CHANG-ES

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Abstract
Context. Galactic magnetic fields regulate star formation and cosmic-ray (CR) transport. Understanding their three-dimensional structure is key to constraining galactic CR transport. Aims. By stacking the linearly polarised signal from a sample of star-forming edge-on galaxies, we aim to analyse the average magnetic field structure of star-forming galaxies. Methods. Using synthetic data, we explored the validity of stacking Stokes Q and U spectra to infer the intrinsic polarisation characteristics of star-forming galaxies. Before stacking, we aligned, scaled, convolved, and reprojected C -band (6 GHz, λ = 0.05 m) Stokes Q and U cubes of 27 star-forming edge-on galaxies. For the stacked cubes, we performed an RM-synthesis and discussed the derived polarised intensity (PI), polarisation angle ( χ 0 ), and RM maps. Results. Synthetic data tests demonstrate that stacking Stokes Q and U spectra is valid for tightly constrained underlying distributions of PI, χ 0 , and RM. We identify the spread of the underlying RM distribution ( σ RM ) as a critical parameter and establish a threshold of σ RM thresh = 180 rad m –2 ( σ RM thresh λ 2 = 0.45 rad). For conditions that represent star-forming galaxies, stacking introduces a systematic uncertainty of δ RM sys = 90 rad m –2 and underestimates the recovered PI. Stacking results reveal an X-shaped pattern, and polarised emission is detected up to 7 kpc above the galactic disc. We find higher PI on the approaching side of galaxies and a decrease in PI in the galactic halo of ∼60% near the galaxy’s minor axis. For an X-shaped halo field, this aligns with the fitted opening angle of these galaxies and suggests that there is energy equipartition between CRs and the magnetic field rather than a uniform CR density. A global RM pattern, as reported in a previous study, cannot be confirmed. Conclusions. We present stacking of Stokes Q and U cubes as an effective tool to recover faint polarised emission in the halo of nearby galaxies, if the underlying distributions of PI, χ 0 , and RM are tightly constrained.
Funding Information
  • Deutsche Forschungsgemeinschaft (SFB1491)

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