# The HI Velocity Function: a test of cosmology or baryon physics?

## TL;DR

This study tests dark matter models by comparing simulated and observed HI velocity functions, emphasizing the importance of baryonic physics modeling and survey effects, finding closer agreement than previously reported.

## Contribution

It introduces a detailed modeling approach of HI emission lines in semi-analytic galaxy formation, accounting for survey selection effects and galaxy-halo velocity relations.

## Key findings

- Simulated HI velocity functions agree with observations within 50%.
- Survey selection effects significantly impact velocity function measurements.
- Modeling galaxy-halo velocity relations with scatter reduces discrepancies.

## Abstract

Accurately predicting the shape of the HI velocity function 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 $\Lambda$CDM model predicts an overabundance of low circular velocity galaxies when compared to observed HI velocity functions. 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 HI velocity function predictions. We explore this in detail by modelling HI emission lines of galaxies in the SHARK semi-analytic galaxy formation model, built on the SURFS suite of $\Lambda$CDM $N$-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 HI velocity width distributions, finding differences of $\lesssim 50$%, 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 HI mass-maximum circular velocity relation of galaxies is characterised by a large scatter. These biases are complex enough that building a velocity function from the observed HI line widths cannot be done reliably.