We present a dynamical measurement of the tangential motion of the Andromeda system , the ensemble consisting of the Andromeda Galaxy ( M31 ) and its satellites .
The system is modelled as a structure with cosmologically-motivated velocity dispersion and density profiles , and we show that our method works well when tested using the most massive substructures in high-resolution \Lambda Cold Dark Matter ( \Lambda CDM ) simulations .
Applied to the sample of 40 currently-known galaxies of this system , we find a value for the transverse velocity of 164.4 \pm 61.8 { km s ^ { -1 } } ( v _ { East } = -111.5 \pm 70.2 { km s ^ { -1 } } and v _ { North } = 99.4 \pm 60.0 { km s ^ { -1 } } ) , significantly higher than previous estimates of the proper motion of M31 itself .
This result has significant implications on estimates of the mass of the Local Group , as well as on its past and future history .