The Westerbork SINGS Survey II. Polarization, Faraday Rotation, and Magnetic Fields

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.

An Analysis of Magnetic Fields through Polarimetric Observations in SINGS Galaxies

The paper describes an extensive survey of polarized radio continuum emissions observed in galaxies from the Spitzer Infrared Nearby Galaxies Survey (SINGS) using the Westerbork Synthesis Radio Telescope (WSRT). It primarily focuses on 28 northern galaxies, from which 21 were successfully detected in polarized emissions at 18- and 22-cm wavelengths. This study employs the rotation measure synthesis (RM-Synthesis) method to efficiently analyze Faraday rotations and magnetic field orientations within these galaxies. This marks the inaugural systematic exploration of Faraday rotation down to a sensitivity of approximately $10~\mu\text{Jy beam}^{-1}$ across a significant galaxy sample.

The findings reveal ubiquitous non-zero Faraday rotations across the fields surveyed, originating from both Galactic foregrounds and the galaxies themselves. A notable trend detected from this study is the modulation of polarized intensity with galactic azimuth, observing a global decrease in polarized intensity towards the kinematically receding major axis of the galaxies. The paper discusses the implications of these observations in terms of the magnetic field geometries, elucidating underlying large-scale magnetic structures and geometrical configurations within these galaxies.

Methodologies and Results

• RM-Synthesis in Polarimetric Study: The RM-Synthesis technique, a critical analytical method employed in this study, allowed for coherent detection of polarized emissions over substantial bandwidths while assessing Faraday rotation at each line-of-sight. This technique mitigates $n\pi$ ambiguities that occur in traditional methods of rotation measure calculations, thereby providing a more accurate assessment of polarized emissions even for faint signals.
• Deconvolution of Faraday Dispersion Functions: To resolve issues stemming from sidelobe complications in the RMSF, similar to dirty beam effects in radio imaging, the researchers adopted a deconvolution approach akin to the Högbom CLEAN algorithm. This process enabled the attainment of credible Faraday depth reconstructions, which are essential for tracing the component of the magnetic field along the line-of-sight ($B_{\parallel}$) and dissecting the perpendicular component ($B_{\perp}$).
• Galaxy Observations and Interpretations: Across the 21 galaxies detected with polarized emissions, researchers identified consistent structural modulation in polarized intensity aligned with the receding axis. Additionally, multiple Faraday depth components were detected near galactic nuclei exhibiting both positive and negative RM shifts between $[-200, +200]$. This unexpected result potentially unveils complex magnetic field and electron density distributions around galactic nuclei deserving further exploration.

Implications and Future Developments

This survey elucidates critical insights into large-scale magnetic fields within galaxies and their influence on star formation and galaxy evolution. Understanding the modulation and geometry of these magnetic fields holds significant implications in astrophysics. The study’s methodological rigor provides a framework that could be extended and honed with future radio telescopes, facilitating broader and deeper insight into interstellar magnetic fields.

The paper suggests further investigations into nuclear Faraday dispersion functions and proposes higher frequency polarimetric observations to delineate Faraday thickness in these regions more definitively. The advent of highly sensitive future instruments, such as the Square Kilometre Array (SKA) and its pathfinders, will empower even more refined exploration of magnetic fields in a wider range of galactic environments. These observations will be instrumental in characterizing magnetic fluctuations at various scales, contributing to comprehensive models in cosmic magnetism.

Through the systematic and innovative approaches described within this paper, the authors contribute substantially to the field of polarimetric radio astronomy and foster anticipation of upcoming technological advancements and their myriad possibilities for research in this domain.