WALLABY pilot survey: properties of HI-selected dark sources and low surface brightness galaxies

TL;DR

This study analyzes the properties of HI-selected low surface brightness and dark galaxies in the WALLABY survey, revealing their diversity, low star formation efficiencies, and the presence of previously uncatalogued objects, including potential isolated dark galaxies.

Contribution

First comprehensive analysis of optically dark and low surface brightness galaxies in WALLABY, identifying new objects and assessing their properties and origins.

Findings

17% of HI detections are optically low surface brightness galaxies

3% of detections are optically dark sources

75% of LSBGs and all dark HI sources are newly catalogued

Abstract

We examine the optical counterparts of the 1829 neutral hydrogen (HI) detections in three pilot fields in the Widefield ASKAP L-band Legacy All-sky Blind surveY (WALLABY) using data from the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Surveys DR10. We find that 17 per cent (315) of the detections are optically low surface brightness galaxies (LSBGs; mean $g$-band surface brightness within 1 $ R_e$ of $> 23$ mag arcsec$^{-2}$) and 3 per cent (55) are optically 'dark'. We find that the gas-rich WALLABY LSBGs have low star formation efficiencies, and have stellar masses spanning five orders of magnitude, which highlights the diversity of properties across our sample. 75 per cent of the LSBGs and all of the dark HI sources had not been catalogued prior to WALLABY. We examine the optically dark sample of the WALLABY pilot survey to verify the fidelity of the catalogue and investigate the implications for the full survey for identifying dark HI sources. We assess the HI detections without optical counterparts and identify 38 which pass further reliability tests. Of these, we find that 13 show signatures of tidal interactions. The remaining 25 detections have no obvious tidal origin, so are candidates for isolated galaxies with high HI masses, but low stellar masses and star-formation rates. Deeper HI and optical follow-up observations are required to verify the true nature of these dark sources.