Spatially Resolved Stellar Spectroscopy of the Ultra-diffuse Galaxy Dragonfly 44. III. Evidence for an Unexpected Star-Formation History

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Alexa Villaume, Aaron J. Romanowsky, Jean Brodie, Pieter van Dokkum, Charlie Conroy, Duncan Forbes, Shany Danieli, Christopher Martin, Matt Matuszewski

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 (mage∼+0.01+0.07−0.08 log Gyr kpc−1) and a flat-to-positive metallicity gradient (m[Fe/H]∼+0.08+0.11−0.11 dex kpc−1), which are inconsistent with the gradients measured in similarly pressure-supported dwarf galaxies. We also measure a flat-to-negative [Mg/Fe] gradient (m[Mg/Fe]∼−0.18+0.17−0.17 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.

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