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

TL;DR

This study reveals that a cluster merger shock in Abell 1367 causes the longest known ram-pressure-stripped tails in star-forming galaxies, with evidence of cosmic ray re-acceleration and impact on galaxy evolution.

Contribution

It provides the first detailed spectral analysis of extended RPS tails influenced by a cluster shock, including the longest tail and evidence of particle re-acceleration.

Findings

UGC 6697 has a 300 kpc tail, the longest known for a star-forming galaxy.

Spectral analysis suggests cosmic ray re-acceleration due to ICM shock.

First tentative detection of particle acceleration at an infalling galaxy's leading edge.

Abstract

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. We investigate a complex of three galaxies UGC 6697, CGCG 097-073, and CGCG 097-079, that are currently undergoing extreme RPS, as evident from their multi-wavelength-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. We analyzed LOFAR and MeerKAT observations at frequencies of 54, 144, 817, and 1270 MHz to perform a detailed spectral analysis of the tails. We found that all three tails are significantly more extended than in previous radio studies, with lengths of $\geq70$ 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 construct a model of the tail that includes compression and re-acceleration 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 discover a radio plume at the leading edge of UGC 6697 that connects to a narrow filament. These sources exhibit extremely steep ($\alpha\approx-1.7$) and highly curved spectra. We speculate that this emission arises from cosmic rays re-energized by UGC 6697's rapid infall that propagate along magnetic filaments in the cluster center. 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.