The longest known tails of ram-pressure-stripped star-forming galaxies are caused by an intracluster medium shock in Abell 1367

¹ ASTRON, Netherlands Institute for Radio Astronomy Oude Hoogeveensedijk 4 7991 PD Dwingeloo, The Netherlands
² Hamburger Sternwarte, University of Hamburg Gojenbergsweg 112 D-21029 Hamburg, Germany
³ Thüringer Landessternwarte Sternwarte 5 D-07778 Tautenburg, Germany
⁴ Max-Planck-Institut für Radioastronomie Auf dem Hügel 69 53121 Bonn, Germany
⁵ Center for Astrophysics, Harvard & Smithsonian 60 Garden Street Cambridge MA 02138, USA
⁶ Department of Physics & Astronomy, University of Alabama in Huntsville 301 Sparkman Dr NW Huntsville AL 35899, USA
⁷ INAF – Istituto di Radioastronomia Via P. Gobetti 101 Bologna, Italy
⁸ INAF – Astronomical Observatory of Padova Vicolo dell’Osservatorio 5 IT-35122 Padova, Italy
⁹ Waterloo Centre for Astrophysics, University of Waterloo 200 University Ave W Waterloo ON N2L 3G1, Canada
¹⁰ Department of Physics & Astronomy, University of Waterloo Waterloo ON N2L 3G1, Canada
¹¹ Leiden Observatory, Leiden University PO Box 9513 2300 RA Leiden, The Netherlands

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

Received: 6 November 2025
Accepted: 16 December 2025

Abstract

Context. The environment plays an important role in shaping the evolution of cluster galaxies through mechanisms such as ram pressure stripping (RPS), whose effect may be enhanced in merging clusters.

Aims. We investigate a complex of three galaxies – UGC 6697, CGCG 097−073, and CGCG 097−079 – that are currently undergoing extreme RPS, as is evident from their multiwavelength-detected tails. The galaxies are members of the nearby (d = 92 Mpc) merging cluster Abell 1367 and are located in proximity to an intracluster medium (ICM) shock that is traced by X-ray observations and the presence of a radio relic.

Methods. We analyzed LOFAR and MeerKAT observations at frequencies of 54, 144, 817, and 1270 MHz to perform a detailed spectral analysis of the tails.

Results. We found that all three tails are significantly more extended than in previous radio studies, with lengths of ≥70 kpc. For UGC 6697, we detected a tail of 300 kpc, making it the longest known RPS tail of a star-forming galaxy at any wavelength. The length and spectral variations of the tail cannot be explained purely by the spectral aging of stripped cosmic rays. We constructed a model of the tail that includes compression and reacceleration due to the encounter with the nearby ICM shock, which can plausibly account for the extreme RPS as well as the length and spectral variation of the tail. We further discovered a radio plume at the leading edge of UGC 6697 that connects to a narrow filament. These sources exhibit extremely steep (α ≈ −1.7) and highly curved spectra. We speculate that this emission arises from cosmic rays reenergized by UGC 6697’s rapid infall that propagate along magnetic filaments in the cluster center.

Conclusions. Our findings represent direct evidence of a cluster merger shock impacting the evolution of member galaxies. Furthermore, we report the first tentative detection of particle acceleration at the leading edge of an infalling galaxy.