Primordial magnetic fields: consistent initial conditions and impact on high-z structures

TL;DR

This paper presents the first hydrodynamical simulations incorporating primordial magnetic fields (PMFs) to study their effects on small-scale structures and baryon fractions in high-redshift galaxies, revealing significant impacts and uncertainties.

Contribution

It introduces a novel simulation approach that accounts for PMFs' differential impacts on baryons and dark matter, highlighting their role in early galaxy formation and matter power spectrum enhancement.

Findings

PMFs can significantly boost baryon perturbations at high redshift.

Halo mass functions can be accurately modeled with Sheth-Torman formalism.

High-redshift galaxies may have baryon fractions several times above the cosmic average.

Abstract

Primordial magnetic fields (PMFs) can enhance matter power spectrum on small scales ($\lesssim$ Mpc) and still agree with bounds from cosmic microwave background (CMB) and Faraday rotation measurements. As modes on scales smaller than Mpc have already become non-linear today, exploring PMFs' impact on small-scale structures requires dedicated cosmological simulations. Here, for the first time, we perform a suite of hydrodynamical simulations that take into account the different impacts of PMFs on baryons and dark matter. Specifically, in the initial conditions we displace particles according to the Lorentz force from PMFs. We also highlight the large theoretical uncertainty in the peak enhancement of the matter power spectrum due to PMFs, which was not considered in previous studies. We present halo mass functions and show that they can be accurately reproduced using Sheth-Torman formalism. Moreover, we show that PMFs can generate galaxies with baryon fraction several times larger than the cosmic average at high redshifts. This is simply a consequence of the fact that PMFs enhance baryon perturbations, causing them to be larger than dark matter perturbations. We argue that this scenario could be tested soon by obtaining accurate estimates of the baryon fraction in high redshift galaxies.