The structure of voids

Using high-resolution N-body simulations, we address the problem of the emptiness of giant ~20 h^-1 Mpc diameter voids found in the distribution of bright galaxies. Are the voids filled by dwarf galaxies? Do cosmological models predict too many small dark matter haloes inside the voids? Can the problems of cosmological models on small scales be addressed by studying the abundance of dwarf galaxies inside voids? We find that voids in the distribution of 10¹²h^-1 M_solar haloes (expected galactic magnitudes ~M_*) are almost the same as the voids in 10¹¹h^-1 M_solar haloes. Yet, much smaller haloes with masses 10⁹h^-1 M_solar and circular velocities v_circ~ 20 km s^-1 readily fill the voids: there should be almost 1000 of these haloes in a 20 h^-1 Mpc diameter void. A typical void of diameter 20 h^-1 Mpc contains about 50 haloes with v_circ > 50 km s^-1. The haloes are arranged in a pattern, which looks like a miniature universe: it has the same structural elements as the large-scale structure of the galactic distribution of the Universe. There are filaments and voids; larger haloes are at the intersections of filaments. The only difference is that all masses are four orders of magnitude smaller. There is severe (anti)bias in the distribution of haloes, which depends on halo mass and on the distance from the centre of the void. Large haloes are more antibiased and have a tendency to form close to void boundaries. The mass function of haloes in voids is different from the `normal' mass function. It is much steeper for high masses, resulting in very few M33-type galaxies (v_circ~ 100 km s^-1). We present an analytical approximation for the mass function of haloes in voids.