Signatures of bulge triaxiality from kinematics in Baade's window

Abstract

We study a sample of 62 Baade's Window (1,-4) K giants that have published proper motions, radial velocity and metallicity. We construct the velocity ellipsoids for the metal rich stars and metal poor stars. We find a vertex deviation characteristic of a non-axisymmetric system. The long axes of the velocity ellipsoids of two samples point to two nearly perpendicular directions (lv,bv)=(-65+-9,+14+-9) and (25+-14,-11+-14) respectively. The vertex deviations of the velocity ellipsoids cannot be consistently explained by ANY oblate model. We populate orbits in a realistic bar potential with a Gaussian velocity distribution, allowing us to simulate and interpret observations. We conclude that the data are consistent with a triaxial bulge pointing towards (l,b) with l<0 and b=0 as suggested by earlier work. We also predict that low latitude abs(b)<4 deg bulge fields should show the vertex deviation more strongly and would therefore be the best locations for future proper motion studies. The metal rich and metal poor stars appear to occupy the direct boxy and retrograde loop orbits. The correlations between the metallicity and the orbit families can develop if the Bulge forms dissipatively on a sufficiently long time scale. However, it is difficult to reconcile the existence of such correlations with a violent history for the potential of the inner Galaxy.