Asymmetric mass models of disk galaxies - I. Messier 99

TL;DR

This paper introduces a new method to model asymmetric mass distributions in disk galaxies, revealing significant nonuniformities in circular velocities and implications for dark matter profiles, applied specifically to Messier 99.

Contribution

It develops a methodology to derive 3D gravitational potentials accounting for asymmetries, improving mass modeling of disk galaxies beyond axisymmetric assumptions.

Findings

Circular velocities in M99 are highly nonuniform, especially in the inner disk.

A cuspy dark matter density profile is consistent with the data.

A shifted, core-dominated halo better explains the observed kinematical lopsidedness.

Abstract

Mass models of galactic disks traditionally rely on axisymmetric density and rotation curves, paradoxically acting as if their most remarkable asymmetric features, such as lopsidedness or spiral arms, were not important. In this article, we relax the axisymmetry approximation and introduce a methodology that derives 3D gravitational potentials of disk-like objects and robustly estimates the impacts of asymmetries on circular velocities in the disk midplane. Mass distribution models can then be directly fitted to asymmetric line-of-sight velocity fields. Applied to the grand-design spiral M99, the new strategy shows that circular velocities are highly nonuniform, particularly in the inner disk of the galaxy, as a natural response to the perturbed gravitational potential of luminous matter. A cuspy inner density profile of dark matter is found in M99, in the usual case where luminous and dark matter share the same center. The impact of the velocity nonuniformity is to make the inner profile less steep, although the density remains cuspy. On another hand, a model where the halo is core dominated and shifted by 2.2-2.5 kpc from the luminous mass center is more appropriate to explain most of the kinematical lopsidedness evidenced in the velocity field of M99. However, the gravitational potential of luminous baryons is not asymmetric enough to explain the kinematical lopsidedness of the innermost regions, irrespective of the density shape of dark matter. This discrepancy points out the necessity of an additional dynamical process in these regions: possibly a lopsided distribution of dark matter.