Evidence of a bisymmetric spiral in the Milky Way

Charles Francis, Erik Anderson

Published 2012-02-07Version 1

Context: Because of our viewing point within the Galactic disc, it is extremely difficult to observe the spiral structure of the Milky Way. Aims: To clarify the structure of the Galaxy by re-examination of gas distributions and data from 2MASS; to determine stream memberships among local stars and to show the relationship between streaming motions and spiral structure. Methods: 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 is 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. Results: We find evidence in 2MASS of a bar of length 4.2 \pm 0.1 kpc at angle 30 \pm 10\degree. We extend spiral structure by more than a full turn toward the Galactic centre, and confirm that the Milky Way is a two-armed grand-design bisymmetric spiral with pitch angle 5.56 \pm 0.06\degree. Memberships of kinematic groups are assigned to 98% of local disc stars and it is seen that the large majority of local stars have orbits aligned with this spiral structure.