Self-consistent modelling of the Milky Way's Nuclear Stellar Disc

The Nuclear Stellar Disc (NSD) is a flattened high-density stellar structure that dominates the gravitational field of the Milky Way at Galactocentric radius $30\lesssim R\lesssim 300$ pc. We construct axisymmetric self-consistent equilibrium dynamical models of the NSD in which the distribution function is an analytic function of the action variables. We fit the models to the normalised kinematic distributions (line-of-sight velocities + VIRAC2 proper motions) of stars in the NSD survey of Fritz et al., taking the foreground contamination due to the Galactic Bar explicitly into account using an $N$-body model. The posterior marginalised probability distributions give a total mass of $M_{\mathrm{N}\mathrm{S}\mathrm{D}}=10.5_{-1.0}^{+1.1}\times {10}^8{\mathrm{M}}_{\odot }$, roughly exponential radial and vertical scale-lengths of $R_{\mathrm{d}\mathrm{i}\mathrm{s}\mathrm{c}}=88.6_{-6.9}^{+9.2}$ pc and $H_{\mathrm{d}\mathrm{i}\mathrm{s}\mathrm{c}}=28.4_{-5.5}^{+5.5}$ pc respectively, and a velocity dispersion $\sigma \simeq 70$ km/s that decreases with radius. We find that the assumption that the NSD is axisymmetric provides a good representation of the data. We quantify contamination from the Galactic Bar in the sample, which is substantial in most observed fields. Our models provide the full 6D (position+velocity) distribution function of the NSD, which can be used to generate predictions for future surveys. We make the models publicly available as part of the software package AGAMA.