# How to Reconcile the Observed Velocity Function of Galaxies with Theory

## TL;DR

This study uses high-resolution cosmological simulations and mock HI data to reconcile the discrepancy between observed dwarf galaxy velocities and the predictions of the LCDM model, highlighting observational biases and dark matter profile effects.

## Contribution

It demonstrates that accounting for observational methods and dark matter profiles resolves the velocity function discrepancy in LCDM.

## Key findings

- Observed velocity functions align with LCDM when using mock observations.
- Underestimation of halo velocities by HI linewidth measurements explains part of the discrepancy.
- Faint, low-mass halos are often undetected in current surveys.

## Abstract

Within a Lambda Cold Dark Matter (LCDM) scenario, we use high resolution cosmological simulations spanning over four orders of magnitude in galaxy mass to understand the deficit of dwarf galaxies in observed velocity functions. We measure velocities in as similar a way as possible to observations, including generating mock HI data cubes for our simulated galaxies. We demonstrate that this apples-to-apples comparison yields an "observed" velocity function in agreement with observations, reconciling the large number of low-mass halos expected in a LCDM cosmological model with the low number of observed dwarfs at a given velocity. We then explore the source of the discrepancy between observations and theory, and conclude that the dearth of observed dwarf galaxies is primarily explained by two effects. The first effect is that galactic rotational velocities derived from the HI linewidth severely underestimate the maximum halo velocity. The second effect is that a large fraction of halos at the lowest masses are too faint to be detected by current galaxy surveys. We find that cored dark matter density profiles can contribute to the lower observed velocity of galaxies, but only for galaxies in which the velocity is measured interior to the size of the core (~3 kpc).