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The cold dark matter (CDM) model predicts galaxies have 100 times more darkmatter mass than stars. Nevertheless, recent observations report the existenceof dark-matter-deficient galaxies with less dark matter than expected. To solvethis problem, we investigate the physical processes of galaxy formation inhead-on collisions between gas-containing dark matter subhaloes (DMSHs).Analytical estimation of the collision frequency between DMSHs associated witha massive host halo indicates that collisions frequently occur within 1/10th ofthe virial radius of the host halo, with a collision timescale of about 10 Myr,and the most frequent relative velocity increases with increasing radius. Usinganalytical models and numerical simulations, we show the bifurcation channel ofthe formation of dark-matter-dominated and dark-matter-deficient galaxies. Inthe case of low-velocity collisions, a dark-matter-dominated galaxy is formedby the merging of two DMSHs. In the case of moderate-velocity collisions, thetwo DMSHs penetrate each other. However the gas medium collides, and starformation begins as the gas density increases, forming a dwarf galaxy withoutdark matter at the collision surface. In the case of high-velocity collisions,shock-breakout occurs due to the shock waves generated at the collision surfacereaching the gas surface, and no galaxy forms. For example, the simulationdemonstrates that a pair of DMSHs with a mass of 10^9 Msun containing gas of0.1 solar metallicity forms a dark-matter-deficient galaxy with a stellar massof 10^7 Msun for a relative velocity of 200 km/s.

Frequency of the dark matter subhalo collisions and bifurcation sequence arising formation of dwarf galaxies