Still Missing Dark Matter: KCWI High-resolution Stellar Kinematics of NGC1052-DF2

The velocity dispersion of the ultra diffuse galaxy NGC1052-DF2 was found to be ${\sigma }_{\mathrm{gc}}={7.8}_{-2.2}^{+5.2}$ km s−1, much lower than expected from the stellar mass–halo mass relation and nearly identical to the expected value from the stellar mass alone. This result was based on the radial velocities of 10 luminous globular clusters that were assumed to be associated with the galaxy. A more precise measurement is possible from high-resolution spectroscopy of the diffuse stellar light. Here we present an integrated spectrum of the diffuse light of NGC1052-DF2 obtained with the Keck Cosmic Web Imager (KCWI), with an instrumental resolution of σinstr ≈ 12 km s−1. The systemic velocity of the galaxy is vsys = 1805 ± 1.1 km s−1, in very good agreement with the average velocity of the globular clusters ($\langle {v}_{\mathrm{gc}}\rangle =1803\pm 2$ km s−1). There is no evidence for rotation within the KCWI field of view. We find a stellar velocity dispersion of ${\sigma }_{\mathrm{stars}}={8.5}_{-3.1}^{+2.3}$ km s−1, consistent with the dispersion that was derived from the globular clusters. The implied dynamical mass within the half-light radius r1/2 = 2.7 kpc is Mdyn = (1.3 ± 0.8) × 108 M⊙, similar to the stellar mass within that radius (Mstars = (1.0 ± 0.2) × 108 M⊙). With this confirmation of the low velocity dispersion of NGC1052-DF2, the most urgent question is whether this "missing dark matter problem" is unique to this galaxy or applies more widely.