Complete census of massive slow rotators in ten large galaxy clusters

TL;DR

This study provides the first comprehensive mapping of massive slow rotators in ten large galaxy clusters, confirming they are rare relics of violent hierarchical formation, primarily located at cluster density peaks.

Contribution

It offers the first complete census of massive slow rotators in multiple clusters, linking their presence to dense cluster environments and hierarchical formation processes.

Findings

Slow rotators are extremely rare in the studied clusters.

They predominantly trace the density peaks of clusters.

Results confirm slow rotators as relics of violent hierarchical formation.

Abstract

Galaxy interactions leave imprints in the motions of their stars, and so observing the two-dimensional stellar kinematics allows us to uncover their formation process. Slow rotators, which have stellar orbits dominated by random motions, are thought to be the fossil relics of a sequence of multiple gas-poor mergers, in an environment where the cold gas required to form new stars is nearly absent. Indeed, observations of a handful of nearby galaxy clusters have indicated that slow rotators are preferentially found in the gas-poor, dense cores of clusters, which themselves must form by merging of smaller groups. However, the generality of this result and connection between kinematics and environment is currently unclear, as recent studies have suggested that, at given stellar mass, the environment does not influence the formation of slow rotators. Here we address this issue by combining a careful quality-assessed sample selection with two-dimensional stellar kinematics from a large galaxy survey and a novel photometric classification approach where kinematics are unavailable. We obtain the first complete census of the location of massive slow rotators in ten large clusters: in all cases, slow rotators are extremely rare and generally trace the clusters density peaks. This result unambiguously establishes that massive slow rotators are the relics of violent hierarchical cluster formation.