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.
Highlights
Rotation curves and surface density profiles of galactic disks are the observational pillars most models of extragalactic dynamics are based on
Rotation curves are needed to constrain the total mass distribution, the parameters of dark matter haloes, or the characteristics of modified Newtonian dynamics, while surface density profiles are helpful to constrain the structural parameters of disks and bulges, and generate the velocity contributions of luminous matter essential to mass models
The vR and vz components are usually omitted in kinematical studies because they are generally assumed to be negligible. It is the case for vR but since one of our goals is to study the impact of the radial motions on the mass modeling, we decided to fit them
Summary
Rotation curves and surface density profiles of galactic disks are the observational pillars most models of extragalactic dynamics are based on. Though attractive for its simplicity, this approach remains a reductive exploitation of velocity fields and multiwavelength images of stellar and gaseous disks, which are information rich. It prevents one from measuring the rotational support through perturbations (spiral arms, lopsidedness, etc.), which are obviously the most striking features of galactic disks. In an era of conflict between observations and expectations from cold dark matter (CDM) simulations, the cusp-core controversy (see the review of de Blok 2010, and references therein; but see Governato et al 2010), it appeared fundamental to assess the impact of such perturbations on the shape of rotation curves, and more generally on mass models and density profiles of dark matter
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