How to Reconcile the Observed Velocity Function of Galaxies with Theory

Understand

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.