Magnetizing the Cosmic Web during Reionization

Mathieu Langer,Jean-Baptiste Durrive

doi:10.3390/galaxies6040124

Abstract

Increasing evidence suggests that cosmological sheets, filaments, and voids may be substantially magnetized today. The origin of magnetic fields in the intergalactic medium (IGM) is, however, currently uncertain. It seems well known that non-standard extensions to the physics of the standard model can provide mechanisms susceptible of magnetizing the universe at large. Perhaps less well known is the fact that standard, classical physics of matter–radiation interactions actually possesses the same potential. We discuss a magnetogenesis mechanism based on the exchange of momentum between hard photons and electrons in an inhomogeneous IGM. Operating in the neighborhood of ionizing sources during the epoch of reionization, this mechanism is capable of generating magnetic seeds of relevant strengths over scales comparable to the distance between ionizing sources. In addition, summing up the contributions of all ionizing sources and taking into account the distribution of gas inhomogeneities, we show that this mechanism leaves the IGM, at the end of reionization, with a level of magnetization that might account, when amplification mechanisms take over, for the magnetic fields strengths in the current cosmic web.

Highlights

From planets to galaxies through stars and the interstellar medium, the existence of magnetic fields in astrophysical objects and environments has been detected by various, complementary techniques
According to the current paradigm, they were first generated as weak seeds that were later amplified and reshaped, perhaps first on small scales in the post-recombination intergalactic medium (IGM) through collisionless plasma instabilities (e.g., [17]), and/or through adiabatic compression and various dynamo mechanisms during or after structure formation [18,19,20]
We summarize here the basic features and results of an astrophysical mechanism, based on the photoionization of the IGM, that is bound to have contributed to the magnetization of the cosmic web during the first billion years of the universe, in the epoch of reionization

Summary

Introduction

From planets to galaxies through stars and the interstellar medium, the existence of magnetic fields in astrophysical objects and environments has been detected by various, complementary techniques (see, e.g., [1,2,3,4]). According to the current paradigm, they were first generated as weak seeds that were later amplified and reshaped, perhaps first on small scales in the post-recombination intergalactic medium (IGM) through collisionless plasma instabilities (e.g., [17]), and/or through adiabatic compression and various dynamo mechanisms during or after structure formation [18,19,20] Had they been generated with strengths larger than a few nano-Gauss, magnetic fields would have noticeably affected subsequent structure formation [21,22,23,24,25]. Its principles have been explored in [47,53], and the resulting, average strength of the magnetic field that permeates the universe by the end of reionization has been estimated in [54]

Outline of the Physical Mechanism

Resulting Magnetic Fields

Average Magnetic Energy Density Seeded in the IGM

Discussion