Cosmological magnetogenesis: the Biermann battery during the Epoch of reionization

Abstract

We investigate the effect of the Biermann battery during the Epoch of Reionization (EoR) using cosmological Adaptive Mesh Refinement simulations within the framework of the Sphinx project. We develop a novel numerical technique to solve for the Biermann battery term in the Constrained Transport method, preserving both the zero divergence of the magnetic field and the absence of Biermann battery for isothermal flows. The structure-preserving nature of our numerical method turns out to be very important to minimize numerical errors during validation tests of the propagation of a Str”omgren sphere and of a Sedov blast wave. We then use this new method to model the evolution of a 2.5 and 5 co-moving Mpc cosmological box with a state-of-the-art galaxy formation model within the Ramses code. Contrary to previous findings, we show that three different Biermann battery channels emerge: the first one is associated with linear perturbations before the EoR, the second one is the classical Biermann battery associated with reionization fronts during the EoR, and the third one is associated with strong, supernova-driven outflows. While the two former channels generate spontaneously volume-filling magnetic fields with a strength on the order or below 10−20 G, the latter, owing to the higher plasma temperature and a marginally-resolved turbulent dynamo, reaches a field strength as high as 10−18 G in the intergalactic medium around massive haloes.

A Subgrid Turbulent Mean Field Dynamo Model for Cosmological Galaxy Formation Simulations

“…The most popular model to explain the origin of seed magnetic fields in the Universe is the Biermann battery process (Biermann 1950). Small velocity drifts between electrons and ions in presence of misaligned electron density and electron pressure gradients generate microscopic currents and fields during the Epoch of Reionization at the level of 10 −20 G (Kulsrud et al 1997;Gnedin 2000;Garaldi et al 2021;Attia et al 2021). This is very far from the observed 10 −6 G observed in galaxies today and even 10 −4 G in the past.…”

“…Another caveat of our model is how we set up the initial magnetic field. We used a uniform field with a strength of 10 −20 G, mimicking The topology of the Biermann generated fields is far more complex (see Attia et al 2021, for a recent study of the process). In the context of the subgrid turbulent dynamo, the topology and coherence length of the initial field is particularly important.…”

“…The primordial magnetic seed field predicted by the linear perturbation theory of the Biermann battery is very weak, around 10 −25 G (Naoz & Narayan 2013). The non-linear evolution of the Biermann battery during cosmological ionization fronts gives rise to slightly higher field strength around 10 −20 G (Gnedin et al 2000;Attia et al 2021). To mimic these weak seed fields, we have initialised the magnetic field as a constant field parallel to the 𝑧-direction with a strength of 10 −20 G in physical unit.…”

Introducing SPHINX-MHD: the impact of primordial magnetic fields on the first galaxies, reionization, and the global 21-cm signal

“…In future work (e.g. Attia et al 2021), we will look to model these alternative scenarios to determine their impact on the history of reionization and the first galaxies. Our simulations are among the first to simulate the epoch of reionization with fully coupled radiationmagnetohydrodynamics and thus represent an exciting milestone for cosmological numerical simulations.…”

“…The Biermann battery will generate magnetic fields as long as there is a gradient in the electron density that is perpendicular to a temperature gradient, for example, in an ionisation front sweeping over a gas filament. Gnedin et al (2000) post-processed cosmological simulations and found that this mechanism can generate seed magnetic fields on the order of 10 −21 in the IGM (see also Attia et al 2021) and the simulations of Garaldi et al (2020) show that the Durrive battery generates magnetic fields slightly weaker than those produced by the Biermann battery.…”