Asymmetric mass models of disk galaxies

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 M 99, 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 M 99, 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 M 99. 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.

Galactic rotation from Cepheids with Gaia DR2 and effects of non-axisymmetry

“…where F R (x, y) is the gravitational force in the radial direction toward the Galactic centre at the position of (x, y) in the Galactic plane. The local centrifugal speed can be different at the different azimuthal angles even at the same Galactocentric radius (Chemin et al 2015(Chemin et al , 2016Trick et al 2017).…”

“…Quillen 2003;Antoja et al 2009;Minchev & Famaey 2010). Trick et al (2017) study how the Galactic parameters they determine with an advanced, action-based Galactic disc dynamical model are affected by the spiral arms (see also Chemin et al 2016). They conclude that the dynamical models can be biased due to the influence of spiral arms, but if the sample of stars covers a large enough volume (with a radius of at least 3 kpc), the dynamical model can recover the global Galactic parameters.…”

Kinematic clues to the origins of starless HI clouds : dark galaxies or tidal debris?

“…• in Chemin et al (2016) and Makarov et al (2014), based on the stellar component; Gavazzi et al (2003) optical data gives 28…”

Implications of Geometry and the Theorem of Gauss on Newtonian Gravitational Systems and a Caveat Regarding Poisson’s Equation

“…The direct link of F normal with interior mass is used here to specify how density in Poisson's equation must be computed for compliance with Newton's law. Modern RC models do not actually use Poisson's equation, but instead have adopted a modified form proposed by [30], e.g., [17,55,56]:…”

“…Addition of densities in (11) implies that solutions sum. Use of linear superposition is specifically stated in RC literature [55]. However, superposition (summing) is only appropriate for a homogeneous equation, which Poisson's equation most definitely is not [57].…”