Beyond the halo: redefining environment with unbound matter in N -body simulations

Approximately half of the matter in the Universe is ‘unbound at z = 0, according to N-body simulations such as the Millennium Run. Here, we use the milli-Millennium Simulation to examine the distribution of unbound matter in relation to the dark matter haloes which host galaxies. We measure the unbound matter within two types of windows, using a halo-dependent radius and a fixed radius at several different scales. We also consider the time-scales over which a halo can accrete the local unbound matter at z = 2 and 0. Finally, we compare the unbound matter to observable properties of galaxies, such as local galaxy-count environment and stellar mass. We find that haloes at z = 2 can accrete far more of the nearby unbound matter over a Hubble time than haloes at z = 0 and that 78 per cent of particles within 5 Rvir of a halo at z = 2 will be accreted by z = 0, compared to 36 per cent of particles within 5 h−1 Mpc of the halo. We also find that galaxy-count environment is closely related to the amount of nearby unbound matter when measured on the same scale.