A dynamo amplifies the magnetic field of a Milky-Way-like galaxy
  The magnetic fields of spiral galaxies are so strong that they cannot be
primordial. Their typical values are over one billion times higher than any
value predicted for the early Universe. Explaining this immense growth and
incorporating it in galaxy evolution theories is one of the long-standing
challenges in astrophysics. So far, the most successful theory for the
sustained growth of the galactic magnetic field is the alpha-omega dynamo. This
theory predicts a characteristic dipolar or quadrupolar morphology for the
galactic magnetic field, which has been observed in external galaxies. However,
so far, there has been no direct demonstration of a mean-field dynamo operating
in direct, multi-physics simulations of spiral galaxies. We do so in this work.
We employ numerical models of isolated, star-forming spiral galaxies that
include a magnetized gaseous disk, a dark matter halo, stars, and stellar
feedback. Naturally, the resulting magnetic field has a complex morphology that
includes a strong random component. Using a smoothing of the magnetic field on
small scales, we are able to separate the mean from the turbulent component and
analyze them individually. We find that a mean-field dynamo naturally occurs as
a result of the dynamical evolution of the galaxy and amplifies the magnetic
field by an order of magnitude over half a Gyr. Despite the highly dynamical
nature of these models, the morphology of the mean component of the field is
identical to analytical predictions. This result underlines the importance of
the mean-field dynamo in galactic evolution. Moreover, by demonstrating the
natural growth of the magnetic field in a complex galactic environment, it
brings us a step closer to understanding the cosmic origin of magnetic fields.
