Caught in the web: galaxy mergers along cosmic filaments

TL;DR

This study shows that galaxy mergers predominantly occur near cosmic filaments, especially outside cluster cores, indicating filaments facilitate pre-processing of galaxies before they enter clusters.

Contribution

It introduces a deep-learning approach to identify galaxy mergers and demonstrates their spatial correlation with cosmic web filaments in a large galaxy sample.

Findings

Galaxy mergers are closer to filaments than non-merging galaxies.

The trend of mergers near filaments is strongest beyond the cluster virial radius.

Filaments likely promote galaxy pre-processing through mergers.

Abstract

Galaxy clusters grow through the accretion of galaxies from groups, filaments, and other clusters. During this process, galaxies may undergo pre-processing in lower-density environments, where galaxy-galaxy mergers and other interactions can significantly alter their properties prior to cluster infall. We investigate the role of galaxy mergers in the pre-processing of galaxies prior to cluster infall by studying the spatial distribution of mergers across the cosmic web. We use a sample of 43,922 galaxies targeted by the 4MOST CHANCES survey in and around 33 low-redshift clusters (z < 0.07). Using Zoobot, a deep-learning framework trained on Galaxy Zoo data, we identify 698 galaxy mergers. We measure their distances to cosmic web filaments and compare them with those of non-merging galaxies. We find that galaxy mergers are significantly closer to filaments than the non-merging galaxy population, with this trend being strongest beyond the cluster virial radius. This suggests that filaments provide conditions conducive to mergers, possibly moderating relative velocities and enhancing gas availability. Our findings support a scenario in which filaments play a key role in transforming galaxies through pre-processing by promoting mergers before they enter cluster cores where star formation quenches.