Tidal Torquing of Elliptical Galaxies in Cluster Environments

TL;DR

This study investigates how tidal forces in galaxy clusters cause shape distortions and alignments in elliptical galaxies, explaining discrepancies between observations and dark matter simulations.

Contribution

It demonstrates that external tidal fields induce internal shape twisting in galaxies, reconciling observational anisotropy with simulation results.

Findings

Tidal forces cause significant internal shape twisting in galaxies.

Shape twisting depends on orbital phase and eccentricity.

Internal misalignments impact galaxy evolution and mass modeling.

Abstract

Observational studies of galaxy isophotal shapes have shown that galaxy orientations are anisotropic: a galaxy's long axis tends to be oriented toward the center of its host. This radial alignment is seen across a wide range of scales, from galaxies in massive clusters to small Milky Way type satellite systems. Recently, this effect has also been detected in dark matter simulations of cosmological structure, but the degree of alignment of dark matter substructures in these studies is significantly stronger than seen in observations. In this paper we attempt to reconcile these two results by performing high-resolution numerical experiments on N-body multi-component models of triaxial galaxies orbiting in an external analytical potential. The large number of particles employed allows us to probe deep into the inner structure of the galaxy: we show that the discrepancy between observed galaxies and simulated dark matter halos is a natural consequence of induced radial shape twisting in the galaxy by the external potential. The degree of twisting depends strongly on the orbital phase and eccentricity of the satellite, and it can, under certain conditions, be significant at radii smaller than the dark matter scale radius. Such internal misalignments will have important consequences, both for the dynamical evolution of the galaxy itself, and for mass modeling of galaxies in clustered environments.