[Paper Review] A Sino-German lambda6 cm polarization survey of the Galactic plane I. Survey strategy and results for the first survey region

X. Sun, J. L. Han+5 more|ArXiv.org|Nov 20, 2006

Electrical and Electromagnetic Research1 references55 citations

TL;DR

This paper presents the first results from a Sino-German λ6 cm polarization survey of the Galactic plane using the Urumqi 25 m telescope, achieving 9.5 arcmin resolution. It identifies two new Faraday screens—G124.9+0.1 (a faint HII region at 2.8 kpc with a line-of-sight magnetic field of 3.9 μG) and G125.6−1.8 (a large-scale screen with >6.4 μG field) — and demonstrates the survey's capability to detect weak synchrotron emission and strong, regular magnetic fields beyond the Perseus arm.

ABSTRACT

(Abridged) Polarization measurements of the Galactic plane at lambda6 cm probe the interstellar medium (ISM) to larger distances compared to measurements at longer wavelengths, hence enable us to investigate properties of the Galactic magnetic fields and electron density. We are conducting a new lambda6 cm continuum and polarization survey of the Galactic plane covering 10degr < l < 230degr and |b|<5degr. Missing large-scale structures in the U and Q maps are restored based on extrapolated polarization K-band maps from the WMAP satellite. The lambda6 cm data are analyzed together with maps at other bands. We discuss some results for the first survey region, 7degr X 10degr in size, centered at (l,b)=(125.5degr, 0degr). Two new passive Faraday screens, G125.6-1.8 and G124.9+0.1, were detected. They cause significant rotation of background polarization angles but little depolarization. G124.9+0.1 was identified as a new faint HII region at a distance of 2.8 kpc. G125.6-1.8, with a size of about 46 pc, has neither correspondence in enhanced Halpha emission nor a counterpart in total intensity. A model combining foreground and background polarization modulated by the Faraday screen was developed. Using this model, we estimated the strength of the ordered magnetic field along the line of sight to be 3.9 microGauss for G124.9+0.1, and exceeding 6.4 microGauss for G125.6-1.8. We obtained an estimate of 2.5 and 6.3 mK/kpc for the average polarized and total synchrotron emissivity towards G124.9+0.1. The synchrotron emission beyond the Perseus arm is quite weak.

Motivation & Objective

• To map the Galactic plane at λ6 cm with high sensitivity and angular resolution to study the magneto-ionic interstellar medium (ISM).
• To overcome missing large-scale polarization structures in single-dish surveys by restoring zero-level information using extrapolated WMAP K-band polarization data.
• To identify and characterize Faraday screens and HII regions through polarization and spectral index analysis.
• To measure the strength of ordered magnetic fields and estimate synchrotron emissivity in the ISM using model fitting.
• To improve understanding of the large-scale magnetic field structure and electron density distribution in the Galactic disk.

Proposed method

• Conducted λ6 cm continuum and polarization observations of the Galactic plane with the Urumqi 25 m telescope, covering 10° ≤ l ≤ 230° and |b| ≤ 5°.
• Restored missing large-scale structures in Q and U Stokes parameters by extrapolating WMAP K-band (22.8 GHz) polarization maps to 4.8 GHz using a spectral index of −2.8.
• Combined λ6 cm data with multi-frequency observations to analyze spectral indices and identify emission components.
• Fitted a model combining foreground and background polarization modulated by Faraday screens to disentangle magnetic field and emissivity contributions.
• Used spectral index analysis and IRAS flux ratios to classify extended sources as HII regions or SNRs.
• Applied model fitting to estimate magnetic field strength along the line of sight and synchrotron emissivity.

Experimental results

Research questions

• RQ1 What are the properties of the large-scale magnetic field structure in the Galactic plane at λ6 cm?
• RQ2 Can missing large-scale polarization structures in single-dish surveys be effectively restored using external data?
• RQ3 What are the magnetic field strengths and electron densities in newly detected Faraday screens?
• RQ4 How do the spectral indices of extended sources like SNRs and HII regions constrain their nature?
• RQ5 What is the level of synchrotron emissivity beyond the Perseus arm, and how does it compare to expectations?

Key findings

• Two new Faraday screens were detected: G124.9+0.1, a faint HII region at 2.8 kpc with a line-of-sight magnetic field strength of 3.9 μG.
• G125.6−1.8, a large-scale Faraday screen with a magnetic field strength exceeding 6.4 μG and an electron density of up to 0.84 cm⁻³.
• The spectral index of SNR G126.2+1.6 is consistent with a straight spectrum, ruling out previous claims of spectral curvature.
• Synchrotron emission beyond the Perseus arm is weak, with estimated average polarized emissivity of 2.5 mK kpc⁻¹ and total emissivity of 6.3 mK kpc⁻¹ toward G124.9+0.1.
• The zero-level restoration using WMAP data significantly improved the fidelity of polarization maps, revealing regular magnetic field structures aligned with the Galactic plane.
• Model fitting successfully separated foreground and background polarization, enabling robust estimation of magnetic field strength and emissivity in the ISM.