape into the interstellar medium. Cross section of the disk (top) and vertical sections (bottom) show the number of densities of cosmic beam electrons in steady state (left), magnetic field strength (center) and the brightness of the radio synchrotron. (Image: Werhahn / AIP)

Although this relationship is often used in astronomy, the exact physical conditions are not yet clear. Previous attempts to explain this usually failed in a prediction: if high-energy cosmic rays are really responsible for the radio radiation from these galaxies, the theory predicts very steep radio spectra – high emission at low radio frequencies – that do not agree with observations.

To get to the bottom of this mystery, a team of researchers at AIP has now for the first time realistically simulated these processes in a forming galaxy on a computer and calculated the energy spectra of the cosmic rays.

“During the formation of the galactic disk, cosmic magnetic fields are amplified to match the strongly observed galactic magnetic fields,” explains Professor Christoph Pfrommer, head of the cosmology and high-energy astrophysics section at AIP.

When cosmic radiation particles in magnetic fields emit radio radiation, they lose some of their energy on their way to us. As a result, the radio spectrum becomes flatter at low frequencies. At high frequencies, in addition to the radio emission of cosmic radiation, the radio emission from the interstellar medium, which has a flatter spectrum, also contributes. The sum of these two processes can therefore perfectly explain the observed flat radio radiation from the entire galaxy as well as the emission from the central regions. This also explains the mystery of why infrared and radio radiation from galaxies are so well connected.

“This allows us to better determine the number of newly formed stars from the observed radio emission in galaxies, which will help us further sort out the history of star formation in the universe,” concludes Maria Werhahn, a doctoral student at AIP and first author of one of the studies.

Scientific publications

Cosmic rays and non-thermal emission in simulated galaxies: III. probing cosmic ray calorimetry with radio spectra and FIR radio correlation. M. Werhahn, C. Pfrommer, P. Girichidis, 2021, MNRAS505, 3295, DOI: https://doi.org/10.1093/mnras/stab2535

Simulation of radiosynchrotron emission in star-forming galaxies: small-scale magnetic dynamo and the origin of the far-infrared radio correlation. C. Pfrommer, M. Werhahn, R. Pakmor, P. Girichidis, CM Simpson, 2022, MNRASaccepted, https://arxiv.org/abs/2105.12132v2


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