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The magnetic fields of spiral galaxies are so strong that they cannot beprimordial. Their typical values are over one billion times higher than anyvalue predicted for the early Universe. Explaining this immense growth andincorporating it in galaxy evolution theories is one of the long-standingchallenges in astrophysics. So far, the most successful theory for thesustained growth of the galactic magnetic field is the alpha-omega dynamo. Thistheory predicts a characteristic dipolar or quadrupolar morphology for thegalactic magnetic field, which has been observed in external galaxies. However,so far, there has been no direct demonstration of a mean-field dynamo operatingin direct, multi-physics simulations of spiral galaxies. We do so in this work.We employ numerical models of isolated, star-forming spiral galaxies thatinclude a magnetized gaseous disk, a dark matter halo, stars, and stellarfeedback. Naturally, the resulting magnetic field has a complex morphology thatincludes a strong random component. Using a smoothing of the magnetic field onsmall scales, we are able to separate the mean from the turbulent component andanalyze them individually. We find that a mean-field dynamo naturally occurs asa result of the dynamical evolution of the galaxy and amplifies the magneticfield by an order of magnitude over half a Gyr. Despite the highly dynamicalnature of these models, the morphology of the mean component of the field isidentical to analytical predictions. This result underlines the importance ofthe mean-field dynamo in galactic evolution. Moreover, by demonstrating thenatural growth of the magnetic field in a complex galactic environment, itbrings us a step closer to understanding the cosmic origin of magnetic fields.

A dynamo amplifies the magnetic field of a Milky-Way-like galaxy