[Paper Review] Future Observations of Cosmic Magnetic Fields with the SKA and its Precursors

R. Beck|arXiv (Cornell University)|Nov 24, 2011

Radio Astronomy Observations and Technology1 references3 citations

TL;DR

This paper proposes that the Square Kilometre Array (SKA) and its precursors—LOFAR, ASKAP, and MeerKAT—will revolutionize the study of cosmic magnetic fields through deep, wide-area radio surveys. By mapping synchrotron emission and Faraday rotation measures (RMs) from polarized sources, the SKA will trace magnetic fields in galaxies, clusters, and the intergalactic medium with unprecedented sensitivity and resolution, enabling the first direct constraints on the origin and evolution of cosmic magnetism across cosmic time.

ABSTRACT

The origin of magnetic fields in the Universe is an open problem in astrophysics and fundamental physics. Polarization observations with the forthcoming large radio telescopes, especially the Square Kilometre Array (SKA), will open a new era in the observation of magnetic fields and should help to understand their origin. Low-frequency radio synchrotron emission, to be observed with LOFAR, MWA and the SKA, traces low-energy cosmic ray electrons and allows us to map the structure of weak magnetic fields in the outer regions and halos of galaxies, in halos and relics of galaxy clusters and in the Milky Way. Polarization at higher frequencies (1-10 GHz), to be observed with the SKA and its precursors ASKAP and MeerKAT, will trace magnetic fields in the disks and central regions of galaxies and in cluster relics in unprecedented detail. All-sky surveys of Faraday rotation measures towards a dense grid of polarized background sources with ASKAP (project POSSUM) and the SKA are dedicated to measure magnetic fields in intervening galaxies, clusters and intergalactic filaments, and will be used to model the overall structure and strength of magnetic fields in the Milky Way. "Cosmic Magnetism" is key science for LOFAR, ASKAP and the SKA.

Motivation & Objective

• To investigate the origin and evolution of cosmic magnetic fields across cosmic time.
• To develop observational strategies using the SKA and its precursors to map magnetic fields in galaxies, galaxy clusters, and the intergalactic medium.
• To enable high-precision 3D reconstruction of magnetic field structures in the Milky Way and nearby galaxies using RM grids from polarized sources.
• To detect or constrain the presence of large-scale intergalactic magnetic fields with coherence lengths of a few Mpc.
• To test theoretical models of cosmic magnetism, including dynamo theory and primordial field generation, using deep all-sky surveys.

Proposed method

• Utilize low-frequency radio synchrotron emission (70–450 MHz) from LOFAR, MWA, and SKA to trace weak magnetic fields in galactic halos and intergalactic filaments.
• Apply high-frequency polarization observations (1–10 GHz) with SKA, ASKAP, and MeerKAT to map ordered magnetic fields in galactic disks and cluster relics.
• Conduct all-sky surveys of Faraday rotation measures (RMs) toward a dense grid of polarized background sources using ASKAP (POSSUM) and SKA to map intervening magnetic fields.
• Employ pulsar surveys with the SKA to obtain thousands of high-precision RM measurements for mapping the Milky Way’s magnetic field in 3D.
• Use statistical analysis of RM power spectra and cross-correlations with large-scale structure to detect weak, diffuse intergalactic magnetic fields.
• Combine data from multiple SKA frequency bands and array types (aperture arrays and parabolic dishes) to achieve wide field of view, high sensitivity, and sub-arcsecond resolution.

Experimental results

Research questions

• RQ1 What is the strength and spatial structure of magnetic fields in the halos of galaxies and in the intergalactic medium, as traced by low-frequency synchrotron emission?
• RQ2 Can the SKA detect and map ordered magnetic fields in distant galaxies and cluster relics through polarized emission and Faraday rotation?
• RQ3 What constraints can deep all-sky RM surveys (e.g., POSSUM and SKA) place on the existence and coherence of intergalactic magnetic fields?
• RQ4 How do magnetic field strengths and configurations evolve over cosmic time, as inferred from RM grids toward high-redshift sources (z ≈ 5)?
• RQ5 Can statistical methods such as power spectrum analysis of RMs detect a diffuse, primordial intergalactic magnetic field with coherence lengths of several Mpc?

Key findings

• The SKA will detect at least 1.5 × 10^7 Faraday rotation measures per square degree at 1 GHz, with a mean source spacing of ~90 arcseconds, enabling detailed 3D mapping of magnetic fields in the Milky Way and intervening galaxies.
• ASKAP’s POSSUM survey will measure ~100 RMs per square degree per 10-hour integration, providing high-density RM grids for modeling magnetic fields in the Milky Way and extragalactic structures.
• The SKA pulsar survey is expected to discover ~20,000 new pulsars, yielding over 10,000 RM values in the M31 region, enabling high-precision 3D reconstruction of the Galactic magnetic field.
• The SKA will detect magnetic fields in the intergalactic medium with a strength of ~0.1 nG and coherence length of a few Mpc, provided the RM density reaches ~1000 sources per square degree.
• For intergalactic magnetic fields with coherence lengths of 1–3 Mpc and electron densities of ~10^{-5} cm^{-3}, Faraday rotation measures between 0.1 and 1 rad m^{-2} are expected, detectable with statistical analysis of RM data.
• The method of RM grids will allow detection of regular magnetic fields of several μG in distant galaxies up to redshift z ≈ 3, consistent with mean-field dynamo theory predictions.