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

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 $\sim 8$ Gyr ago. Using integral-field spectroscopy obtained with the Keck Cosmic Web Imager (KCWI), we measure a stellar velocity dispersion of $\sigma =15.8\pm 2.0$ km s${}^{-1}$ within the half-light radius ($R_{\mathrm{e}}$). We derive a dynamical mass within the 3D half-light radius of $M_{\mathrm{d}\mathrm{y}\mathrm{n}}(<r_{1\mathrm{/}2})=(1.5\pm 0.4)\times {10}^8{M}_{\odot }$, a factor of $\sim 3$ larger than the enclosed stellar mass, implying a DM fraction of $f_{\mathrm{D}\mathrm{M}}=0.7\pm 0.1$ at $r_{1\mathrm{/}2}$. Full-spectrum stellar population fitting yields an old ($t\approx 13$ Gyr) and metal-poor ($[\mathrm{M}\mathrm{/}\mathrm{H}]\approx -1.2$ 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.