The influence of baryons on low-mass haloes

ABSTRACT The Voids-within-Voids-within-Voids project used dark-matter-only (DMO) simulations to study the abundance and structure of dark matter (DM) haloes over the full mass range populated in the standard Lambda cold dark matter cosmology. Here, we explore how baryonic effects modify these results for z=0 halo masses in the range 10⁴–10⁷~M_, below the threshold for galaxy formation. Our main study focuses on three simulations from identical initial conditions at z=127, one following DMO, one including non-radiative gas, and one additionally including the baryonic physics relevant in this halo mass range (cooling and photoheating). In the non-radiative simulation, above 10^5. 5~M_, halo abundance and internal structure are very similar to the DMO simulation, and the baryon to DM ratio is everywhere close to the cosmic value. At lower mass, this ratio drops and haloes are less concentrated and less massive in the non-radiative case. Test simulations at higher resolution show this to be mainly a resolution effect; the expected drop in baryon content due to residual pressure effects only becomes substantial for z=0 haloes below 10^2. 7~M_. However, gas is heated by reionization at z=6 in our ‘full physics’ run, and this results in almost complete expulsion of gas from all haloes in our simulated mass range. This suppresses the halo mass function by 30{\ per\ cent}, lowers halo concentration, and consequently weakens the DM annihilation signal by 40-60{\ per\ cent}.