Testing the correlation between bending angle and polarization properties of bent radio galaxies

TL;DR

This study probes whether the bending of radio galaxy jets in clusters correlates with polarization properties by analyzing 24 polarized bent radio galaxies with VLA L-band data. Using RM synthesis and a Burn–Sokoloff external Faraday-screen model, the authors measure RM, RM dispersion, and intrinsic polarization, applying redshift corrections and radiative-transfer modeling to derive per-source polarization properties. They find no statistically significant correlations between bending angle and polarization parameters, though there is a notable tendency for magnetic-field vectors to align with the bending direction, suggesting coherent field structures persist through jet bending. The results support bent radio galaxies as reliable probes of intracluster magnetic fields at current resolutions, and projections for POSSUM and SKA indicate orders-of-magnitude larger samples suitable for more robust RM-grid studies of cluster magnetism.

Abstract

The bending of radio galaxies in galaxy clusters is expected to be caused by interactions with the local environment. The physical processes responsible for jet bending, and their influence on the polarization properties of radio galaxies, remain poorly understood, leading to the question of whether jet properties in bent radio galaxies differ from those in linear radio galaxies. Using a sample of 24 polarized bent radio galaxies, observed with the Karl G. Jansky Very Large Array at 1--2 GHz, we test for correlation of bending angle with polarization parameters measuring Faraday rotation, intrinsic fractional polarization, and Faraday rotation dispersion, used here as a measure of turbulence along the line of sight. We find no statistically significant correlations. At the spatial resolution of our dataset (3--46 kpc, median 18.4 kpc), our results indicate that we are primarily probing larger-scale intracluster medium effects not related to bending angle. The absence of a statistically significant correlation suggests that bent radio galaxies are reliable probes of intracluster magnetic fields, because their intrinsic properties do not appear to introduce systematic biases into measured polarization parameters. We do detect a preference for source magnetic field vectors to align with the direction of jet bending. Finally, we estimate that the POSSUM and SKA surveys will contain $\gtrsim$300 and $\gtrsim$1000 polarized radio galaxies, respectively, providing large future samples with a range of bending angles and similar redshift distribution and number of beams per source as in our sample, enabling our results to be tested with greater statistical power.

Figures (14)

• Figure 1: Distributions of bending angles for our sample of 48 radio galaxies. The distribution of bending angles for the BCG galaxies are overlaid in green.
• Figure 2: Cutouts of the Stokes I MFS images of the first 24 radio galaxies in our sample, ordered by increasing bending angle. The cutouts have been rotated such that the bisector is pointing approximately toward the top of the page. The grayscale maps total intensity, and the red outlines indicate the 10$\sigma$ contour lines in the image. The black arrow indicates the direction of increasing declination (north), and the gray arrow points east. The position of the host galaxy is indicated by a cyan circle. The black circles in the top right of the cutouts show the synthesized beam area and the black line at the top of the cutouts span 30 arcsec. The bending angles are noted in the top left corners, with the source integration method ("SL", "BL", or "FS") indicated below (see text in Section \ref{['sec:data red']} for a description of these integration methods). Sources with bending angle 180$^{\circ}$ are in brackets to indicate that these are head-tail sources for which we are not able to resolve the individual jet/lobe structures.
• Figure 3: continued. The remaining 24 radio galaxies in our sample.
• Figure 4: Example 2D maps of polarized intensity (top) and RM (bottom) for source J$004150-092547$, derived from 3D RM synthesis. The 10$\sigma$ contours from the Stokes I MFS image is shown in red, and the position of the host galaxy is indicated by the green circle. Masked pixels are colored light yellow for clarity.
• Figure 5: Model parameters versus bending angle for the full-source integrated spectra (top row), the individual lobe-integrated spectra (middle row), and the lobe ratios (bottom row). In each panel, the Kendall's tau correlation coefficient, $\tau$, and corresponding P-value for the data is given in the top left corner. We use a modified P-value of $3.1\times10^{-5}$ as our threshold for correlation.