Dependence of Halo Properties on Interaction History, Environment and Cosmology

I present results from numerical N-body simulations regarding the effect of merging events on the angular momentum distribution of galactic halos as well as a comparison of halo growth in semi-analytic vs. N-body methods. A total of six simulations are used spanning 3 cosmologies: a standard flat Ω0 = 1 model, an open Ω0 = 0.3 model and a “tilted” flat Ω0 = 1 model with spectral index n = 0.8. In each model, one run was conducted using a spatially uniform grid of and one using a refined grid in a large void. In all three models and all environments tested, the mean angular momentum of merger remnants (halo interaction products with mass ratios 3:1 or less) is greater than non-merger remnants. Furthermore, the dispersion in the merger-remnant angular momentum distribution is smaller than the dispersion of the non-merger distribution. The interpretation most consistent with the data is that the orbital angular momentum of the interactors is important in establishing the final angular momentum of the merger product, although this conclusion is at odds with previous studies. I give the proper angular momentum distribution that should be used to describe merger remnants. I trace the most massive progenitor of L∗ galactic-mass halos from redshift z = 0 back to z = 5 in the uniform grid simulations and for 10M⊙ halos in void regions. Semi-analytic mass histories match reasonably well for the latter sample. However, I find that for halos of mass 10 <∼ M <∼ 10M⊙, the semi-analytic method underestimates the mass of progenitors and hence the formation redshift of halos in simulations. These results are independent of δc. I conclude that the tested current semi-analytic implementation of halo growth and interaction does not reproduce absolute masses of progenitors or formation times above 10M⊙.