A Dwarf Disrupting -- Andromeda XXVII and the North West Stream

Abstract

We 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 $\sim$3$^{\circ}$ 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 $\pm$ 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} 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}. In the upper segment of the North West Stream we measure mean values for the metallicity = -1.8$\pm$0.4, systemic velocity = -519.4 $\pm$4.0{\kms} and velocity dispersion = 10.0$\pm$4.0{\kms}. We also detect a velocity gradient of 1.7$\pm$0.3 {\kms} kpc$^{-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.