N-body simulation insights into the X-shaped bulge of the Milky Way: Kinematics and distance to the Galactic Centre

Using simulations of box/peanut- (B/P-) shaped bulges, we explore the nature of the X-shape of the Milky Ways bulge. An X-shape can be associated with a B/P-shaped bulge driven by a bar. By comparing in detail the simulations and the observations we show that the principal kinematic imprint of the X-shape is a minimum in the difference between the near and far side mean line-of-sight velocity along the minor axis. This minimum occurs at around |b| = 4{deg}, which is close to the lower limit at which the X-shape can be detected. No coherent signature of an X-shape can be found in Galactocentric azimuthal velocities, vertical velocities, or any of the dispersions. After scaling our simulations, we find that a best fit to the BRAVA data leads to a bar angle of 15{deg}. We also explore a purely geometric method for determining the distance to the Galactic Centre by tracing the arms of the X-shape. We find that we are able to determine this ill-known distance to an accuracy of about 5% per cent with sufficiently accurate distance measurements for the red clump stars in the arms.

Stellar populations in the Milky Way bulge region : Towards solving the Galactic bulge and bar shapes using 2MASS data

We re-analyse photometric near-infrared data in order to investigate why it is so hard to get a consensus for the shape and density law of the bulge, as seen from the literature. To solve the problem we use the Besancon Galaxy Model to provide a scheme for parameter fitting of the structural characteristics of the bulge region. The fitting process allows the determination of the global shape of the bulge main structure. We explore various parameters and shape for the bulge/bar structure based on Ferrers ellipsoids and fit the shape of the inner disc in the same process. The results show that the main structure is a quite standard triaxial boxy bar/bulge with an orientation of about 13 degree with respect to the Sun-centre direction. But the fit is greatly improved when we add a second structure, which is a longer and thicker ellipsoid. We emphasize that our first ellipsoid represent the main boxy bar of the Galaxy, and that the thick bulge could be either a classical bulge slightly flattened by the effect of the bar potential, or a inner thick disc counterpart. We show that the double clump seen at intermediate latitudes can be reproduced by adding a slight flare to the bar. In order to better characterize the populations, we further simulate several fields which have been surveyed in spectroscopy and for which metallicity distribution function (MDF) are available. The model is in good agreement with these MDF along the minor axis if we assume that the main bar has a mean solar metallicity and the second thicker population has a lower metallicity. It then creates naturally a vertical metallicity gradient by the mixing of the two poulations. (abridged)