# Jellyfish: Ram-pressure stripping as a diagnostic tool in studies of cluster collisions

## TL;DR

This study uses ram-pressure stripping phenomena in galaxy clusters, especially A1758N, to diagnose the collision geometry and understand galaxy evolution in dense environments, revealing a directional bias linked to merger dynamics.

## Contribution

It demonstrates that ram-pressure stripping can serve as a diagnostic tool for understanding the orientation and dynamics of cluster collisions.

## Key findings

- RPS candidates show debris trails and increased star formation.
- Most RPS candidates are moving toward us, indicating a directional bias.
- RPS events can help constrain the geometry of cluster mergers.

## Abstract

Prompted by the discovery of A1758N_JFG1, a spectacular case of ram-pressure stripping (RPS) in the galaxy cluster A1758N, we investigate the properties of other galaxies suspected to undergo RPS in this equal-mass, post-collision merger. Exploiting constraints derived from Hubble Space Telescope images and Keck longslit spectroscopy, our finding of apparent debris trails and dramatically enhanced star formation rates in an additional seven RPS candidates support the hypothesis that RPS, and hence rapid galaxy evolution in high-density environments, is intricately linked to cluster collisions. Unexpectedly, we find the vast majority of RPS candidates in A1758N to be moving toward us, and in a shared direction as projected on the plane of the sky. We hypothesize that this directional bias is the result of two successive events: (1) the quenching, during and after the first core passage, of star formation in galaxies with an approximately isotropic velocity distribution within the central region of the merger, and (2) RPS events triggered in late-type galaxies falling into the merging system along a filament, possibly enhanced by a shock front expanding into the outskirts of the south-eastern subcluster. Since this explanation implies that the merger axis of A1758N must be significantly inclined with respect to the plane of the sky, our findings open the possibility of RPS events becoming important diagnostic tools to constrain the geometry of cluster collisions that, due to the orientation of the merger axis, lack the classic observational signatures of face-on mergers.