The HI velocity function: A test of cosmology or baryon physics?

Accurately predicting the shape of the HI velocity function (VF) of galaxies is regarded widely as a fundamental test of any viable dark matter model. Straightforward analyses of cosmological N-body simulations imply that the Δ cold dark matter (ΔCDM) model predicts an overabundance of low circular velocity galaxies when compared to observed H I VFs. More nuanced analyses that account for the relationship between galaxies and their host haloes suggest that how we model the influence of baryonic processes has a significant impact on H I VF predictions. We explore this in detail by modelling H I emission lines of galaxies in the SHARK semi-analytic galaxy formation model, built on the SURFS suite of ΔCDM N-body simulations. We create a simulated ALFALFA survey, in which we apply the survey selection function and account for effects such as beam confusion, and compare simulated and observed H I velocity width distributions, finding differences of ≲ 50 per cent, orders of magnitude smaller than the discrepancies reported in the past. This is a direct consequence of our careful treatment of survey selection effects and, importantly, how we model the relationship between galaxy and halo circular velocity - the H I mass-maximum circular velocity relation of galaxies is characterized by a large scatter. These biases are complex enough that building a VF from the observed H I linewidths cannot be done reliably.