LAUSR

Using the Widefield ASKAP L-band Legacy All-sky Blind surveY (WALLABY) we performed an untargeted search for H i-bearing ultra-diffuse galaxies (UDGs). We identified a core sample of 10 UDGs defined by μg, 0 ≥ 24 mag arcsec−2 and Re ≥ 1.5 kpc, and a broader sample including 12 additional faint diffuse galaxies (μg, 0 ≥ 23.7 mag arcsec−2 and Re ≥ 1.3 kpc). Within the core sample, we highlight the first discovery of a UDG pair. Their projected separation is just 75 arcsec (22 kpc at 61.9 Mpc), with a central H i velocity difference of 34 km s−1. The North-Western UDG (WALLABY J104513-262755-UDG-1) has a larger H i reservoir, $\log _{10}(M_{HI}/\rm M_{\odot }) = 8.95\pm 0.03$, compared to the South-Eastern UDG (WALLABY J104513-262755-UDG-2), $\log _{10}(M_{HI}/\rm M_{\odot }) = 8.60\pm 0.04$. UDG-1’s stellar mass and star formation rate are also approximately an order of magnitude larger at $\log _{10}(M_*/\rm M_{\odot }) = 8.07\pm 0.12$ and $\log _{10}(SFR/\rm M_{\odot }~yr^{-1}) = -1.26\pm 0.12$ respectively. The pair has an isolated local environment, with no other galaxies or H i sources within 30 arcmin (525 kpc) and ±1000 km s−1. However, in the context of the larger-scale structure, the pair is located outside the virial radius of the Hydra cluster, with its position on the phase-space diagram indicating that it is infalling into the cluster. The identification of this H i-bearing UDG pair raises important questions around the formation of such a unique system and the evolution of UDGs in a transitional phase before ram pressure stripping and cluster infall.