Optically low surface brightness galaxies have been predicted to represent a significant galaxy population. They may make up 85% of the number density for galaxies with stellar masses > 107 solar masses, but they have been incredibly difficult to detect in large numbers. Yet our understanding of galaxy formation has been largely shaped by those galaxies that we have been able to observe, making the study of these faint galaxies key to refining our knowledge of the processes driving the evolution of galaxies.
O’Beirne et al. have used ASKAP WALLABY (Widefield ASKAP L-band Legacy All-sky Blind surveY) data to search for HI-bearing ultra-diffuse galaxies (UDGs). They identified a core sample of 10 UDGs, including the first discovery of a UDG pair, with a projected separation of just 75 arcsec (22 kpc at 61.9 Mpc). The pair has an isolated local environment, with no other galaxies or HI sources within 30 arcmin (525 kpc). 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 HI-bearing UDG pair raises important questions around the formation of such a unique system. The image above is a three-colour optical Legacy Survey image of WALLABY J104513-262755. The HI contours levels are overlaid in white. The dashed contour represents the edge of the SoFiA mask. The north-western galaxy of the pair is labelled UDG-1 and the south-eastern galaxy is labelled UDG-2. The WALLABY beam (30 arcsec) is shown in the lower left corner.