Comparing dynamical effects of the central bar and the spiral arms in the solar neighborhood

Abstract

The dynamical effects on the stellar motion produced by the Galactic central bar and the spiral arms perturbations are investigated separately and compared. The stars from the Gaia DR3 catalog are selected in the region of observable completeness, which we estimate as $\sim$1 kpc from the Sun. We apply the 2D model of the Galactic potential consisting of three axisymmetric components, the disk, the bulge, and the dark matter halo, and two non-axisymmetric components, the central bar and the spiral arms. The stellar dynamics is studied using analytical and numerical techniques, such as Hamiltonian topology analysis, the construction of dynamical maps on the representative planes, dynamic spectra, and Poincaré sections. We identify the main dynamical features in the solar neighborhood (SNd), the corotation (CR) and Lindblad resonances (LRs). By assuming that the main moving groups (MGs) in the SNd originate from the resonances, we compare their locations, structures, and intensities with the theoretical predictions and provide a description of the process involved in the formation of the MGs. In addition, we explore parametric planes by adjusting the values of the pattern rotation speed $Ω_{p}$ with the positions of the MGs, for both the spiral arms and bar models, and conclude that the spiral arms model shows better results when compared to those of the bar, under the hypothesis of the dynamical origin of MGs.

Figures (10)

• Figure 1: The sample used in this work, on the $R$--$L_z$ plane (top panel), and the Normalized Cumulative Distribution (NCD) of the stars from the sample as a function of the distance to the Sun (bottom panel). Top: The assumed limits of the sample's observable completeness are indicated by two vertical ($R$--range between 7.1 kpc and 9.1 kpc) and two horizontal ($L_z$--range between 1.6 kpc$^{2}$ Myr$^{-1}$ and 2.2 kpc$^{2}$ Myr$^{-1}$) black continuous lines. The blue and red dashed lines indicate the nominal locations of the main LRs, produced by spiral arms and the central bar, respectively (see in Sect. \ref{['sec:mapping-resonances']}). The continuous red curve shows the location of the rotation curve on the $R$--$L_z$ plane, while the star symbol is the location of the Sun on this plane. Bottom: The cumulative distribution is normalized by the maximum value, defined at $\sim 6$ kpc from the Sun. The solid black and red lines are two linear fits of the NCD; their intersection at $\sim 0.97$ kpc is chosen as a starting point for the decrease in the accretion rate of the NCD.
• Figure 2: Distribution of the stars on the $p_R$--$L_z$ plane (left panel) and the $R$--$L_z$ plane (right panel). Left: The main MGs calculated within $|R-R_\odot| < 200$ pc, are schematically indicated by black ellipses (I - Sirius, II - Coma Berenice, III - Hyades & Pleiades, IV - HMG I, V - HMG II). The color bar on the top of the panel shows, in logarithmic scale, the density of the stars calculated in bins 0.8 km s$^{-1}$$\times$0.005 kpc$^2$Myr$^{-1}$. Right: The DRs calculated in bins 0.012 kpc$\times$0.007 kpc$^2$ Myr$^{-1}$ show, in color scale, the distribution of the median of radial velocity $p_R$. The vertical dashed lines delimit the vicinity of the Sun inside $200$ pc. Finally, the horizontal arrows set up a correspondence between the diagonal ridges and the MGs, while the star symbols in both panels shows the position of the Sun in these planes for reference.
• Figure 3: Force functions of the Galactic components: the axisymmetric structure (red), the spiral arms (blue) and the central bar (black), calculated along the radial Galactocentric distance in the direction of the Sun ($\varphi=90$ deg). The dashed vertical line indicates the actual radial distance of the Sun.
• Figure 4: The topology of the Hamiltonian (\ref{['eq:Hamiltonian']}) on the $X$--$Y$ plane, and the co-rotation zones originated by the spiral arms (black curves) and the central bar perturbations (red curves). The stable and unstable centers (equilibrium solutions) are indicated by the dot and cross symbols, respectively. The black dashed lines intersecting denote the Sun's position, whereas the dotted lines show the orientation of the central bar, which is symbolically depicted as an ellipsoid. We also plot the azimuthal minima and maxima of spiral potential (light green line and orange dashed line, respectively), calculated through Eq.(\ref{['eq:PhiArms']}).
• Figure 5: The azimuthal velocity of the initially circular orbits affected by the spirals (top panel) and the bar (bottom panel) perturbations, as a function of the Galactocentric distance. The maximal/minimal values (black dots) of the velocity $V_\varphi$ were calculated over 10 Gyr, for $\varphi=90$ deg. Its averaged values (red dots) is superposed the observation rotation curve (solid gray line). The CR zones on both panels are indicated by the blue dots, while the locations of the main LRs are indicated by vertical dashed lines. The star symbol indicates the initial position of the Sun for reference, while the vertical black lines mark the $R$--interval of completeness as defined in Fig. \ref{['fig:RLz-distribution']}.