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Evidence of a bisymmetric spiral in the Milky Way




It is extremely difficult to observe the spiral structure of the Milky Way because of our viewing point within the Galactic disc. The aims of this paper are to clarify the structure of the Galaxy by re-examination of gas distributions and data from the Two Micron All Sky Survey (2MASS), to determine stream memberships among local stars and to show the relationship between streaming motions and spiral structure. To achieve these aims, we extend the spiral pattern found from neutral gas towards the Galactic Centre using data from 2MASS. We select a population of 23 075 local disc stars for which complete kinematic data are available. We plot eccentricity against the true anomaly for stellar orbits and identify streams as dense regions of the plot. We reconstruct the spiral pattern by replacing each star at a random position of the inward part of its orbit.

As a result of this study, we find evidence in 2MASS of a bar of length 4.2 ± 0.1 kpc at angle 30°± 10°. We extend spiral structure by more than a full turn towards the Galactic Centre, and confirm that the Milky Way is a two-armed grand-design bisymmetric spiral with pitch angle 5bsl0006456 ± 0bsl0006406. Memberships of kinematic groups are assigned to 98 per cent of local disc stars and it is seen that the large majority of local stars have orbits aligned with this spiral structure.