Off-centred dark matter halo leading to strong central disc lopsidedness

TL;DR

This paper demonstrates that a small off-centred dark matter halo in a galaxy can induce significant central disc lopsidedness and kinematic asymmetries, persisting over several billion years.

Contribution

It introduces a perturbation model showing how a minor halo offset causes strong central lopsidedness in galaxy discs, supported by orbital and kinematic analysis.

Findings

A 350 pc halo offset causes ~40% orbital deviation at 1.5 kpc.

The induced lopsidedness persists for several Gyr.

Small halo offsets can significantly impact galaxy central dynamics.

Abstract

There is increasing evidence now from simulations and observations that the centre of dark matter halo in a Milky Way type galaxy could be off-centred by a few 100 pc w.r.t. the galactic disc. We study the effect of such an offset halo on the orbits and kinematics in the central few kpc of the disc. The equations of motion in the disc plane can be written in terms of the disc and halo potentials when these two are concentric and a perturbation term due to the offset halo. This perturbation potential shows an m=1 azimuthal variation, or is lopsided, and its magnitude increases at small radii. On solving these equations, we find that the perturbed orbit shows a large deviation of ~ 40 % in radius at R = 1.5 kpc, and also strong kinematical lopsidedness. Thus even a small halo offset of 350 pc can induce surprisingly strong spatial and kinematical lopsidedness in the central region within ~ 3 kpc radius. Further, the disc would remain lopsided for several Gyr, as long as the halo offset lasts. This would have important implications for the dynamical evolution of this region.