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The formation of dark-matter-deficient galaxies through galaxy collisions

In the standard galaxy formation model, cold dark matter (CDM) drives the hierarchical structure formation. However, recently it is suggested that the satellite galaxy NGC1052-DF2, a member of the elliptical galaxy NGC1052 group, has a very little dark matter component compared to the theoretical predictions. 27 dark-matter-deficient galaxies have been discovered, but it is difficult to explain their formation process in the CDM model. We investigate the formation process through dark matter subhalo (DMSH) collisions using three-dimensional numerical simulations. As a result, we show the bifurcation sequence of the formation of dark-matter-dominated galaxies and dark-matter-deficient galaxies. In the case of low-speed collisions, a merger of the two DMSHs produces dark-matter-dominated galaxies. However, the two DMSHs penetrate each other, but the gaseous mediums collide for the middle-speed encounter. The enhancement of the gas density induces a burst of star formation, and then a dwarf galaxy without a dark matter halo forms at the interface of the DMSH collision.