DYNAMICS OF THE OUTER ARM HIGH-VELOCITY CLOUD AND THE TRIAXIALITY OF THE MILKY-WAY

Among high-velocity clouds of neutral hydrogen (HVCs) a so-called Outer Arm Cloud, often interpreted as a part of the general warp of the Milky Way disc, can be distinguished, which is located at 49-degrees less-than-or-equal-to l less-than-or-equal-to 161-degrees and 4-degrees less-than-or-equal-to b less-than-or-equal-to 31-degrees and has quite smooth velocity variation along its length. Closer inspection of the arm shows that its velocity field can be explained in two ways: (i) gas is moving in a circular orbit around the Galactic Centre, but the Sun has a galactocentric radial velocity component with respect to the Local Standard of Rest (LSR) of 28.5 km s-1 rather than -9.2 km s-1 as usually accepted; (ii) gas is moving in an elliptic orbit around the Galactic Centre. From these two possibilities only the second one seems to be acceptable, if we assume that the gravitational potential of our Galaxy is slightly triaxial. The best representation of the observations can be obtained in a triaxial model with an asymptotically flat (on the level equal to 200 km s-1) rotation curve, where the gas orbits have an axial ratio epsilon greater-than-or-equal-to 0.8, and the longest axis of the potential isosurfaces points towards l = 124-degrees. The distance of the gas from the Galactic Centre is r = 15.5 kpc and the galactic mass distribution core radius, r(c) = 13.3 kpc.