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tokens: the IC file, which sets two haloes + star system simultaneously. The NGC 1052 system and the NGC 1052-DF2 system are always initially placed 400 kpc apart, with a fixed relative velocity of 250 km s –1 towards each other, which is consistent with the gravitational acceleration of free-falling from infinity and checked later to satisfy the velocity projection requirement. In order to control the NGC 1052 stellar distribution precisely to match the deprojection of the cored n = 3.4 Sérsic profile, we use a static baryonic potential kept at the centre of NGC 1052 instead of living particles. The NGC 1052 halo and NGC 1052-DF2 halo are simulated by 5.54 million and 0.06 million halo particles, respectively, and 0.1 million star particles simulate the luminous NGC 1052-DF2 profile. [Such initial configuration and simulation set-up is shared by the simulation for an alternative dark matter model; see the last paragraph. The physics for the pure CDM model is later tested by higher resolution simulation (Yang, Yu An 2020 ).] The gravitational softening length is 114 pc for halo particle and 14.4 pc for star particle. The bulk of the parameter scan is 2D, namely on the impact parameter b and the stellar Plummer radius r P (equivalent to its central density ρ c ). 3 RESULTS AND DISCUSSION In Fig. 3 , we plot the information extracted from the snapshots, which can be consistent with the observation after the encounter; here and later, d is the true 3D distance between the centres of the two galaxies. First, in the left-hand panel, we can see that with a scan of impact parameter at a spacing of 0.2 kpc, all simulations within this b range give small enclosed DM mass within the 7.6 kpc, which can be consistent with the observation, at the level of M / L 8. This is a consequence of the initial radial distribution, since before encounter the enclosed mass at the corresponding radius has a similar ratio. We can see that generally the smaller the impact parameter, the larger the stretch, and the smaller the enclosed mass in the 7.6 kpc. In the lower subpanel, we check the visual velocity relative to the central NGC 1052, and in a region, it can be consistent with the observed value. Figure 3. The combination of the two effects explains the observed lack of DM in NGC 1052-DF2. Left-hand panel: DM enclosed within the 7.6 kpc from several simulation snapshots after the encounter, in which we scan the impact parameter from 3.0 to 4.0 kpc. In the bottom subpanel, we also check the radial velocity relative to NGC 1052 and confirm that the observed 293 km s –1 can be reproduced within several snapshots. The two groups of curves are for NGC 1052-DF2 passing visually in front of or behind the central NGC 1052, respectively. They correspond to the two extremes in their final velocity angles with the line of sight, when considering the non-zero impact parameter and the bending of trajectory. Right-hand panel: The probability histogram by randomly selecting 10 star