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This paper presents a study of the specific merger rate as a function ofgroup membership, local environment, and redshift in a very large, $500h^{-1}Mpc$, cosmological N-body simulation, the \textit{Millennium Simulation}. Thegoal is to provide environmental diagnostics of major merger populations inorder to test simulations against observations and provide further constraintson major merger driven galaxy evolution scenarios. A halo sample is definedusing the maximum circular velocity, which is both well defined for subhalosand closely correlated with galaxy luminosity. Subhalos, including theprecursors of major mergers, are severely tidally stripped. Major mergersbetween subhalos are therefore extremely rare. Tidal stripping also suppressesdynamical friction, resulting in long major merger time scales when the moremassive halo does not host other subhalos. In contrast, when other subhalos arepresent major merger time scales are several times shorter. This enhancement islikely due to inelastic unbound collisions between subhalos. Following theseresults, we predict that major mergers in group environments are dominated bymergers involving the central galaxy, that the specific merger rate issuppressed in groups, and that the frequency of fainter companions is enhancedfor mergers and their remnants. We also observe an `assembly bias' in the majormerger rate in that mergers of galaxy-like halos are slightly suppressed inoverdense environments while mergers of group-like halos are slightly enhanced.A dynamical explanation for this trend is advanced which calls on both tidaleffects and interactions between bound halos beyond the virial radii of locallydynamically dominant halos.

Dark Matter Halo Mergers I: Dependence on Environment & Redshift Evolution