The H IX galaxy survey IV: What does the molecular gas reveal in H
i
eXtreme galaxies?
V A Capa, H Dénes, K A Lutz, T H Brown Abstract
Galaxies with extreme amounts of neutral atomic hydrogen (H i), known as H i-eXtreme (H ix) 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 H ix survey galaxies: ESO208-G026, ESO378-G003, ESO381-G005, ESO417-G018, and ESO055-G013. While these systems possess massive, extended H i discs, their molecular gas discs are compact with normal H2 masses relative to their stellar mass. Dynamical modelling with 3DBarolo reveals that the H i 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 H i discs, rendering them globally stable against collapse. This stability suppresses the atomic-to-molecular gas conversion, as evidenced by extremely long H i depletion times contrasted with typical H2 depletion times. The high angular momentum and H i excess likely originate from past gas accretion or recent tidal interactions; this is especially apparent in ESO378-G003 and ESO381-G005, which exhibit extended H i structures and positive tidal indices.