WALLABY: an untargeted search for H i-bearing ultra-diffuse galaxies uncovers the first known ultra-diffuse galaxy pair

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 μ_0,g ≥ 24 mag arcsec⁻² and R_e ≥ 1.5 kpc, and a broader sample including 12 additional faint diffuse galaxies (μ_0,g ≥ 23.7 mag arcsec−2 and R_e ≥ 1.3 kpc). Within the core sample, we highlight the first discovery of a pair of UDGs. 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, log10(M_{H I}/M₀) = 8.95 ± 0.03, compared with the south-eastern UDG (WALLABY J104513−262755-UDG-2), log10(M_{H I}/M₀) = 8.60 ± 0.04. The stellar mass and star formation rate of UDG-1 are also approximately an order of magnitude larger at log10(M_∗/M₀) = 8.07 ± 0.12 and log10(SFR/M₀ y^r−1) = −1.26 ± 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.