UC Santa Cruz

We use results from the Bolshoi-Planck cosmological dark matter simulation to (1) study the evolution of dark matter halo properties in regions of different local environment densities, (2) investigate whether dark matter halo spin is a reliable predictor of galaxy size, and (3) determine the causes and consequences of virial mass loss in distinct halos. We find that halos in high density regions typically have higher NFW concentrations ($C = R_{\mathrm{vir}}/R_{\mathrm{s}}$), lower spin parameters, and much lower mass accretion rates than median halos at z = 0. These trends are a result of strong tidal forces (due to nearby massive halos), which inhibit relatively lower mass halos from accreting material and strip particles from their outer regions. Surprisingly, we find that lower mass halos in the lowest density regions also have higher concentrations and lower spin parameters. We provide possible explanations for these trends: (i) halos in low density regions must form early if they are to form at all, and (ii) less violent accretion histories lead to less increase in $R_{\mathrm{s}}$, implying higher concentrations; (iii) these halos have fewer massive neighbors to spin them up, implying lower spin parameters. If halo spin parameter is proportional to galaxy size, this predicts that galaxies in low density regions should have less extended disks. We test this prediction using SDSS data. We additionally identify two primary causes of halo mass loss: tidal stripping by a massive neighboring halo (a dominating effect in high density regions), and virial relaxation after a major merger. Tidal stripping causes halos to become less prolate and have lower spins and higher NFW concentrations. Tidally stripped halos often lose a large fraction of their peak mass ($>20\%$) and most never recover (or even reattain a positive accretion rate). Major mergers initially boost Mvir and typically cause the final halo to become more prolate and less relaxed and to have higher spin and lower NFW concentration. As the halo relaxes, high energy material from the recent merger gradually escapes beyond the virial radius, temporarily resulting in a net negative accretion rate that reduces the halo mass by $5-15\%$ on average. Halos that experience a major merger around z=0.4 typically reach a minimum mass near z=0.