tokens: the satellite at two specific times, and where its spin is indicated by a curled arrow. When cold rotationally supported disc galaxies merge they produce complex tidal tails ( Toomre Toomre 1972 ; Eneev et al. 1973 ; Quinn 1984 ; Barnes 1988 ). These tidal tails can be quite different from those originating in spherical dispersion-supported systems. This is also what we find in our simulations, as shown in Fig. 4 . The bottom row of this figure zooms in on the discy satellite and reveals that some of the stars in green (i.e. the ridge in the bottom panel of Fig. 3 ), are lost first. These are followed by some stars in cyan with lower final eccentricity and slightly less retrograde motions since they are part of the ridge in the middle panel of Fig. 3 . We also see that there is some alternation in green and cyan stars amongst the material that becomes unbound slightly later. Since a discy satellite has internal and orbital angular momentum, mass loss depends on the exact configuration of the merger (e.g. inclination, spin) and not only on the internal binding energy of the stars as in the case of a spheroidal system. For example, the presence of a single extended counter-tail can be explained by the alignment of the spins of the two galaxies ( Struck Smith 2012 ), i.e. although the secondary moves on a retrograde orbit, its sense of rotation is prograde with respect to the primary. A difference in eccentricity also arises from these additional degrees of freedom. For a given internal binding energy stars have a range of internal angular momenta, which translates in debris with high and low orbital angular momentum, resulting respectively in more circular and more radial final orbits (the latter being typical of shell-like features). In addition to these effects, dynamical friction causes the satellite’s core to spiral inwards. This enhances the prominence of the only tidal arm apparent in Fig. 4 , which trails behind the core. 3.2. Some implications We have just shown that the stars in the velocity arch constitute debris lost early on in the merger. This probably also has implications for the chemical abundances of its stars since dwarf galaxies are known to possess metallicity gradients. For M ⋆ ∼ 10 9.6 M ⊙ the gradients have an amplitude ∼ − 0.064 dex kpc −1 according to Ho et al. (2015) . Thus, if Gaia -Enceladus had a physical extent of 5 kpc, this could imply a metallicity difference of 0.32 dex between its centre and outskirts. Myeong et al. (2019) studied some of the stars in the velocity arch, and argued that these stars, together with other retrograde stars, were part of a different accreted galaxy that they named Sequoia. These authors have shown that the stars are more metal poor on average than the debris of Gaia -Enceladus by ∼0.30 dex, which suggests they formed in an object that is ten times smaller ( Matsuno et al. 2019 ). However, an alternative explanation supported by our simulations is that these stars (or some of them)
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