Transverse gradients of azimuthal velocity in a global disc model of the Milky Way

Abstract

In this paper, we aim to estimate the vertical gradients in the rotational velocity of the Galaxy. This is carried out in the framework of a global thin disc model approximation. The predicted gradient values coincide with the observed vertical fall-off in the rotation curve of the Galaxy. The gradient is estimated based on a statistical analysis of trajectories of test bodies in the gravitational field of the disc and in an analytical way using a quasi-circular orbit approximation. The agreement of the results with the gradient measurements is remarkable in view of other more complicated, non-gravitational mechanisms used for explaining the observed gradient values. Finally, we find that models with a significant spheroidal component give worse vertical gradient estimates than the simple disc model. In view of these results, we can surmise that, apart from the central spherical bulge and Galactic halo, the gross mass distribution in the Galaxy forms a flattened rather than spheroidal figure.

The role of large-scale magnetic fields in galaxy NGC 891: can magnetic fields help to reduce the local mass-to-light ratio in the galactic outskirts?

“…In Jalocha et al (2010) we modelled the vertical gradient for the galaxy NGC 891 for the global disc model and quasi‐circular‐orbit approximation, assuming that the motion of gas above the disc plane is governed mainly by the gravitation of the column mass density distributed in the mid‐plane. Our gradient estimate was in agreement with the above measurements.…”

Constraining the vertical structure of the Milky Way rotation by microlensing in a finite-width global disk model

“…The expression for R ∂ R Ψ(R, Z) can be used to compute the rotation velocity v φ (R, Z) in the quasi-circular orbits approximation studied in Jałocha et al (2010). In this approximation, v 2 φ (R, Z) = R ∂ R Ψ, and the resulting vertical gradient of rotation is ∂ Z v φ (R, Z).…”

“…It is interesting to see the effect of the disk thickness on the gradient value prediction and to compare it with the analogous prediction in an infinitesimally thin global disk model (Jałocha et al 2010).…”

“…In our several previous studies, e.g., Jałocha et al (2010Jałocha et al ( , 2011, we modeled spiral galaxies in the approximation of an infinitesimally thin disk. In this framework, we easily obtained high values of the vertical gradient of rotation, in accord with gradient measurements.…”

Gravitational microlensing as a test of a finite-width disk model of the Galaxy

“…For these galaxies, a thin disk model approximation has proven to be quite satisfactory (Jałocha et al , 2010b. There was also a successful attempt at explaining high transverse gradients in the Galaxy rotation in the same disk model framework (Jałocha et al 2010c), whereas including a significant spheroidal mass component proved to reduce the predicted gradient to values that are inconsistent with measurements. There is also a recent result (Moni Bidin et al 2012) showing that the kinematics of the thick disk stars in the solar neighborhood is consistent with the visible mass alone, and there is no place for a spheroidal distribution of NDM, although the assumptions of this result are the subject of debate (Bovy & Tremaine 2012).…”