Vol. 601
In section 1. Letters

The HI content of isolated ultra-diffuse galaxies: A sign of multiple formation mechanisms?

Ultra-diffuse galaxies (UDG) are distinctive because of their extremely low surface density of stars, with respect to the mass and size of their dark matter halo. The latter are derived from their relatively large rotational velocity, and large size. UDG are very abundant in galaxy clusters, and one of the best scenarios is that they have been stripped of their gas by the cluster environment, which stopped their star formation. In isolation, they could be quenched by starburst-driven outflows. The present article reports of 21cm observations of the HI component in four isolated UDG, leading to three upper limits and one detection. Combining with the literature, this means six field UDG have been observed, with three upper limits. The observed dichotomy in the gaseous properties of field UDGs is interpreted as a sign of the existence of multiple mechanisms for their formation, with the formation of the quiescent gas-poor UDGs remaining a mystery.