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 $σ= 15.8 \pm 2.0$ km s$^{-1}$ within the half-light radius ($R_\mathrm{e}$). We derive a dynamical mass within the 3D half-light radius of $M_\mathrm{dyn}(<r_{1/2}) = (1.5 \pm 0.4) \times 10^8\,M_\odot$, a factor of $\sim 3$ larger than the enclosed stellar mass, implying a DM fraction of $f_\mathrm{DM} = 0.7 \pm 0.1$ at $r_{1/2}$. Full-spectrum stellar population fitting yields an old ($t \approx 13$ Gyr) and metal-poor ($[\mathrm{M/H}] \approx -1.2$ 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.

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Authors

Daniel Vaz; Arsen Levitskiy; Duncan Forbes; Aaron J. Romanowsky; Jonah Gannon; Jean P. Brodie; Yimeng Tang; Kenji Bekki; Warrick J. Couch

Published

Oct 5, 2026

Paper type

preprint

Source

Arxiv