Measuring cosmic magnetic fields by rotation measure-galaxy cross-correlations in cosmological simulations

TL;DR

This study uses cosmological MHD simulations to assess the potential of cross-correlation of Faraday Rotation Measures with galaxy density to measure cosmic magnetic fields, highlighting challenges with current data and the need for future instruments.

Contribution

It evaluates the feasibility of detecting cosmic magnetic fields via RM-galaxy cross-correlations using simulations and discusses the limitations of current data and models.

Findings

Detection is only feasible with future instruments like SKA and ASKAP.

Current data cannot constrain magnetic field amplitudes in large-scale structures.

Mapping RM signals to cosmic magnetization is highly challenging due to model ambiguities.

Abstract

Using cosmological MHD simulations of the magnetic field in galaxy clusters and filaments we evaluate the possibility to infer the magnetic field strength in filaments by measuring cross-correlation functions between Faraday Rotation Measures (RM) and the galaxy density field. We also test the reliability of recent estimates considering the problem of data quality and Galactic foreground (GF) removal in current datasets. Besides the two self-consistent simulations of cosmological magnetic fields based on primordial seed fields and galactic outflows analyzed here, we also explore a larger range of models scaling up the resulting magnetic fields of one of the simulations. We find that, if an unnormalized estimator for the cross-correlation functions and a GF removal procedure is used, the detectability of the cosmological signal is only possible for future instruments (e.g. SKA and ASKAP). However, mapping of the observed RM signal to the underlying magnetization of the Universe (both in space and time) is an extremely challenging task which is limited by the ambiguities of our model parameters, as well as to the weak response of the RM signal in low density environments. Therefore, we conclude that current data cannot constrain the amplitude and distribution of magnetic fields within the large scale structure and a detailed theoretical understanding of the build up and distribution of magnetic fields within the Universe will be needed for the interpretation of future observations.