Revealing the intricacies of radio galaxies and filaments in the merging galaxy cluster Abell 2255. II. Properties of filaments using multi-frequency radio data

TL;DR

The paper investigates non-thermal filaments in the merging cluster Abell 2255 by combining LOFAR-VLBI 144 MHz data with 1.26 and 1.52 GHz observations from uGMRT and VLA to map spectral shapes at ~2.3 kpc resolution. Through spectral-index mapping, radiative-aging calculations, color-color analyses, and RM synthesis, the authors reveal a complex, multi-component filamentary network intimately linked to the Original TRG, including very steep Trail features with $\alpha > 2$ and integrated filament spectra in the $1.1-1.7$ range. Polarization is detected along the tail and bright filaments up to ~22%, with RM indicating a magnetized intracluster medium; no single aging track can account for all components, implying magnetic-field inhomogeneities and multiple particle populations. The authors discuss formation scenarios where instabilities in the downstream wake of the Original TRG and turbulence-driven magnetic-field amplification play central roles, suggesting that the TRG acts as the main electron injector for cluster-scale filaments. These results advance understanding of particle acceleration, magnetic-field amplification, and filament dynamics in cluster environments and motivate future X-ray confirmations and deeper high-resolution radio studies.

Abstract

In this paper, we aim to further analyze the filaments in Abell 2255 combining LOFAR data with uGMRT (1260 MHz) and VLA (1520 MHz) data to constrain the spectral shape of the filaments. This allows studying their morphological properties, required to understand their origin, at unprecedentedly high resolution (~2.3 kpc), crucial to disentangle the different cosmic ray components that populate the Original TRG. We produced a LOFAR-VLBI map at 1.5" resolution using the wide-field technique with 56 hours of observations. This was the first time this technique was used for a galaxy cluster, especially for such deep observations. uGMRT and VLA data have been calibrated and imaged to produce spectral index maps and to apply further techniques to extract additional information, such as the radiative ages of the filaments or their equipartition magnetic field. Polarization information was also obtained using VLA through the rotation measure synthesis technique. Thanks to the LOFAR-VLBI wide-field image at 144 MHz, we revealed additional, very steep ($α> 2$) filaments beyond those attached to the radio galaxy, extending for around 250 kpc and previously known as the Trail. Combining LOFAR-VLBI with uGMRT and VLA, we found integrated spectral values between 1.1-1.7 for the filaments. Spectral analysis revealed also that the Original TRG has a complex structure, showing overlapping features with distinct spectral indices that extend throughout its tail. Polarized emission emerges only from the tail and the brightest part of the filaments, with values up to $22\%$. Although there is no clear scenario regarding the formation of filaments, we highlight the importance of the Original TRG as the main driver of such structures, even at larger distances from the core.

Figures (13)

• Figure 1: LOFAR-VLBI wide-field image of the central region (size $9.2^\prime \times 7.5^\prime$) of A2255 at 144 MHz (from Fig \ref{['fig:lofar_widefield']}). The final image has a resolution of $1.5^{\prime\prime}$, with rms noise of $38~\rm{\muup Jy~beam^{-1}}$. The restoring beam is shown in the bottom-left corner. Top panel: zoom on the Original TRG with the main features labeled as in Paper I. The contour levels from the sub-arcsecond resolution image from Paper I (Fig. 4 in that paper) are drawn in red at $3, 9, 27, 81, 243\sigma$ levels, with $\sigma=18~\rm{\muup Jy~beam^{-1}}$.
• Figure 2: Spectral index map using LOFAR-VLBI (144 MHz), uGMRT (1260 MHz), and VLA (1520 MHz) data for the Original TRG. Grey contours represent $3,~6,~12,~24\ldots\sigma_{\rm{rms}}$ levels, with $\sigma_{\rm{rms}} = 38~\rm{\muup Jy~beam^{-1}}$ being the noise from the LOFAR-VLBI, wide-field image at $1.5^{\prime\prime}$ resolution in Fig. \ref{['fig:lofar_widefield_zoom']}.
• Figure 3: Polarization results for the Original TRG at $2^{\prime\prime}$ resolution using the VLA. From left to right: polarized flux, fractional polarization, and rotation measure map. Contours in black are from the VLA map in total intensity used for the polarization analysis at $3, 6, 12, 24, 48, 96, 192~\sigma$ noise level, with $\sigma = 10~\rm{\muup Jy~beam^{-1}}$.
• Figure 4: Radio features observed from the LOFAR-VLBI image in Fig. \ref{['fig:lofar_widefield']}. Radio emission is shown in orange/yellow, superimposed on an optical image from the Panoramic Survey Telescope & Rapid Response System panstarrs2016. Cyan rhombi represent cluster member galaxies from yuan2003, shim2011, and tyler2014, which may have provided electrons to generate these features. Top: Comma, Counter-comma, and tethers. Center: the Ghost. Bottom: the Trail and T-bone.
• Figure 5: Surface brightness and spectral index trends for the Original TRG and the related filaments. The regions used to evaluate the average surface brightness and spectral index, with different color corresponding to different features, are shown in Fig. \ref{['fig:trends_regions']}, as well as the arrows that indicate the direction of the trends. These are the main tail (cyan), the horizontal (orange) and vertical (black) filaments, F1 (pink), F2 (green), and the Trail (red). Left: Average surface brightness trends along the features' extension at 144 MHz. Right: Average spectral index trends along the features' extension, considering only pixels above a $3\sigma_{\rm{rms}}$ threshold. The spectral index is not shown for the vertical filament and for the Trail given the lack of enough pixels above the chosen threshold.