Magnetic field evolution in cosmic filaments with LOFAR data

Abstract

ABSTRACT Measuring the magnetic field in cosmic filaments reveals how the Universe is magnetized and the process that magnetized it. Using the Rotation Measures (RM) at 144 MHz from the LoTSS DR2 data, we analyse the rms of the RM extragalactic component as a function of redshift to investigate the evolution with redshift of the magnetic field in filaments. From previous results, we find that the extragalactic term of the RM rms at 144 MHz is dominated by the contribution from filaments (more than 90 per cent). Including an error term to account for the minor contribution local to the sources, we fit the data with a model of the physical filament magnetic field, evolving as $B_f = B_{f,0}\, (1+z)^\alpha$ and with a density drawn from cosmological simulations of five magnetogenesis scenarios. We find that the best-fitting slope is in the range α = [ − 0.2, 0.1] with uncertainty of σα = 0.4–0.5, which is consistent with no evolution. The comoving field decreases with redshift with a slope of γ = α − 2 = [ − 2.2, −1.9]. The mean field strength at z = 0 is in the range Bf, 0 = 39–84 nG. For a typical filament gas overdensity of δg = 10 the filament field strength at z = 0 is in the range $B_{f,0}^{10}=8$–26 nG. A primordial stochastic magnetic field model with initial comoving field of BMpc = 0.04–0.11 nG is favoured. The primordial uniform field model is rejected.