Kinematic diagnostics for non-axisymmetry in the Milky Way's nuclear stellar disc

Abstract

There is now strong evidence that the Milky Way (MW) hosts a nuclear stellar disc (NSD). However, whether the NSD is purely axisymmetric or contains a nuclear bar remains unresolved. Since approximately $50\%$ of barred galaxies with MW-like mass in the local Universe host a nuclear bar, investigating whether the MW hosts one is of interest. We conduct a systematic analysis to identify robust kinematic diagnostics capable of determining whether the MW hosts a nuclear bar. Using N-body simulations, we explore the kinematic signatures indicative of a nuclear bar. Using the phase-space coordinates longitude $(\ell)$, latitude $(b)$, proper motions ($μ_\ell$ and $μ_{\rm b})$ and line-of-sight velocity $(v_{\rm los})$, we test various diagnostics assuming different nuclear bar orientations. We also evaluate how sample size, dust extinction and bar amplitude influence the efficacy of the diagnostics. We identify two independent kinematic diagnostics capable of revealing a nuclear bar in the MW: (1) the vertex deviation, $l_{\rm v}$, of the ($v_{\ell}-v_{\rm los}$) velocity ellipse; and (2) The asymmetry in the $μ_{\ell}$ vs $\ell$ distribution. While both are impacted by the sample size and extinction, the vertex deviation proves more robust, especially when combining stars from multiple observational fields. We also assess the correlation between the line-of-sight velocity and the $h_3$ Gauss-Hermite moment ("skewness") of the line-of-sight velocity but find no clear distinction between an NSD and a nuclear bar based on this metric. Our results suggest that data from the current KMOS survey may allow a marginal detection of a nuclear bar using the vertex deviation method. A companion paper provides further validation and detailed analysis of this approach. Nonetheless, future surveys will provide the high quality data necessary to fully exploit the diagnostics outlined in this study.

Figures (19)

• Figure 1: The face-on (bottom) and side-on (top) stellar density distribution of the models. Left to right: the axisymmetric NSD, isolated strong NSB, isolated weak NSB, and large-scale bar. The large-scale bar model is oriented at $-27^\circ$ relative to the Solar-GC line (dotted line). The nuclear bar angle, $\alpha$, is a free parameter. In all cases, the Solar position is $(x,y,z) = (-8.2,0,0.012)$ kpc.
• Figure 2: The surface density of the large-scale MW model with no nuclear component (left), the isolated NSD (middle), and the ratio between the two (right). The ratio illustrates how prominent the MW’s NSD would be if we were to see it from the Sun (see Fig. \ref{['fig:model_xy']}).
• Figure 3: Evolution of the NSD model. Left: the bar strength, measured by the Fourier $A_2$ parameter, as a function of time. $A_2$ is calculated considering all stars within the radius $R_{\rm max}$ that encloses $98\%$ of the model's mass. Right: the evolution of $R_{\rm max}$ with time. The initially stable NSD starts forming a bar at $t\approx0.5 \hbox{$\>{\rm Gyr}$}$, which reaches maximum strength at $t\approx1.7 \hbox{$\>{\rm Gyr}$}$. Black and blue dashed lines indicate the times at which the S-NSB and W-NSB models are extracted, respectively.
• Figure 4: Plots in Galactocentric Cartesian coordinates of the isolated NSD (left), strong NSB (middle) and weak NSB (right) models oriented at $-60^\circ$ relative to the Sun-GC line. Overlaid are the velocity ellipses coloured by the vertex deviation, $l_{\rm v}$. We overlay an ellipse in each bin with $N>25$ stars. The dashed lines reflect a $2^\circ$ field-of-view centred on the Solar position at $(x,y) = (-8.2, 0)\,\hbox{$\>{\rm kpc}$}$. The NSD shows radially oriented ellipses, while the S-NSB model ellipses show strong alignment with the bar's major axis. The alignment is weaker for the W-NSB model.
• Figure 5: Velocity ellipses for the models, with the major and minor axes shown in solid and dashed lines, respectively. We uniformly sample $N=2,400$ stars from the region $|\ell|<0.9^\circ$ and $|b|<0.25^\circ$. In each panel we denote the vertex deviation ($l_{\rm v}$), the anisotropy ($\beta$), the bar angle $\alpha$ (except for the NSD case), and the velocity dispersions in the line of sight and longitude directions. The top left panel shows the ellipse for the large-scale MW model with no nuclear component. As expected, the direction of highest dispersion is in the line of sight direction. The bottom left shows the ellipse for the axisymmetric NSD model. The other panels show the ellipses for the S-NSB model with various nuclear bar angles. In this case, the direction of highest dispersion depends on the nuclear bar angle.