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 d ark matter haloes are affected by their environment. We look for correlatio ns between the masses, formation redshifts, concentrations, shapes and spin s of haloes and the overdensity of their local environment. We also look fo r correlations of these quantities with the local tidal field. Our conclusi on 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 dep endence of any other halo property on environment. Our results do not depen d 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 embedde d. If these models are correct, clustering variations between galaxies of d iffering morphological types, luminosities, colours and surface brightnesse s 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.