Revealing the intricacies of radio galaxies and filaments in the merging galaxy cluster Abell 2255

II. Properties of filaments using multi-frequency radio data

¹ INAF – Istituto di Radioastronomia di Bologna Via Gobetti 101 40129 Bologna, Italy
² Dipartimento di Fisica e Astronomia, Università di Bologna Via Gobetti 93/2 I-40129 Bologna, Italy
³ Leiden Observatory, Leiden University PO Box 9513 NL-2300 RA Leiden, The Netherlands
⁴ ASTRON, The Netherlands Institute for Radio Astronomy Postbus 2 7990 AA Dwingeloo, The Netherlands
⁵ Minnesota Institute for Astrophysics, University of Minnesota 116 Church St. SE Minneapolis MN 55455, USA
⁶ Hamburger Sternwarte, Universität Hamburg Gojenbergsweg 112 21029 Hamburg, Germany
⁷ Centre for Extragalactic Astronomy, Department of Physics, Durham University South Road Durham DH1 3LE, UK
⁸ Institute for Computational Cosmology, Department of Physics, Durham University South Road Durham DH1 3LE, UK
⁹ Center for Astrophysics | Harvard & Smithsonian 60 Garden St. Cambridge MA 02138, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 10 October 2025
Accepted: 22 December 2025

Abstract

Context. Thin, elongated, non-thermal filaments in galaxy clusters and groups are nowadays ubiquitous in sensitive radio maps. Despite the large (and increasing) number of cases, their origin is still unclear. In a previous work, we revealed a wealth of filaments surrounding the main member radio galaxy of Abell 2255: a head-tail named “Original Tailed Radio Galaxy” (Original TRG). We did this using 56 hours of sub-arcsecond resolution LOFAR-VLBI observations at 144 MHz.

Aims. 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 enables the study of their morphological properties, which is required to understand their origin, at an unprecedentedly high resolution (∼2.3 kpc), crucial for disentangling the different cosmic ray components that populate the Original TRG.

Methods. 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.

Results. 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.