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

TL;DR

This study investigates how primordial magnetic fields, present before recombination, alter the linear matter power spectrum and leave imprints on the 21 cm intensity mapping signal. By modeling the magnetic field as a stochastic Gaussian background with a damped power spectrum $P_B(k)$ and incorporating the compensated magnetic mode, the authors generate simulated 21 cm maps using modified CLASS and CRIME pipelines and compute multifrequency angular power spectra (MAPS) across frequencies corresponding to uGMRT Band 3, MeerKAT L band, and the future SKA1-MID. They analyze the MAPS $C_{\ell}(\nu_*,\Delta\nu)$ and quantify decorrelation with $|\Delta\nu|$, as well as the detectability via signal-to-noise estimates for current arrays and SKA1-MID configurations, including a Wide Band 1 survey. The results indicate that, with optimistic foreground removal, MAPS offer a viable path to constrain magnetic-field parameters ($B_0$, $n_B$), particularly for small $\Delta\nu$ and higher multipoles, highlighting the practical relevance of 21 cm tomography for primordial magnetism.

Abstract

The cosmic 21 cm line signal of neutral hydrogen provides the possibility to constrain the matter power spectrum. Primordial magnetic fields present before decoupling have an effect on the linear matter power spectrum. The redshifted 21 cm line signal is observed at different frequencies allowing to determine multifrequency angular power spectra. These are presented 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 multifrequency angular power spectra frequency ranges have been set around central frequencies of uGMRT Band 3 data as well as MeerKAT L band data. For these signal-over-noise ratios are obtained as well as for SKA1-MID which is part of the currently under construction SKAO.

Figures (3)

• Figure 1: Absolute values of the multi frequency angular power spectra of the theoretical model 21 cm signal $C_{\ell}^{MAPS}(\Delta\nu)\equiv C_{\ell}(\nu_c,\Delta\nu)$ as a function of $\Delta\nu$ for the total adiabatic and compensated magnetic mode are shown for uGMRT Band 3 at $\nu_c=432.8$ MHz ($z=2.28$) ( first row), for MeerKAT L-band (1) at $\nu_c=986$ MHz ($z=0.44$) ( second row, left) and for MeerKAT L-band (2) at $\nu_c=1077.5$ MHz ($z=0.32$) ( second row, right) in the upper panels. The fractional change of the MAPS ${\cal M}^{[ad,CMM(B_0,n_B)]}_{\ell}(\nu_{c,conf},\Delta\nu)$ resulting from the contribution of the adiabatic as well as the magnetic mode with respect to the MAPS of the adiabatic mode only are reported in the corresponding lower panels. These are shown for different magnetic field parameters ($B_0=0$ ( solid), $B_0=4$nG: $n_B=-2.9$ ( light dashed), $n_B=-2.5$ ( thick dashed)). Cosmological parameters are chosen to be the best fit parameter of the Planck 2018 baseline $\Lambda$CDM model.
• Figure 2: Signal-over-noise ratios of the multi frequency angular power spectra $C_{\ell}^{MAPS}(\Delta\nu)\equiv C_{\ell}(\nu_c,\Delta\nu)$ at different values of $\Delta\nu=\Delta\nu_*$ for the total adiabatic and compensated magnetic mode for ( first row), uGMRT Band 3 at $\nu_c=432.8$ MHz ($z=2.28$) ( left), for MeerKAT L-band (1) at $\nu_c=986$ MHz ($z=0.44$) ( centre) and for MeerKAT L-band (2) at $\nu_c=1077.5$ MHz ($z=0.32$) ( right). The second row shows results for the proposed SKA1-MID Wide Band 1 Survey at central frequencies for uGMRT ( left) and MeerKAT [( centre), ( right), respectively]. These are shown for different magnetic field parameters ($B_0$=0 ( solid), $B_0=4$nG: $n_B=-2.9$ ( light dashed), $n_B=-2.5$ ( thick dashed)). Further technical characteristics are given in the text.
• Figure 3: Cumulative signal-over-noise ratios of the multi frequency angular power spectra $C_{\ell}^{MAPS}(\Delta\nu)\equiv C_{\ell}(\nu_c,\Delta\nu)$ at different values of $\Delta\nu=\Delta\nu_*$ for the total adiabatic and compensated magnetic mode for ( first row), uGMRT Band 3 at $\nu_c=432.8$ MHz ($z=2.28$) ( left), for MeerKAT L-band (1) at $\nu_c=986$ MHz ($z=0.44$) ( centre) and for MeerKAT L-band (2) at $\nu_c=1077.5$ MHz ($z=0.32$) ( right). The second row shows results for the proposed SKA1-MID Wide Band 1 Survey at central frequencies for uGMRT ( left) and MeerKAT [( centre), ( right), respectively]. These are shown for different magnetic field parameters ($B_0$=0 ( solid), $B_0=4$nG: $n_B=-2.9$ ( light dashed), $n_B=-2.5$ ( thick dashed)). Further technical characteristics are given in the text.