Influence of primordial magnetic fields on 21 cm emission

TL;DR

This paper models how primordial magnetic fields influence the 21 cm emission signal from the early universe, affecting thermal history, ionization, and structure formation, thus offering a new way to constrain such magnetic fields.

Contribution

It introduces a semi-analytic model incorporating magnetic fields into early universe evolution and predicts their impact on 21 cm signals and fluctuations.

Findings

Magnetic fields increase the 21 cm signal at high redshifts due to extra heating.

Magnetic fields delay star formation and Lyman alpha background buildup.

Magnetic fields can decrease the 21 cm signal at lower redshifts.

Abstract

Magnetic fields in the early universe can significantly alter the thermal evolution and the ionization history during the dark ages. This is reflected in the 21 cm line of atomic hydrogen, which is coupled to the gas temperature through collisions at high redshifts, and through the Wouthuysen-Field effect at low redshifts. We present a semi-analytic model for star formation and the build-up of a Lyman alpha background in the presence of magnetic fields, and calculate the evolution of the mean 21 cm brightness temperature and its frequency gradient as a function of redshift. We further discuss the evolution of linear fluctuations in temperature and ionization in the presence of magnetic fields and calculate the effect on the 21 cm power spectrum. At high redshifts, the signal is increased compared to the non-magnetic case due to the additional heat input into the IGM from ambipolar diffusion and the decay of MHD turbulence. At lower redshifts, the formation of luminous objects and the build-up of a Lyman alpha background can be delayed by a redshift interval of 10 due to the strong increase of the filtering mass scale in the presence of magnetic fields. This tends to decrease the 21 cm signal compared to the zero-field case. In summary, we find that 21 cm observations may become a promising tool to constrain primordial magnetic fields.