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We study the star formation history (SFH) of the ultra-diffuse galaxy (UDG)Dragonfly 44 (DF44) based on the simultaneous fit to near-ultraviolet tonear-infrared photometry and high signal-to-noise optical spectroscopy. Infitting the observations we adopt an advanced physical model with a flexibleSFH, and we discuss the results in the context of the degeneracies betweenstellar population parameters. Through reconstructing the mass-assembly historywith a prior for extended star formation (akin to methods in the literature) wefind that DF44 formed 90 per cent of its stellar mass by $z\sim 0.9$ ($\sim7.2$ Gyr ago). In comparison, using a prior that prefers concentrated starformation (as informed by previous studies of DF44's stellar populations)suggests that DF44 formed as early as $z\sim 8$ ($\sim 12.9$ Gyr ago).Regardless of whether DF44 is old or very old, the SFHs imply early starformation and rapid quenching. This result, together with DF44's large size andevidence that it is on its first infall into the Coma cluster, challenges UDGformation scenarios from simulations that treat all UDGs as contiguous with thecanonical dwarf population. While our results cannot confirm any particularformation scenario, we can conclude from this that DF44 experienced a rarequenching event.

Still at Odds with Conventional Galaxy Evolution: The Star Formation History of Ultra-Diffuse Galaxy Dragonfly 44