The Rogue One: A Dark Matter-Dominated Dwarf in the Trail of Dark Matter-Deficient Galaxies

Abstract We present the first measurement of the stellar velocity dispersion of LEDA 4014647, a compact dwarf galaxy spatially aligned with the NGC 1052 trail, a linear chain of dark matter-deficient galaxies (DMDGs) proposed to have formed in a single high-speed dwarf–dwarf collision ∼8 Gyr ago. Using integral-field spectroscopy obtained with the Keck Cosmic Web Imager (KCWI), we measure a stellar velocity dispersion of σ = 15.8 ± 2.0 km s−1 within the half-light radius (Re). We derive a dynamical mass within the 3D half-light radius of Mdyn( < r1/2) = (1.5 ± 0.4) × 108 M⊙, a factor of ∼3 larger than the enclosed stellar mass, implying a DM fraction of fDM = 0.7 ± 0.1 at r1/2. Full-spectrum stellar population fitting yields an old (t ≈ 13 Gyr) and metal-poor ([M/H] ≈ −1.2 dex) stellar population, consistent with independent photometric estimates and predating the proposed collision epoch. LEDA 4014647 is therefore a DM-dominated system, and cannot have formed from the DM-free, shock-compressed gas that produced the proposed trail DMDGs DF2, DF4, and DF9. Its nature, however, remains an open question: while a chance-projection along the trail axis is a possible explanation, its old stellar population, compact morphology, and DM content are also consistent with the surviving collisionless remnant of one of the progenitor dwarfs of the dwarf–dwarf collision. Distinguishing between these two scenarios will ultimately require deeper spectroscopy to tighten the stellar population and DM halo constraints, and targeted N-body and hydrodynamical simulations tracking the post-encounter fate of the progenitors.