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 $\sigma_{\rm gc}=7.8^{+5.2}_{-2.2} \ \mathrm{kms^{-1}}$, 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 ten 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, with an instrumental resolution of $\sigma_{\rm
instr}\approx 12 \ \mathrm{kms^{-1}}$. The systemic velocity of the galaxy is
$v_{\rm sys}=1805\pm 1.1 \ \mathrm{kms^{-1}}$, in very good agreement with the
average velocity of the globular clusters ($\langle v_{\rm gc}\rangle = 1803\pm
2 \ \mathrm{kms^{-1}}$). There is no evidence for rotation within the KCWI
field of view. We find a stellar velocity dispersion of $\sigma_{\rm stars}=8.4
\pm 2.1 \ \mathrm{kms^{-1}}$, consistent with the dispersion that was derived
from the globular clusters. The implied dynamical mass within the half-light
radius $r_{1/2}=2.7 \ \mathrm{kpc}$ is $M_{\rm dyn}= (1.3 \pm 0.8) \times 10^8
$ M$_{\odot}$, similar to the stellar mass within that radius ($M_{\rm
stars}=(1.0 \pm 0.2) \times 10^8 \ \mathrm{M}_{\odot}$). 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.
