[Paper Review] A Sino-German $λ$6\ cm polarization survey of the Galactic plane VI. Discovery of supernova remnants G178.2-4.2 and G25.1-2.3

X. Y. Gao, Xiaohui Sun+4 more|Jul 4, 2011

Astrophysics and Cosmic Phenomena36 references15 citations

TL;DR

This study presents the discovery of two new, large, faint supernova remnants (SNRs) — G178.2−4.2 and G25.1−2.3 — using high-sensitivity λ6 cm polarization data from the Sino-German Galactic plane survey. The identification was confirmed via non-thermal radio spectra, shell-like morphology, and ordered magnetic fields in multi-wavelength radio observations, establishing them as genuine SNRs despite their low surface brightness.

ABSTRACT

Supernova remnants (SNRs) were often discovered in radio surveys of the Galactic plane. Because of the surface-brightness limit of previous surveys, more faint or confused SNRs await discovery. The Sino-German $λ$6\ cm Galactic plane survey is a sensitive survey with the potential to detect new low surface-brightness SNRs. We want to identify new SNRs from the $λ$6\ cm survey map of the Galactic plane. We searched for new shell-like objects in the $λ$6\ cm survey maps, and studied their radio emission, polarization, and spectra using the $λ$6\ cm maps together with the $λ$11\ cm and $λ$21\ cm Effelsberg observations. Extended polarized objects with non-thermal spectra were identified as SNRs. We have discovered two new, large, faint SNRs, G178.2-4.2 and G25.1-2.3, both of which show shell structure. G178.2-4.2 has a size of 72 arcmin x 62 arcmin with strongly polarized emission being detected along its northern shell. The spectrum of G178.2-4.2 is non-thermal, with an integrated spectral index of $α= -0.48\pm0.13$. Its surface brightness is $Σ_{1 GHz} = 7.2 x 10^{-23}{Wm^{-2} Hz^{-1} sr^{-1}}$, which makes G178.2-4.2 the second faintest known Galactic SNR. G25.1-2.3 is revealed by its strong southern shell which has a size of 80 arcmin x 30\arcmin. It has a non-thermal radio spectrum with a spectral index of $α= -0.49\pm0.13$. Two new large shell-type SNRs have been detected at $λ$6\ cm in an area of 2200 deg^2 along the the Galactic plane. This demonstrates that more large and faint SNRs exist, but are very difficult to detect.

Motivation & Objective

• To identify previously undetected, low-surface-brightness supernova remnants (SNRs) in the Galactic plane using high-sensitivity radio surveys.
• To overcome limitations of previous surveys, which often miss faint or confused SNRs due to surface-brightness and resolution constraints.
• To validate the SNR nature of newly detected shell-like radio objects through multi-wavelength radio data, including polarization and spectral analysis.
• To assess the detectability of faint SNRs and estimate the potential number of undiscovered remnants in the Galaxy.
• To investigate associations with H I cavities and infrared emission to constrain distance and physical properties.

Proposed method

• Conducted a deep λ6 cm polarization survey of the Galactic plane (10° ≤ ℓ ≤ 230°, |b| ≤ 5°) with 9.5′ angular resolution and sensitivity of ~4.9×10⁻²³ W m⁻² Hz⁻¹ sr⁻¹ at 1 GHz.
• Combined λ6 cm data with newly obtained λ11 cm and archival λ21 cm continuum and polarization data from the Effelsberg 100-m telescope.
• Identified shell-like radio structures via total intensity and polarization maps, focusing on regions with ordered magnetic fields and non-thermal spectra.
• Applied spectral index analysis using the flux density relation Sν ∝ ν^α to determine the spectral behavior of extended sources.
• Used TT-plot spectral analysis to derive integrated spectral indices and assess non-thermal emission characteristics.
• Searched for associations with H I cavities (from LAB survey), infrared emission (from IRIS 60 μm), and pulsars to infer distance and physical context.

Experimental results

Research questions

• RQ1 Can faint, large supernova remnants with low surface brightness be detected in high-sensitivity λ6 cm polarization surveys?
• RQ2 Do newly discovered shell-like radio objects exhibit non-thermal spectra and ordered polarization, indicating a supernova remnant origin?
• RQ3 What is the distance and physical size of the newly discovered SNRs, and how do they compare with known SNR scaling relations?
• RQ4 Are there associated H I cavities or infrared counterparts that support the SNR nature and help constrain distance?
• RQ5 How do the spectral indices and polarization properties of these remnants compare with those of known Galactic SNRs?

Key findings

• Two new shell-type supernova remnants, G178.2−4.2 and G25.1−2.3, were discovered in the λ6 cm survey, both showing non-thermal radio spectra with spectral indices α = −0.48 ± 0.13 and α = −0.49 ± 0.13, respectively.
• G178.2−4.2 has a surface brightness of 7.2×10⁻²³ W m⁻² Hz⁻¹ sr⁻¹ at 1 GHz, making it the second faintest known Galactic SNR, with a size of 72′ × 62′ and strong polarization along its northern shell.
• G25.1−2.3 exhibits a large southern shell of 80′ × 30′, with a non-thermal spectrum and a spectral index consistent with typical SNRs, and shows a potential H I cavity at 39.2 km/s with a kinematic distance of ~2.9 kpc.
• No significant infrared counterparts were found for either SNR in 60 μm IRIS images, suggesting no strong dust emission or association with dense clouds.
• The distance estimate for G25.1−2.3 (3.1 kpc) derived from the Σ−D relation is consistent with the H I cavity distance, supporting its SNR nature and physical size of ~72 pc.
• The discovery demonstrates that many large, faint SNRs remain undetected due to sensitivity and confusion limits, implying a potentially high total number of undetected Galactic SNRs.