NIHAO X: Reconciling the local galaxy velocity function with Cold Dark Matter via mock HI observations

TL;DR

This study uses high-resolution hydrodynamical simulations to show that biases in HI velocity measurements explain the apparent discrepancy between observed and predicted galaxy velocity functions in the Cold Dark Matter model.

Contribution

It demonstrates that accounting for HI velocity bias resolves the mismatch between observations and CDM predictions, supporting the model's validity.

Findings

Bias between HI velocities and Vmax causes underestimation of halo velocities.

When bias is corrected, CDM matches observed velocity functions.

Simulations reproduce Tully-Fisher relation and HI sizes.

Abstract

We used 87 high resolution hydrodynamical cosmological simulations from the NIHAO suite to investigate the relation between the maximum circular velocity (Vmax) of a dark matter halo in a collisionless simulation and the velocity width of the HI gas in the same halo in the hydrodynamical simulation. These two quantities are normally used to compare theoretical and observational velocity functions and have led to a possible discrepancy between observations and predictions based on the Cold Dark Matter (CDM) model. We show that below 100 km/s, there is clear bias between HI based velocities and Vmax, that leads to an underestimation of the actual circular velocity of the halo. When this bias is taken into account the CDM model has no trouble in reproducing the observed velocity function and no lack of low velocity galaxies is actually present. Our simulations also reproduce the linewidth - stellar mass (Tully-Fisher) relation and HI sizes, indicating that the HI gas in our simulations is as extended as observed. The physical reason for the lower than expected linewidths is that, in contrast to high mass galaxies, low mass galaxies no longer have extended thin HI rotating disks, as is commonly assumed.