The Sphericalization of Dark Matter Halos by Galaxy Disks

TL;DR

This study uses N-body simulations to show that galaxy disks can make dark matter halos more spherical, especially when the disk's gravitational influence is significant, impacting how we observe halo shapes.

Contribution

It provides a quantitative analysis of how galaxy disks influence the shape of dark matter halos, highlighting the conditions under which halos become more spherical.

Findings

Disks with less than 50% of circular velocity do not significantly alter halo shape.

Halo triaxiality may be observable in dwarf and low surface brightness galaxies.

The complex kinematics of NGC 2976 suggest a triaxial halo influence.

Abstract

Cosmological simulations indicate that cold dark matter (CDM) halos should be triaxial. Verifying observationally this theoretical prediction is, however, less than straightforward because the assembly of galaxies is expected to modify the halo shapes and to render them more axisymmetric. We use a suite of N-body simulations to investigate quantitatively the effect of the growth of a central disk galaxy on the shape of triaxial dark matter halos. As expected, the halo responds to the presence of the disk by becoming more spherical. The net effect depends only weakly on the orientation of the disk relative to the halo principal axes or the timescale of disk assembly, but strongly on the overall gravitational importance of the disk. Our results show that exponential disks whose contribution peaks at less than ~50% of their circular velocity are unable to modify noticeably the shape of the gravitational potential of their surrounding halos. Many dwarf and low surface brightness galaxies are expected to be in this regime, and therefore their detailed kinematics could be used to probe halo triaxiality, one of the basic predictions of the CDM paradigm. We argue that the complex disk kinematics of the dwarf galaxy NGC 2976 might be the reflection of a triaxial halo. Such signatures of halo triaxiality should be common in galaxies where the luminous component is subdominant.