The puzzling link between star formation and radio emission in galaxies


Simulation of a forming disk galaxy, in which cosmic rays are accelerated by supernova remnants and then escape into the interstellar medium. Disc cross-sections (top) and vertical sections (bottom) show steady-state cosmic-ray electron number density (left), magnetic field strength (middle), and radio synchrotron luminosity . Credit: Werhahn/AIP

On the 50th anniversary of the discovery of a close link between star formation in galaxies and their infrared and radio radiation, researchers at the Leibniz Institute for Astrophysics in Potsdam (AIP) have now deciphered the underlying physics. To this end, they used new computer simulations of galaxy formation with comprehensive cosmic ray modeling.

To understand the formation and evolution of galaxies like our Milky Way, it is particularly important to know the amount of newly formed stars in nearby and distant galaxies. To this end, astronomers often use a link between the infrared and radio radiation of galaxies, which was already discovered 50 years ago: the energetic radiation of young massive stars that form in the densest regions of galaxies is absorbed by surrounding dust clouds and re-emitted as low energy infrared radiation. Eventually, when their fuel supply is exhausted, these massive stars explode into supernovae at the end of their lives. In this explosion, the outer stellar envelope is ejected 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, moving almost at the speed of light, emit very low energy radio radiation with a wavelength of a few centimeters to a few meters. Through this chain of processes, newly formed stars, infrared radiation and radio radiation of galaxies are closely linked.

Although this relationship is often used in astronomy, the exact physical conditions are not yet clear. Previous attempts to explain it have generally failed in a prediction: if high-energy cosmic rays are indeed responsible for the radio radiation of these galaxies, the theory predicts very steep radio spectra – high emission at low radio frequencies – which do not correspond not to observations. To get to the bottom of this mystery, a team of AIP researchers has, for the first time, realistically simulated these galaxy formation processes on a computer and calculated the energy spectra of cosmic rays. Their results are published in Royal Astronomical Society Monthly Notices.

“During the formation of the galactic disk, the cosmic magnetic fields are amplified so that they match the strong galactic magnetic fields observed,” explains Professor Christoph Pfrommer, head of the High Energy Cosmology and Astrophysics section at the AIP. When cosmic ray 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 lower frequencies. At high frequencies, in addition to the radio emission of cosmic rays, the radio emission of the interstellar medium, which has a flatter spectrum, also contributes. The sum of these two processes can therefore perfectly explain the flat radio radiation observed from the entire galaxy as well as the emission from the central regions.

It also explains the mystery of why infrared and radio radiation from galaxies are so well linked. “This allows us to better determine the number of newly formed stars from the radio emission observed in galaxies, which will help us better understand the history of star formation in the universe”, concludes Maria Werhahn , Ph.D. student at AIP and first author of one of the studies.

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More information:
Maria Werhahn et al, Cosmic Rays and Nonthermal Emission in Simulated Galaxies—III. Probing cosmic calorimetry with radio spectra and FIR-radio correlation, Royal Astronomical Society Monthly Notices (2021). DOI: 10.1093/mnras/stab2535

Christoph Pfrommer, Maria Werhahn, Rüdiger Pakmor, Philipp Girichidis, Christine M. Simpson, Simulation of synchrotron radio emission in star-forming galaxies: small-scale magnetic dynamo and origin of far-infrared-radio correlation. arXiv:2105.12132v2 [astro-ph.GA]

Provided by the Leibniz Institute for Astrophysics Potsdam

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