Environmental influences on dark matter haloes and consequences for the galaxies within them

We use large N-body simulations of dissipationless gravitational clustering in cold dark matter (CDM) cosmologies to study whether the properties of dark matter haloes are affected by their environment. We look for correlations between the masses, formation redshifts, concentrations, shapes and spins of haloes and the overdensity of their local environment. We also look for correlations of these quantities with the local tidal field. Our conclusion is extremely simple. Only the mass distribution varies as a function of environment. This variation is well described by a simple analytic formula based on the conditional Press-Schechter theory. We find no significant dependence of any other halo property on environment. Our results do not depend on our choice of cosmology. According to current hierarchical models, the structure and evolutionary history of a galaxy are fully determined by the structure and evolutionary history of the dark halo in which it is embedded. If these models are correct, clustering variations between galaxies of differing morphological types, luminosities, colours and surface brightnesses must all arise because the halo mass function is skewed towards high-mass objects in overdense regions of the Universe and towards low-mass objects in underdense regions.