Mysterious relationship between star formation and radio emission in galaxies

(ORDO NEWS) — To understand the formation and evolution of galaxies like the Milky Way, it is especially important to know the number of newly formed stars in both nearby and distant galaxies.

To do this, astronomers often use the link between infrared and radio emission, which was discovered 50 years ago: the energetic radiation of young massive stars that form in the densest regions of galaxies is absorbed by the surrounding dust clouds and re-emitted as low-energy infrared radiation.

Eventually, when the fuel supply runs out, these massive stars explode at the end of their lives, transforming into supernovae.

In this explosion, the outer shell of the star is ejected into the environment, which accelerates several particles of the interstellar medium to very high energies, giving rise to the so-called cosmic rays.

In the galaxy’s magnetic field, these fast particles, moving at nearly the speed of light, emit very low-energy radio waves with wavelengths ranging from a few centimeters to a meter.

Through this chain of processes, newly formed stars, infrared radiation and radio emission of galaxies are closely linked.

Although this relationship is often used in astronomy, the exact physical conditions are not yet clear. To unravel this mystery, the AIP research team has for the first time realistically modeled the processes of galaxy formation on a computer and calculated the energy spectra of cosmic rays.

“During the formation of the galactic disk, the cosmic magnetic fields are amplified so that they match the strong observed galactic magnetic fields,” explains Prof. Christoph Pfrommer, Head of the Cosmology and High Energy Astrophysics Section at the AIP.

When cosmic ray particles in magnetic fields emit radio emission, it loses some of its energy on its 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 rays, the radio emission of the interstellar medium, which has a flatter spectrum, also contributes.

Therefore, the sum of these two processes can perfectly explain the observed flat radio emission of the entire galaxy, as well as the emission of the central regions.

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