[Paper Review] The Westerbork SINGS Survey II. Polarization, Faraday Rotation, and Magnetic Fields

G. Heald, Róbert Braun+1 more|arXiv (Cornell University)|May 25, 2009

Astrophysics and Cosmic PhenomenaPhysics and Astronomy40 references120 citations

TL;DR

This study presents the first systematic, high-sensitivity survey of polarized radio continuum emission and Faraday rotation in 28 nearby spiral galaxies using the Westerbork Synthesis Radio Telescope. Employing RM-Synthesis, it reveals a ubiquitous azimuthal modulation of polarized intensity with a minimum along the receding major axis, indicating a large-scale, ordered magnetic field geometry, and detects multiple nuclear Faraday depth components offset by ±100–200 rad m⁻², suggesting complex magnetic structures near galactic nuclei.

ABSTRACT

A sample of large northern Spitzer Infrared Nearby Galaxies Survey (SINGS) galaxies has recently been observed with the Westerbork Synthesis Radio Telescope (WSRT). We present observations of the linearly polarized radio continuum emission in this WSRT-SINGS galaxy sample. Of the 28 galaxies treated in this paper, 21 are detected in polarized radio continuum at 18- and 22-cm wavelengths. We utilize the rotation measure synthesis (RM-Synthesis) method, as implemented by Brentjens & de Bruyn, to coherently detect polarized emission from a large fractional bandwidth, while simultaneously assessing the degree of Faraday rotation experienced by the radiation along each line-of-sight. This represents the first time that the polarized emission and its Faraday rotation have been systematically probed down to ~10 microJy/beam RMS for a large sample of galaxies. Non-zero Faraday rotation is found to be ubiquitous in all of the target fields, from both the Galactic foreground and the target galaxies themselves. In this paper, we present an overview of the polarized emission detected in each of the WSRT-SINGS galaxies. The most prominent trend is a systematic modulation of the polarized intensity with galactic azimuth, such that a global minimum in the polarized intensity is seen toward the kinematically receding major axis. The implied large-scale magnetic field geometry is discussed in a companion paper. A second novel result is the detection of multiple nuclear Faraday depth components that are offset to both positive and negative RM by 100-200 rad/m^2 in all targets that host polarized (circum-)nuclear emission.

Motivation & Objective

• To systematically probe the magnetic field structure in nearby spiral galaxies using polarized radio continuum emission.
• To measure Faraday rotation across a large sample of galaxies to infer magnetic field geometry and electron density along the line of sight.
• To determine the role of ordered magnetic fields in shaping galactic structure and regulating star formation.
• To investigate the presence of multiple Faraday depth components in galactic nuclei, indicating complex magnetic or plasma configurations.
• To establish a foundation for future high-resolution, wide-bandwidth polarization surveys with next-generation radio telescopes.

Proposed method

• Utilized the Westerbork Synthesis Radio Telescope (WSRT) to observe linearly polarized radio continuum emission at 18- and 22-cm wavelengths.
• Applied RM-Synthesis (Brentjens & de Bruyn) to coherently detect polarized emission over a wide fractional bandwidth and measure Faraday rotation measures (RMs).
• Used rotation measure synthesis to reconstruct Faraday dispersion functions and identify multiple Faraday depth components in the data.
• Corrected polarization angles using Faraday rotation measures to map magnetic field orientations perpendicular to the line of sight.
• Performed azimuthal binning of polarized intensity to detect systematic variations with galactic position angle.
• Accounted for Galactic foreground Faraday rotation to isolate contributions from the target galaxies.

Experimental results

Research questions

• RQ1 What is the large-scale geometry of magnetic fields in nearby spiral galaxies, as revealed by polarized radio emission?
• RQ2 How does the polarized intensity vary with galactic azimuth, and what does this imply about magnetic field structure?
• RQ3 Are there multiple Faraday depth components in the nuclei of galaxies with polarized emission, and what do they indicate about magnetic field topology?
• RQ4 To what extent is Faraday rotation in the target fields due to Galactic foreground versus intrinsic galactic plasma?
• RQ5 Can the observed polarization and RM patterns be explained by axi- or bi-symmetric magnetic field models?

Key findings

• Linearly polarized radio continuum emission was detected in 21 out of 28 galaxies, all with Hubble types between Sab and Sd.
• A systematic azimuthal modulation was observed, with polarized intensity minimized along the kinematically receding major axis, indicating a large-scale, ordered magnetic field.
• Non-zero Faraday rotation was detected in all target fields, with contributions from both Galactic foreground and intrinsic galactic plasma.
• In galaxies with prominent nuclear emission, Faraday dispersion functions showed broadening or splitting, indicating multiple Faraday depth components offset by ±100–200 rad m⁻².
• The presence of both positive and negative net RMs after foreground subtraction suggests a reversal of the line-of-sight magnetic field component near galactic nuclei, consistent with a radially directed field.
• The observed polarization and RM patterns are consistent with axi- or bi-symmetric magnetic field models, including a possible vertical field component.