Spatial density map of the star particles at the end of the merger simulation. The galaxy is rotated and projected onto the sky plane according to the inclination and PA of M31. The selected areas for the M31 disc, the GSS and NE and W shelves are indicated with different colours. Upper row: All simulated stellar particles [left], main progenitor stellar particles [middle], and secondary progenitor stellar particles [right] are shown in independent panels. Middle row: Same differentiation, but only for stars older than 3.5 Gyr. Lower row: Same differentiation, but only for stars younger than 3.5 Gyr.