We present DESI observations of the stellar halo of M31 which reveal the kinematics of a recent merger in exquisite detail. Using data from less than four hours of observations by DESI survey we measure radial velocities of more than 7000 sources in M31. These observations show an intricate coherent kinematic structure in position and velocity space in M31 stellar halo. While hints of coherent structures have been previously detected in M31, this is the first time they have been seen with such detail and clarity in a galaxy beyond the Milky Way. We find clear kinematic evidence for multiple shell structures across M31. The kinematics of these structures are remarkably similar to the predictions of dynamical models of a single merger event that happened ~2 Gyr ago and enable us to constrain the gravitational potential of the M31. Using a model of just a small part of the merger debris observed by DESI we estimate the total mass of M31 within a projected radius of 125 kpc to be log10(M/Msun) = 11.78+/-0.1. This study opens a new era in our ability to map stellar halos and measure masses of nearby galaxies with high multiplex spectrographs.
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