The galaxy-halo connection of disc galaxies over six orders of magnitude in stellar mass

The relations between stellar ( M * ), gas ( M gas ), baryonic ( M bar = M * + M gas ), and dark matter halo mass ( M 200 ) provide unique constraints on galaxy formation and cosmology. The shape of the relations constrains how galaxies regulate their growth through gas accretion, star formation, and feedback, and their scatter probes the stochasticity of galaxy assembly, which depends on the underlying cosmological model. In this paper, we assemble a sample of 49 nearby gas-rich dwarf and massive disc galaxies with unmatched ancillary data. We obtain their gas kinematics and derive their dark matter properties through rotation curve decomposition. Our sample is representative of the regularly rotating gas-rich galaxy population and allowed us to study the galaxy-halo connection across nearly six orders of magnitude in M * . We find that the M gas − M 200 relation rises monotonically, with galaxies having around 4% of the average cosmological baryon fraction in cold gas. Contrastingly, the M * − M 200 relation shows a more complex behaviour. A particularly interesting finding is that of a population of ‘baryon-deficient’ dwarfs (BDDs) with stellar masses ∼1 − 1.5 orders of magnitude lower than expected from current models. Yet, baryon-rich galaxies also exist, and we find a large spread in the baryon retention fraction across our galaxies. We compare our findings with semi-analytic (DarkLight) and hydrodynamical (TNG50, Simba) galaxy formation simulations. While the simulations broadly reproduce most observed features, they struggle to match the BDDs and do not capture the diversity in baryon fractions. Understanding these differences will shed new light on how feedback regulates galaxy formation. Finally, we study the dark matter halo concentration-mass relation. We find that below M 200 ∼ 10 11 M ⊙ , the concentrations are systematically lower than expected from pure-dark matter simulations. We discuss whether these results stem from the influence of baryonic physics or the environment. Understanding this is crucial if gas-rich galaxies are to be used to test cosmological models.