Context. Two main scenarios for the formation of the Galactic bulge are invoked, the first one through gravitational collapse or hierarchical merging of subclumps, the second through secular evolution of the Galactic disc. Aims. We aim to constrain the formation of the Galactic bulge through studies of the correlation between kinematics and metallicities in : the metal-rich population presents bar-like kinematics while the metal-poor population shows kinematics corresponding to an old spheroid or a thick disc. In this context the metallicity gradient along the bulge minor axis observed by Zoccali et al. (2008, A&A, 486, 177), visible also in the kinematics, can be related to a varying mix of these two populations as one moves away from the Galactic plane, alleviating the apparent contradiction between the kinematic evidence of a bar and the existence of a metallicity gradient. Conclusions. We show evidence that the two main scenarios for the bulge formation co-exist within the Milky Way bulge.

Abstract. The Galactic bar causes a characteristic splitting of the disc phase space into regular and chaotic orbit regions which is shown to play an important role in shaping the stellar velocity distribution in the Solar neighbourhood. A detailed orbital analysis within an analytical 2D rotating barred potential reveals that this splitting is mainly dictated by the value of the Hamiltonian H and the bar induced resonances. In the u − v velocity plane at fixed space position, the contours of constant H are circles centred on the local solid rotation velocity of the bar frame and of radius increasing with H. For reasonable bar strengths, the contour H = H12 corresponding to the effective potential at the Lagrangian points L 1/2 marks the average transition from regular to chaotic motion, with the majority of orbits being chaotic at H > H12. On top of this, the resonances generate an alternation of regular and chaotic orbit arcs opened towards lower angular momentum and asymmetric in u for space positions away from the principal axes of the bar. Test particle simulations of exponential discs in the same potential and a more realistic high-resolution 3D N-body simulation reveal how the decoupled evolution of the distribution function in the two kind of regions and the process of chaotic mixing lead to overdensities in the H > ∼ H12 chaotic part of the disc velocity distributions outside corotation. In particular, for realistic space positions of the Sun near or slightly beyond the outer Lindblad resonance and if u is defined positive towards the anti-centre, the eccentric quasi-periodic orbits trapped around the stable x1(1) orbits -i.e. the bar-aligned closed orbits which asymptotically become circular at larger distances -produce a broad u < ∼ 0 regular arc in velocity space extending within the H > H12 zone, whereas the corresponding u > ∼ 0 region appears as an overdensity of chaotic orbits forced to avoid that arc. This chaotic overdensity provides an original interpretation, distinct from the anti-bar elongated quasi-periodic orbit interpretation proposed by Dehnen (2000), for the prominent stream of high asymmetric drift and predominantly outward moving stars clearly emerging from the Hipparcos data. However, the most appropriate interpretation for this stream remains uncertain. The effects of spiral arms and of molecular clouds are also briefly discussed within this context.

We present new results from BRAVA, a large-scale radial velocity survey of the Galactic bulge, using M giant stars selected from the Two Micron All Sky Survey catalog as targets for the Cerro Tololo Inter-American Observatory 4m Hydra multi-object spectrograph. The purpose of this survey is to construct a new generation of self-consistent bar models that conform to these observations. We report the dynamics for fields at the edge of the Galactic bulge at latitudes b = −8 • and compare to the dynamics at b = −4 • . We find that the rotation curve V(r) is the same at b = −8 • as at b = −4 • . That is, the Galactic boxy bulge rotates cylindrically, as do boxy bulges of other galaxies. The summed line of sight velocity distribution at b = −8 • is Gaussian, and the binned longitude-velocity plot shows no evidence for either a (disk) population with cold dynamics or for a (classical bulge) population with hot dynamics. The observed kinematics are well modeled by an edge-on N-body bar, in agreement with published structural evidence. Our kinematic observations indicate that the Galactic bulge is a prototypical product of secular evolution in galaxy disks, in contrast with stellar population results that are most easily understood if major mergers were the dominant formation process.

We determine the values of parameters of an N-body model for the Galaxy developed by Fux via comparison with an unbiased, homogeneous sample of OH/IR stars. Via Monte-Carlo simulation, we find the plausibilities of the best-fitting models, as well as their errors. The parameters that are constrained best by these projected data are the total mass of the model and the viewing angle of the central Bar, although the distribution of the latter has multiple maxima. The best model has a viewing angle of 44 degrees, semi-major axis of 2.5 kpc, a bar mass of 1.7E10 solar masses and a tangential velocity of the local standard of rest of 171 km/s . We argue that the lower values that are commonly found from stellar data for the viewing angle (around 25 degrees) arise when too few coordinates are available, when the longitude range is too narrow or when low latitudes are excluded from the fit. The new constraints on the viewing angle of the galactic Bar from stellar line-of-sight velocities decrease further the ability of the Bar's distribution to account for the observed micro-lensing optical depth toward Baade's window : our model reproduces only half the observed value. The signal of triaxiality diminishes quickly with increasing latitude, fading within approximately one scaleheight. This suggests that Baade's window is not a very appropriate region to sample Bar properties.Comment: 10 pages, 8 figures, TeX, accepted for publication in MNRA