The H IX galaxy survey IV: What does the molecular gas reveal in HI eXtreme galaxies

Galaxies with extreme amounts of neutral atomic hydrogen (HI), known as HI-eXtreme (HIX) galaxies, paradoxically exhibit normal or low star formation rates. To investigate the bottleneck in their star formation, we present ALMA CO(1-0) observations for five HIX survey galaxies: ESO208-G026, ESO378-G003, ESO381-G005, ESO417-G018, and ESO055-G013. While these systems possess massive, extended HI discs, their molecular gas discs are compact with normal H 2 masses relative to their stellar mass. Dynamical modelling with 3DBarolo reveals that the HI and molecular gas are kinematically consistent. Both gas phases show high velocity dispersions, indicating enhanced turbulence compared to regular late-type galaxies. We calculate the specific angular momentum for the stellar, atomic, and molecular components alongside the global stability parameter (q). Most of the system angular momentum is stored in the HI discs, rendering them globally stable against collapse. This stability suppresses the atomic-to-molecular gas conversion, as evidenced by extremely long HI depletion times contrasted with typical H 2 depletion times. The high angular momentum and HI excess likely originate from past gas accretion or recent tidal interactions; this is especially apparent in ESO378-G003 and ESO381-G005, which exhibit extended HI structures and positive tidal indices.