Strong magnetic fields and large rotation measures in protogalaxies by supernova seeding
(Aug 15, 2013)Abstract
We present a model for the seeding and evolution of magnetic fields in
protogalaxies. Supernova (SN) explosions during the assembly of a protogalaxy
provide magnetic seed fields, which are subsequently amplified by compression,
shear flows and random motions. We implement the model into the MHD version of
the cosmological N-body / SPH simulation code GADGET and we couple the magnetic
seeding directly to the underlying multi-phase description of star formation.
We perform simulations of Milky Way-like galactic halo formation using a
standard LCDM cosmology and analyse the strength and distribution of the
subsequent evolving magnetic field. A dipole-shape divergence-free magnetic
field is injected at a rate of 10^-9G / Gyr within starforming regions, given
typical dimensions and magnetic field strengths in canonical SN remnants.
Subsequently, the magnetic field strength increases exponentially on timescales
of a few ten million years. At redshift z=0, the entire galactic halo is
magnetized and the field amplitude is of the order of a few \$\mu\$G in the
center of the halo, and 10^-9 G at the virial radius. Additionally, we
analyse the intrinsic rotation measure (RM) of the forming galactic halo over
redshift. The mean halo intrinsic RM peaks between redshifts z=4 and z=2 and
reaches absolute values around 1000 rad m^-2. While the halo virializes
towards redshift z=0, the intrinsic RM values decline to a mean value below 10
rad m^-2. At high redshifts, the distribution of individual starforming, and
thus magnetized regions is widespread. In our model for the evolution of
galactic magnetic fields, the seed magnetic field amplitude and distribution is
no longer a free parameter, but determined self-consistently by the star
formation process occuring during the formation of cosmic structures.