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

Abstract

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 ${\mu }_{g,0}\ge 24$ mag arcsec${}^{-2}$ and $R_e\ge 1.5$ kpc, and a broader sample including 12 additional faint diffuse galaxies (${\mu }_{g,0}\ge 23.7$ mag arcsec${}^{-2}$ and $R_e\ge 1.3$ 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, ${\log }_{10}(M_{HI}/{\mathrm{M}}_{\odot })=8.95\pm 0.03$, compared to the South-Eastern UDG (WALLABY J104513-262755-UDG-2), ${\log }_{10}(M_{HI}/{\mathrm{M}}_{\odot })=8.60\pm 0.04$. UDG-1's stellar mass and star formation rate are also approximately an order of magnitude larger at ${\log }_{10}(M_{\ast }/{\mathrm{M}}_{\odot })=8.07\pm 0.12$ and ${\log }_{10}(SFR/{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1})=-1.26\pm 0.12$ respectively. The pair has an isolated local environment, with no other galaxies or H I sources within 30 arcmin (525 kpc) and $\pm 1000$ 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.

14 pages, 8 figures, accepted for publication in MNRAS