  The cold dark matter (CDM) model predicts galaxies have 100 times more dark
matter mass than stars. Nevertheless, recent observations report the existence
of dark-matter-deficient galaxies with less dark matter than expected. To solve
this problem, we investigate the physical processes of galaxy formation in
head-on collisions between gas-containing dark matter subhaloes (DMSHs).
Analytical estimation of the collision frequency between DMSHs associated with
a massive host halo indicates that collisions frequently occur within 1/10th of
the virial radius of the host halo, with a collision timescale of about 10 Myr,
and the most frequent relative velocity increases with increasing radius. Using
analytical models and numerical simulations, we show the bifurcation channel of
the formation of dark-matter-dominated and dark-matter-deficient galaxies. In
the case of low-velocity collisions, a dark-matter-dominated galaxy is formed
by the merging of two DMSHs. In the case of moderate-velocity collisions, the
two DMSHs penetrate each other. However the gas medium collides, and star
formation begins as the gas density increases, forming a dwarf galaxy without
dark matter at the collision surface. In the case of high-velocity collisions,
shock-breakout occurs due to the shock waves generated at the collision surface
reaching the gas surface, and no galaxy forms. For example, the simulation
demonstrates that a pair of DMSHs with a mass of 10^9 Msun containing gas of
0.1 solar metallicity forms a dark-matter-deficient galaxy with a stellar mass
of 10^7 Msun for a relative velocity of 200 km/s.
