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

I. Insights from deep LOFAR-VLBI sub-arcsecond resolution images

¹ Dipartimento di Fisica e Astronomia, Università di Bologna, Via Gobetti 93/2, I-40129 Bologna, Italy
² INAF – Istituto di Radioastronomia di Bologna, Via Gobetti 101, I-40129 Bologna, Italy
³ Leiden Observatory, Leiden University, PO Box 9513, 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 Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA

^⋆ Corresponding author: emanuele.derubeis2@unibo.it

Received: 23 April 2025
Accepted: 16 May 2025

Abstract

Context. The high sensitivity of modern interferometers has revealed a plethora of filaments surrounding radio galaxies, especially in galaxy cluster environments. The morphology and spectral characteristics of these thin structures require the combination of high-resolution and low frequency observations, best obtained using LOw Frequency ARray (LOFAR) international stations.

Aims. In this paper, we aim to detect and characterize non-thermal filaments observed close or within the radio galaxies in Abell 2255 using deep LOFAR-VLBI observations at 144 MHz. These structures can be used to disentangle plausible scenarios describing the origin of the non-thermal filaments and connection to the motion of the host galaxy within the dense and turbulent intracluster medium (ICM), as well as the subsequent interactions between the ICM and radio jets.

Methods. Combining multiple observations, we produced the deepest images ever obtained with LOFAR-VLBI targeting a galaxy cluster, using 56 hours of observations, reaching a resolution of 0.3 − 0.5″. We detailed throughout the paper the calibration and imaging strategy for the different targets, as well as the multitude of morphological features discovered.

Results. Thanks to the high-sensitivity of LOFAR-VLBI, we revealed an unprecedented level of detail for the main cluster radio galaxies, recovering in most cases their more extended structure as well, which can only be observed at such low frequencies. In particular, we focused on the Original Tailed Radio Galaxy (Original TRG) where we distinguished many filaments constituting its tail with varying lengths (80 − 110 kpc) and widths (3 − 10 kpc). The final radio images showcase the potential of deep, high-resolution observations for galaxy clusters. With such approach, we enabled the study of hese thin, elongated radio filaments. Following their discovery, these filaments now require spectral studies to determine their formation mechanisms.