Effects of primordial magnetic fields on 21 cm multifrequency angular power spectra

Abstract

The redshifted cosmic 21 cm line signal of neutral hydrogen provides the possibility to constrain the matter power spectrum. Cross correlating temperature maps at different frequencies (corresponding to different redshifts along the line of sight) allows to determine multifrequency angular power spectra. Primordial magnetic fields raise the amplitude of the linear matter power spectrum on small scales dominating over the contribution of the adiabatic primordial curvature mode. Multifrequency angular power spectra could provide a new possibility to probe primordial magnetic fields including its evolution present before decoupling. Here first multifrequency angular power spectra of the 21 cm line signal are obtained for cosmological models including the adiabatic, primordial curvature mode as well as the compensated magnetic mode for different values of the magnetic field parameters. For this temperature maps are simulated with the modified linear matter power spectra for magnetic fields with magnetic field strength $B_0=4$ nG and spectral indices $n_B=-2.9$ and $n_B=-2.5$, respectively, which have been chosen as examples. Moreover foregrounds have not been taken into account. As examples these multifrequency angular power spectra have been calculated for frequency ranges set around central frequencies of uGMRT Band 3 data as well as MeerKAT L band data. For these using only system noise signal-over-noise ratios are obtained as well as for SKA1-MID which is part of the currently under construction SKAO. Within this simple model results of this first study seem to be promising for constraining magnetic field parameters especially for SKA1-MID.