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 km s^-1, lower than what we had inferred before from a DEIMOS spectrum in the Hα region. There is no evidence for rotation, with (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 _o/L _o, similar to other UDGs and higher by a factor of six than smaller galaxies of the same luminosity. This difference between UDGs and other galaxies is, however, sensitive to the aperture that is used, and it is much reduced when the M/L ratios are measured within a fixed radius of 10 kpc. Dragonfly 44 has a rising velocity dispersion profile, from km s^-1 at R = 0.2 kpc to km s^-1 at R = 5.1 kpc. The profile can only be fit with a cuspy Navarro-Frenk-White profile if the orbital distribution has strong tangential anisotropy, with . An alternative explanation is that the dark matter profile has a core: a Di Cintio et al. density profile with a mass-dependent core provides a very good fit to the kinematics for a halo mass of and , i.e., isotropic orbits. This model predicts a slight positive kurtosis, in qualitative agreement with the measured h ₄ parameter. UDGs such as Dragonfly 44 are dark matter dominated even in their centers and can constrain the properties of dark matter in a regime where baryons usually dominate the kinematics: small spatial scales in massive halos. In a companion paper we provide constraints on the axion mass in the context of "fuzzy" dark matter models.