Ultra-diffuse galaxies (UDGs) have been in the spotlight since their (re)discovery in 2015. These galaxies challenge our understanding of the field of galaxy formation and evolution as they have the sizes of giants, but luminosities of dwarfs. Although UDGs have been heavily studied in the past few years, most works have relied on spectroscopy to understand their stellar population properties, which is extremely challenging for such faint sources, requiring unrealistically large amounts of time on the world’s largest telescopes.
In this work, we alternatively employ spectral energy distribution fitting techniques on photometric data to recover the stellar populations of ~90 UDGs distributed across environments. We find evidence of a stellar population dependence on the environment and globular cluster (GC)-richness of UDGs: (1) Field UDGs are systematically younger than their cluster counterparts; (2) The GC-poor UDGs are consistently more metal-rich than GC-rich ones. As a consequence, GC-poor UDGs are consistent with the mass-metallicity relation (MZR) for dwarf galaxies, suggesting that they may be puffed-up dwarfs. However, GC-rich UDGs lie well below the MZR, in the region where failed galaxy scenarios might be expected.