Multiple Beads on a String: Dark-matter-deficient Galaxy Formation in a Mini-Bullet Satellite–Satellite Galaxy Collision

Dark-matter-deficient galaxies (DMDGs) discovered, in apparent conflict with LCDM, may be produced by high-velocity galaxy-galaxy collisions, the Mini-Bullet scenario. Recent observations of an aligned trail of ultra-diffuse galaxies (UDGs) near NGC 1052, including DMDGs DF2 and DF4, suggesting a common formation event, ∼9 Gyr ago, provide a test. Hydro/N-body simulations, supplemented by galaxy orbit integrations, demonstrate that satellite–satellite collisions outside the host-galaxy virial radius can reproduce the observed UDGs in the NGC 1052 group. A trail of ∼10 DMDGs forms, including 2 massive ones that replicate the observed motions of DF2 and DF4. The linear relation, v = Ax + v0, relating positions (x) and velocities (v) of the aligned DMDGs conjectured presiously as a signature of the collision event, is approximately obeyed, but individual DMDGs can deviate significantly from it. The collided progenitors of the DMDGs survive the collision without becoming DMDGs. We predict that one progenitor is located at the end of the trail, which can be tested by observing the difference between its stars, formed pre-collision, from those of the DMDGs, formed post-collision. By contrast, stellar ages and metallicities of the DMDGs are nearly identical. We further offer a hint that host tidal field may make DMDGs diffuse. LCDM simulation in a 100 cMpc box finds required initial conditions ∼10 times at z < 3. These results indicate current observations are consistent with the Mini-Bullet scenario.