Context. In Papers I and II of this series, we have found clear indications of the existence of two distinct populations of stars in the solar neighborhood belonging to the metal-rich end of the halo metallicity distribution function. Based on high-resolution, high S/N spectra, it is possible to distinguish between "high-alpha" and "low-alpha" components using the Results. The "high-alpha" halo stars have ages 2-3 Gyr larger than the "low-alpha" ones, with some probability that the thick-disk stars have ages intermediate between these two halo components. The orbital parameters show very distinct differences between the "high-alpha" and "low-alpha" halo stars. The "low-alpha" ones have r max 's to 30-40 kpc, z max 's to ≈18 kpc, and e max 's clumped at values greater than 0.85, while the "high-alpha" ones, r max 's to about 16 kpc, z max 's to 6-8 kpc, and e max values more or less uniformly distributed over 0.4-1.0. Conclusions. A dual in situ-plus-accretion formation scenario best explains the existence and characteristics of these two metal-rich halo populations, but one remaining defect is that this model is not consistent regarding the r max 's obtained for the in situ "high-alpha" component; the predicted values are too small. It appears that ω Cen may have contributed in a significant way to the existence of the "low-alpha" component; recent models, including dynamical friction and tidal stripping, have produced results consistent with the present mass and orbital characteristics of ω Cen, while at the same time including extremes in the orbital parameters as great as those of the "low-alpha" component.

With the aim of studying the nonlinear stellar and gaseous response to the gravitational potential of a galaxy such as the Milky Way, we have modeled 3D galactic spiral arms as a superposition of inhomogeneous oblate spheroids and added their contribution to an axisymmetric model of the Galactic mass distribution. Three spiral loci are proposed here, based in different sets of observations. A comparison of our model with a tight-winding approximation shows that the self-gravitation of the whole spiral pattern is important in the middle and outer galactic regions. A preliminary self-consistency analysis taking Ω p = 15 and 20 km s −1 kpc −1 for the angular speed of the spiral pattern, seems to favor the value Ω p = 20 km s −1 kpc −1 . As a first step to full 3D calculations the model is suitable for, we have explored the stellar orbital structure in the midplane of the Galaxy. We present the standard analysis in the pattern rotating frame, and complement this analysis with orbital information from the Galactic inertial frame. Prograde and retrograde orbits are defined unambiguously in the inertial frame, then labeled as such in the Poincaré diagrams of the non-inertial frame. In this manner we found a sharp separatrix between the two classes of orbits. Chaos is restricted to the prograde orbits, and its onset occurs for the higher spiral perturbation considered plausible in our Galaxy. An unrealistically high spiral perturbation tends to destroy the separatrix and make chaos pervasive. This may be relevant in other spiral galaxies.

We study the spiral arm influence on the solar neighbourhood stellar kinematics. As the nature of the Milky Way (MW) spiral arms is not completely determined, we study two models: the Tight-Winding Approximation (TWA) model, which represents a local approximation, and a model with self-consistent material arms named sPiral arms potEntial foRmed by obLAte Spheroids (PERLAS). This is a mass distribution with more abrupt gravitational forces. We perform test particle simulations after tuning the two models to the observational range for the MW spiral arm properties. We find that some of the currently observed MW spiral arm properties are not in obvious agreement with the TWA model. We explore the effects of the arm properties and find that a significant region of the allowed parameter space favours the appearance of kinematic groups. The velocity distribution is mostly sensitive to the relative spiral arm phase and pattern speed. In all cases the arms induce strong kinematic imprints for pattern speeds around 17 km s −1 kpc −1 (close to the 4:1 inner resonance) but no substructure is induced close to corotation. The groups change significantly if one moves only ∼0.6 kpc in galactocentric radius, but ∼2 kpc in azimuth. The appearance time of each group is different, ranging from 0 to more than 1 Gyr. Recent spiral arms can produce strong kinematic structures. The stellar response to the two potential models is significantly different near the Sun, both in density and in kinematics. The PERLAS model triggers more substructure for a larger range of pattern speed values. The kinematic groups can be used to reduce the current uncertainty about the MW spiral structure and to test whether this follows the TWA. However, groups such as the observed ones in the solar vicinity can be reproduced by different parameter combinations. Data from velocity distributions at larger distances are needed for a definitive constraint.

The effect of a barred potential (such as the one of the Milky Way) on the Galactic orbits of 48 globular clusters for which absolute proper motions are known is studied. The orbital characteristics are compared with those obtained for the case of an axisymmetric Galactic potential. Tidal radii are computed and discussed for both the better known axisymmetric case and that including a bar. The destruction rates due to bulge and disk shocking are calculated and compared in both Galactic potentials.

We study the phase space available to the local stellar distribution using a Galactic potential consistent with several recent observational constraints. We find that the induced phase space structure has several observable consequences. The spiral arm contribution to the kinematic structure in the solar neighborhood may be as important as the one produced by the Galactic bar. We suggest that some of the stellar kinematic groups in the solar neighborhood, like the Hercules structure and the kinematic branches, can be created by the dynamical resonances of self-gravitating spiral arms and not exclusively by the Galactic bar. A structure coincident with the Arcturus kinematic group is developed when a hot stellar disk population is considered, which introduces a new perspective on the interpretation of its extragalactic origin. A bar-related resonant mechanism can modify this kinematic structure. We show that particles in the dark matter disk-like structure predicted by recent LCDM galaxy formation experiments, with similar kinematics to the thick disk, are affected by the same resonances, developing phase space structures or dark kinematic groups that are independent of the Galaxy assembly history and substructure abundance. We discuss the possibility of using the stellar phase space groups as constraints to non-axisymmetric models of the Milky Way structure.