HI-selected dark sources and low surface brightness galaxies

Our understanding of galaxy evolution is coupled to those galaxies that historically have been observable, but it is only in recent years that we have had the technology to detect optically dark and low surface brightness galaxies in large numbers. It is challenging to search for these sources just using optical data. However, those that are sufficiently gas-rich can be detected in neutral atomic hydrogen (HI) surveys, and we now have the resolution to do optical cross matching. We examine the optical counterparts of the 1829 HI detections in three pilot fields in the Widefield ASKAP L-band Legacy All-sky Blind surveY (WALLABY) using data from the DESI Legacy Imaging Surveys DR10. With its significantly improved resolution compared to the previous large area HI surveys, HIPASS and ALFALFA, WALLABY is better able to localise the source of the emission and resolve tidal features. We find that 17% (315) of the detections are low surface brightness galaxies (LSBGs; mean g-band surface brightness within 1 Re of > 23 mag arcsec−2) and 3% (55) are dark. The LSBGs have low star formation efficiencies, and 75% of them had not been previously catalogued. Of the dark sources, 38 pass reliability tests, with 13 showing tidal interaction signatures, while 25 are candidates for isolated galaxies with high HI masses, but low stellar masses and star-formation rates. Studying dark and LSBGs allows us to learn more about the efficiency of the baryon cycle in these extreme galaxies.