The evolution of baryon density fluctuations in multi-component cosmological simulations

We critically examine how the evolution of the matter density field in cosmological simulations is affected by details of setting up initial conditions. We show that it is non-trivial to realise an initial distribution of matter in N -body/hydrodynamic simulations so that the baryon and dark matter density fluctuations and their velocities evolve consistently as theoretically predicted. We perform a set of cosmological simulations and use them to distinguish and verify an appropriate method for generating initial conditions. We show that a straightforward way of applying the Zel’dovich approximation to each component using distinct transfer functions results in an incorrect growth of density fluctuations and that it is necessary to correct velocities at the initial epoch. The unperturbed uniform particle distribution must be also generated appropriately to avoid tight coupling of the baryonic and dark matter components. We recommend using independent “glass” particle distributions, using distinct transfer functions for baryons and dark matter, and taking into account the difference in the velocity fields at the initialisation epoch. The proposed method will be useful for studies of the evolution of the intergalactic medium and the formation of the first cosmological objects using numerical simulations.