One stream or two – exploring Andromeda’s North West stream

ABSTRACT We present results of our dynamical stream modelling for the North West Stream in the outer halo of the Andromeda galaxy (M31). Comprising two main segments, the North West Stream was thought to be a single structured arching around M31. However, recent evidence suggests that it is two separate, unrelated, streams. To test this hypothesis, we use observational data from six fields associated with the upper segment of the North West Stream together with eight fields and five globular clusters associated with the lower segment to constrain model orbits. We fit both segments of the stream using a fixed potential model for M31 and an orbit integrator to compare orbits with the observed streams. We measure the central tracks and predict proper motions for the upper segment (lower segment) finding ${\mu ^{*}_{\alpha }}$ = 0.078$^{+0.015}_{-0.012}$ (0.085$^{+0.001}_{-0.002}$) mas yr−1 and ${\mu _{\delta }}$ = $-0.05^{+0.008}_{-0.009}$ ($-0.095^{+0.003}_{-0.005}$) mas yr−1. Our results support the hypothesis that the dwarf spheroidal galaxy Andromeda XXVII is the progenitor of the upper segment of the North West Stream and that the upper and lower segments do not comprise a single structure. We propose that the upper segment, which appears to be on an infall trajectory with M31, be renamed the ‘Andromeda XXVII Stream’ and the lower segment, also apparently infalling towards M31, retain the name ‘North West Stream’.