We use the astrometric and photometric data from Gaia Data Release 2 and line-of-sight velocities from various other surveys to study the 3d structure and kinematics of the Sagittarius dwarf galaxy .
The combination of photometric and astrometric data makes it possible to obtain a very clean separation of Sgr member stars from the Milky Way foreground ; our final catalogue contains 2.6 \times 10 ^ { 5 } candidate members with magnitudes G < 18 , more than half of them being red clump stars .
We construct and analyze maps of the mean proper motion and its dispersion over the region \sim 30 \times 12 degrees , which show a number of interesting features .
The intrinsic 3d density distribution ( orientation , thickness ) is strongly constrained by kinematics ; we find that the remnant is a prolate structure with the major axis pointing at \sim 45 ^ { \circ } from the orbital velocity and extending up to \sim 5 kpc , where it transitions into the stream .
We perform a large suite of N -body simulations of a disrupting Sgr galaxy as it orbits the Milky Way over the past 2.5 Gyr , which are tailored to reproduce the observed properties of the remnant ( not the stream ) .
The richness of available constraints means that only a narrow range of parameters produce a final state consistent with observations .
The total mass of the remnant is \sim 4 \times 10 ^ { 8 } M _ { \odot } , of which roughly a quarter resides in stars .
The galaxy is significantly out of equilibrium , and even its central density is below the limit required to withstand tidal forces .
We conclude that the Sgr galaxy is terminally wounded , and will be fully disrupted over the next Gyr .