A closer look at supernovae as seeds for galactic magnetization

Context. Explaining the currently observed magnetic fields in galaxies requires relatively strong seeding in the early Universe. One of the current theories proposes that magnetic seeds on the order of mu G were expelled by supernova (SN) explosions after primordial fields of nG strength or weaker were amplified in stellar interiors. Aims. In this work, we take a closer look at this theory and calculate the maximum magnetic energy that can be injected in the interstellar medium by a stellar cluster of mass M-cl based on what is currently known about stellar magnetism. Methods. We consider early-type stars and adopt either a Salpeter or a top-heavy initial mass function. For their magnetic fields, we adopt either a Gaussian or a bimodal distribution. The Gaussian model assumes that all massive stars are magnetized with 10(3) < < B-*>< 10(4) G, while the bimodal, consistent with observations of MilkyWay stars, assumes only 5 - 10% of OB stars have 10(3) < < B-*> < 10(4) G, while the rest have 10 < < B-*> < 10(2) G. We ignore the e ffect of magnetic di ffusion and assume no losses of magnetic energy. Results. We find that the maximum magnetic energy that can be injected by a stellar population is between 10(-10) and 10(-7) times the total SN energy. The highest end of these estimates is about five orders of magnitude lower than what is usually employed in cosmological simulations, where about 10(-2) of the SN energy is injected as magnetic. Conclusions. Pure advection of the stellar magnetic field by SN explosions is a good candidate for seeding a dynamo, but not enough to magnetize galaxies. Assuming SNe as the main mechanism for galactic magnetization, the magnetic field cannot exceed an intensity of 10(-7) G in the best-case scenario for a population of 10(5) solar masses in a superbubble of 300 pc radius, while more typical values are between 10(-10) and 10(-9) G. Therefore, other scenarios for galactic magnetization at high redshift need to be explored.

Automated summary

This study investigates the impact of supernova explosions on galactic magnetization by examining the maximum magnetic energy that can be injected into the interstellar medium by a stellar cluster. The results indicate that pure advection of stellar magnetic fields by supernovae can serve as a good candidate for seeding a dynamo, but is insufficient to magnetize galaxies. Other mechanisms for galactic magnetization at high redshifts need to be explored.