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We present the Australian Square Kilometre Array Pathfinder (ASKAP) WALLABYpre-pilot observations of two `dark' HI sources (with HI masses of a few times10^8 Msol and no known stellar counterpart) that reside within 363 kpc of NGC1395, the most massive early-type galaxy in the Eridanus group of galaxies. Weinvestigate whether these `dark' HI sources have resulted from past tidalinteractions or whether they are an extreme class of low surface brightnessgalaxies. Our results suggest that both scenarios are possible, and notmutually exclusive. The two `dark' HI sources are compact, reside in relativeisolation and are more than 159 kpc away from their nearest HI-rich galaxyneighbour. Regardless of origin, the HI sizes and masses of both `dark' HIsources are consistent with the HI size-mass relationship that is found innearby low-mass galaxies, supporting the possibility that these HI sources arean extreme class of low surface brightness galaxies. We identified threeanalogues of candidate primordial `dark' HI galaxies within the TNG100cosmological, hydrodynamic simulation. All three model analogues are darkmatter-dominated, have assembled most of their mass 12-13 Gyr ago, and have notexperienced much evolution until cluster infall 1-2 Gyr ago. Our WALLABYpre-pilot science results suggest that the upcoming large area HI surveys willhave a significant impact on our understanding of low surface brightnessgalaxies and the physical processes that shape them.

WALLABY pre-pilot survey: Two dark clouds in the vicinity of NGC 1395