On the 50th anniversary of the discovery of a close relationship between galaxies’ star formation and their infrared and radio radiation, researchers at the Leibniz Institute for Astrophysics Potsdam (AIP) have now deciphered the underlying physics. They used novel computer simulations of galaxy formation with complete modelling of cosmic rays to accomplish this.
Understanding the formation and evolution of galaxies like our Milky Way requires knowing the number of newly formed stars in both nearby and distant galaxies. Astronomers frequently use a link discovered 50 years ago between galaxies’ infrared and radio radiation for this purpose: 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. When their fuel runs out, these massive stars explode as supernovae at the end of their lives.The outer stellar envelope is ejected into the environment during this explosion, which accelerates a few particles of the interstellar medium to extremely high energies, giving rise to cosmic rays. These fast particles, travelling at nearly the speed of light in the galaxy’s magnetic field, emit very low-energy radio radiation with wavelengths ranging from a few centimetres to metres. This chain of processes connects newly formed stars, infrared radiation, and radio radiation from galaxies.
Although this relationship is frequently used in astronomy, the precise physical conditions are not yet known. Previous attempts to explain it failed in one prediction: if high-energy cosmic rays are indeed responsible for these galaxies’ radio radiation, the theory predicts very steep radio spectra (high emission at low radio frequencies) that do not match observations. To solve this mystery, a team of AIP researchers has now realistically simulated the processes of a forming galaxy on a computer and calculated the cosmic ray energy spectra for the first time. Their findings were published in the Royal Astronomical Society’s Monthly Notices.
“During the formation of the galactic disc, cosmic magnetic fields are amplified to match the strong observed galactic magnetic fields,” explains Professor Christoph Pfrommer of the AIP’s section Cosmology and High-Energy Astrophysics. When cosmic ray particles in magnetic fields emit radio radiation, some of their energy is lost on the way to us. As a result, the radio spectrum at low frequencies becomes flatter. At high frequencies, radio emission from the interstellar medium, which has a flatter spectrum, contributes to the radio emission of cosmic rays. As a result, the sum of these two processes perfectly explains the observed flat radio radiation of the entire galaxy, as well as the emission of the central regions.
This also explains why galaxies’ infrared and radio radiation are so closely linked. “This allows us to better determine the number of newly formed stars from radio emission in galaxies, which will help us to further unravel the story of star formation in the universe,” says Maria Werhahn, Ph.D. student at AIP and first author of one of the studies.