Caught in the Act of Quenching? – A Population of Post-Starburst Ultra-Diffuse Galaxies

We report the discovery of post-starburst ultra-diffuse galaxies (UDGs), identified through spectroscopic analysis with KCWI at the Keck II Telescope. Our analysis is based on a sample of 44 candidate UDGs selected from the Systematically Measuring Ultra-Diffuse Galaxies (SMUDGes) program. Our measured spectroscopic redshifts reveal ∼85% of the entire KCWI sample exhibit large physical sizes () and low surface brightnesses ( 24≲μ0,g≲25 mag arcsec −2) which categorize them as UDGs. We find 20% of the confirmed UDG population contain post-starburst (or K+A) features, characterized by minimal to no emission in H β indicative of quenched star formation and a predominant presence of spectral A-type stars. In surveying the local environments of the post-starburst UDGs, we find that nearly half are isolated systems, which is unusual given that isolated UDGs are most commonly found to be star-forming. Two of these systems reside away from potential nearby massive hosts ( M⋆>1010M⊙), indicating the absence of environmental influence. These post-starburst UDGs may represent systems experiencing star formation feedback such that a recent burst may lead to (at least temporary) quenching. Overall, our results highlight the potentially diverse quenching pathways of UDGs in the local Universe.