Off-center dark matter halo leading to strong central disk lopsidedness

¹ Department of Physics, Indian Institute of Technology Bombay, Mumbai 400076, India
e-mail: prasadchatty@phys.iitb.ac.in
² Department of Physics, Indian Institute of Science, Bangalore 560012, India
e-mail: cjjog@physics.iisc.ernet.in

Received: 16 November 2016
Accepted: 6 January 2017

Abstract

There is increasing evidence from simulations and observations that the center of the dark matter halo in a Milky Way-type galaxy could be off-center by a few 100 pc with respect to the galactic disk. We study the effect of such an offset halo on the orbits and kinematics in the central few kpc of the disk via a simple, analytical model. The equations of motion in the disk plane can be written in terms of the potentials of the disk and halo when they are concentric, and a perturbation term arising due to the offset halo. We show that this perturbation potential has 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, ~ 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 a ~3 kpc radius. The disk lopsidedness would have important implications for the evolution of this region; for example, it could help fuel the central active galactic nucleus.