SIMBAD references

We explore the environment of 252 H I-bearing ultra-diffuse galaxies (HUDs) from the 100 per cent ALFALFA survey catalogue in an attempt to constrain their formation mechanism. We select sources from ALFALFA with surface brightnesses, magnitudes, and radii consistent with other samples of ultra-diffuse galaxies (UDGs) without restrictions on their isolation or environment, more than doubling the previously reported ALFALFA sample. We quantify the galactic environment of HUDs using several metrics, including nth nearest neighbour, tidal influence, membership in a group/cluster, and distance from nearest group/cluster or filament. We find that HUDs inhabit the same environments as other samples of H I-selected galaxies and that they show no environmental preference in any metric. We suggest that these results are consistent with a picture of the extreme properties of HUDs being driven by internal mechanisms and that they are largely unperturbed by environmental impacts. While environmental effects may be necessary to convert HUDs into gas-poor cluster UDGs, these effects are not required for diffuse galaxies to exist in the first place.