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 investigates how a cluster merger shock in Abell 1367 influences ram-pressure stripped star-forming galaxies, using deep LOFAR and MeerKAT data across 54, 144, 817, and 1270 MHz to analyze extremely long radio tails. The authors model tail spectra with aging, magnetic-field compression, and diffusive shock acceleration, revealing that pure aging cannot explain the observed extent or spectral trends; a shock-ICM interaction with magnetic-field amplification and reacceleration provides a plausible explanation, including a 300 kpc tail in UGC 6697 and non-monotonic spectral behavior in all three tails. They also report two ultra-steep radio filaments near NGC 3842 that may trace magnetized CR transport in the cluster center, and note a tentative detection of particle energization at a galaxy bow shock, underscoring direct ICM-shock influence on galaxy evolution and CR energetics. These results imply that cluster merger shocks can temporarily enhance ram-pressure stripping and seed CRs, with implications for galaxy evolution in dense environments and for interpreting faint, extended radio features in merging clusters; future high-resolution surveys with LOFAR 2.0, MeerKAT+, and SKA will test the prevalence and impact of such shock–galaxy interactions.

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 ($α\approx-1.7$) 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.

Figures (15)

• Figure 1: Top panel: Northwest of A1367 at 54 MHz, radio sources discussed in the text are highlighted. White and cyan regions were used to measure the total and disk flux densities, respectively. Bottom panel: Same region at 144 MHz, also displaying the XMM-Newton X-ray contours of Ge2019MergerShockAbell, contours are $[5,10,20,40]\,\mathrm{counts\,s^{-1}\,deg^{-2}}$. The purple region marks the detected ICM shock.
• Figure 2: UGC 6697 at $21"$ resolution after subtracting the radio relic model. Gray areas show masked sources. For display purposes, the images are rotated by 40$^\circ$.
• Figure 3: Radio spectrum of UGC 6697 (blue), C 079 (orange), and C 073 (green). The lines are log-parabolic fits to the data, where the solid lines were fit to the total flux density, and the dotted and dashed ones to the flux in the tail and disk, respectively. In red, we also show the filaments close to NGC 3842 discussed in \ref{['sec:filament']}.
• Figure 4: Regions used to measure the flux density along the tails, spaced at $21"$, identical to the resolution of the background map at 144 MHz. Beige regions are masked background sources.
• Figure 5: Flux density (top panel), SFR (top panel, right $y$ axis), spectral index (mid panel), and spectral curvature (bottom panel) as a function of projected distance along the UGC 6697 tail. The vertical purple line shows the location of the ICM shock.