Spatially-resolved stellar kinematics of the ultra diffuse galaxy Dragonfly 44. I. Observations, kinematics, and cold dark matter halo fits

We present spatially resolved stellar kinematics of the well-studied ultra-diffuse galaxy (UDG) Dragonfly 44, as determined from 25.3 hr of observations with the Keck Cosmic Web Imager. The luminosity-weighted dispersion within the half-light radius is sigma(1/2) = 33(-3)(+3) km s(-1), lower than what we had inferred before from a DEIMOS spectrum in the H alpha region. There is no evidence for rotation, with V-max/ < 0.12 (90% confidence) along the major axis, in possible conflict with models where UDGs are the high-spin tail of the normal dwarf galaxy distribution. The spatially averaged line profile is more peaked than a Gaussian, with Gauss-Hermite coefficient h(4) = 0.13 +/- 0.05. The mass-to-light ratio (M/L) within the effective radius is (M-dyn/L-I) (更多