2022ApJ...924...32V

We use the Keck Cosmic Web Imager integral field unit spectrograph to (1) measure the global stellar population parameters for the ultra-diffuse galaxy (UDG) Dragonfly 44 (DF44) to much higher precision than previously possible for any UDG and (2) for the first time measure spatially resolved stellar population parameters of a UDG. We find that DF44 falls below the mass-metallicity relation established by canonical dwarf galaxies both in and beyond the Local Group. We measure a flat radial age gradient (m_logage=+0.01_–0.08^+0.08 log Gyr kpc^–1) and a flat to positive metallicity gradient (m_[Fe/H]=+0.09_–0.12^+0.11 dex kpc^–1), which are inconsistent with the gradients measured in similarly pressure-supported dwarf galaxies. We also measure a negative [Mg/Fe] gradient (m_[Mg/Fe]=-0.20_–0.18^+0.18 ) dex kpc^–1 such that the central 1.5 kpc of DF44 has stellar population parameters comparable to metal-poor globular clusters. Overall, DF44 does not have internal properties similar to other dwarf galaxies and is inconsistent with it having been puffed up through a prolonged, bursty star formation history, as suggested by some simulations. Rather, the evidence indicates that DF44 experienced an intense epoch of "inside-out" star formation and then quenched early and catastrophically, such that star formation was cut off more quickly than in canonical dwarf galaxies.