Probing star formation with galactic cosmic rays

Cosmic-ray energy densities in central regions of starburst galaxies, as inferred from radio and γ-ray measurements of, respectively, non-thermal synchrotron and π⁰-decay emission, are typically , i.e. typically at least an order of magnitude larger than near the Galactic Centre and in other not-very-actively star-forming galaxies. We first show that these very different energy density levels reflect a similar disparity in the respective supernova (SN) rates in the two environments, which is not unexpected given the SN origin of (Galactic) energetic particles. As a consequence of this correspondence, we then demonstrate that there is partial quantitative evidence that the stellar initial mass function (IMF) in starburst nuclei has a low-mass truncation at ∼2 M_⊙, as predicted by theoretical models of turbulent media, in contrast with the much smaller value of 0.1 M_⊙ that characterizes the low-mass cut-off of the stellar IMF in 'normal' galactic environments.