Confirmation of an Anomalously Low Dark Matter Content for the Galaxy NGC 1052-DF4 from Deep, High-resolution Continuum Spectroscopy

NGC 1052-DF4 was found to be the second "galaxy lacking dark matter" in the NGC 1052 group, based on its velocity dispersion of ${\sigma }_{\mathrm{gc}}={4.2}_{-2.2}^{+4.4}$ km s^-1 as measured from the radial velocities of seven of its globular clusters. Here we verify this result by measuring the stellar velocity dispersion of the galaxy. We observed the diffuse stellar light in NGC 1052-DF4 with the Keck Cosmic Web Imager in its highest-resolution mode, with ${\sigma }_{\mathrm{instr}}\approx 7$ km s^-1. With a total science + sky exposure time of 34 hr, the resulting spectrum is exceptional in both its spectral resolution and its signal-to-noise ratio of 23 Å^-1. We find a stellar velocity dispersion of ${\sigma }_{\mathrm{stars}}={8.0}_{-1.9}^{+2.3}$ km s^-1, consistent with the previous measurement from the globular clusters. Combining both measurements gives a fiducial dispersion of ${\sigma }_{{\rm{f}}}={6.3}_{-1.6}^{+2.5}$ km s^-1. The implied dynamical mass within the half-light radius is ${8}_{-4}^{+6}\times {10}^{7}{M}_{\odot }$ . The expected velocity dispersion of NGC 1052-DF4 from the stellar mass alone is 7 ± 1 km s^-1, and for a Navarro-Frenk-White (NFW) halo that follows the stellar mass-halo mass relation and the halo mass-concentration relation, the expectation is ~30 km s^-1. The low velocity dispersion rules out a normal NFW dark matter halo, and we confirm that NGC 1052-DF4 is one of at least two galaxies in the NGC 1052 group that have an anomalously low dark matter content. While any viable model for their formation should explain the properties of both galaxies, we note that NGC 1052-DF4 now poses the largest challenge, as it has the most stringent constraints on its dynamical mass.