Drag-Gravity torques on galaxies in clusters: radial small-scale alignment effects

TL;DR

This paper investigates how gravitational and dynamical friction torques influence galaxy alignments within clusters, suggesting that massive galaxies' alignments can be explained by these mechanisms, especially in high-eccentricity clusters.

Contribution

It introduces a model for galaxy orientation dynamics considering gravity and dynamical friction, highlighting their role in alignment effects, particularly for massive galaxies in clusters.

Findings

Weak radial alignment can be caused by dynamical friction on massive galaxies.

In high-eccentricity clusters, massive galaxies align with the cluster's major axis.

Dynamical friction mainly influences the orientation of the most massive galaxies.

Abstract

We calculate the torque on galaxies in clusters due to gravity and to dynamical friction forces in order to study the possible origin of small-scale alignment effects as the result of interactions with their environment. The equation of motion for the position angle of a galaxy is derived by using a ple model. We find that weak radial alignment effects can be produced by this mechanism involving only the most massive galaxies. We also introduce a dependence on the cluster eccentricity to our equations in order to explore the alignment of galaxies with the cluster's major axis. We find that in the inner regions of high eccentricity clusters, alignments of massive galaxies with the cluster's major axis dominate over the radial ones. This mechanism could account for the observed alignment effects of the most massive galaxies with the major axis of their host cluster. Our results suggest that dynamical friction is a viable generator of alignment only for the most massive cluster galaxies. For the observed alignments of normal galaxies a primordial origin has to be explored.