We computed the gas mass in a tail as the mass at radii larger than 30 kpc from the galaxy center. We chose to consider for the mass of the TDG progenitor all the matter included within a fixed aperture, rather than using a friend-to-friend algorithm that would better delineate the bound tidal object. Indeed this is how the luminosity and mass are usually measured in real observations. Thus, the real mass may have slightly been overestimated, as in the reality. The initial limit of 20 kpc was determined varying the aperture radius, which is also the method used for aperture photometry.

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One fourth on the mass of the condensations of gas measured at t= 470 Myr thus turned out not to belong to the gravitationally linked object.

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