A dwarf disrupting – Andromeda XXVII and the North West Stream

International audienceWe present a kinematic and spectroscopic analysis of 38 red giant branch stars, in 7 fields, spanning the dwarf spheroidal galaxy Andromeda XXVII and the upper segment of the North West Stream. Both features are located in the outer halo of the Andromeda galaxy at a projected radius of 50-80 kpc, with the stream extending for ∼3 • on the sky. Our data is obtained as part of the PAndAS survey and enables us to confirm that Andromeda XXVII's heliocentric distance is 827 ± 47 kpc and spectroscopic metallicity is-2.1 +0.4 −0.5. We also re-derive Andromeda XXVII's kinematic properties, measuring a systemic velocity =-526.1 +10.0 −11.0 kms −1 and a velocity dispersion that we find to be non-Gaussian but for which we derive a formal value of 27.0 +2.2 −3.9 kms −1. In the upper segment of the North West Stream we measure mean values for the metallicity =-1.8±0.4, systemic velocity =-519.4 ±4.0 kms −1 and velocity dispersion = 10.0±4.0 kms −1. We also detect a velocity gradient of 1.7±0.3 kms −1 kpc −1 on an infall trajectory towards M31. With a similar gradient, acting in the same direction, in the lower segment we suggest that the North West Stream is not a single structure. As the properties of the upper segment of the North West Stream and Andromeda XXVII are consistent within 90% confidence limits, it is likely that the two are related and plausible that Andromeda XXVII is the progenitor of this stream