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Interactions such as mergers and flybys play a fundamental role in shaping galaxy morphology. Using the Horizon Run 4 cosmological N-body simulation, we studied the frequency and type of halo interactions, and their redshift evolution as a function of the environment defined by the large-scale density, pair separation, mass ratio, and target halo mass. Most interactions happen at large-scale density contrast δ ≃ 20, regardless of the redshift corresponding to groups and relatively dense part of filaments. However, the fraction of interacting target is maximum at δ ≃ 1000. We provide a new empirical fitting form for the interaction rate as a function of the halo mass, large-scale density, and redshift. We also report the existence of two modes of interactions from the distributions of mass ratio and relative distance, implying two different physical origins of the interaction. Satellite targets lose their mass as they proceed deeper into the host halo. The relative importance of these two trends strongly depends on the large-scale density, target mass, and redshift. [ABSTRACT FROM AUTHOR]

We propose a method using the redshift dependence of the Alcock-Paczynski (AP) test and volume effect to measure the cosmic expansion history. The galaxy two-point correlation function as a function of angle, ν(μ), is measured at different redshifts. Assuming an incorrect cosmological model to convert galaxy redshifts to distances, the shape of ν(μ) appears anisotropic due to the AP effect, and the amplitude is shifted by the change in comoving volume. Due to the redshift dependence of the AP and volume effect, both the shape and amplitude of ν(μ) exhibit redshift dependence. Similar to Li et al. (2014), we find that the redshift-space distortions (RSD) caused by galaxy peculiar velocities, although significantly distorting ν(μ), exhibit much less redshift evolution compared to the AP and volume effects. By focusing on the redshift dependence of ν(μ), we can correctly recover the cosmological parameters despite the contamination of RSD. The method is tested by using the Horizon Run 3 N-body simulation, from which we made a series of 1/8-sky mock surveys having eight million physically self-bound haloes and sampled to have roughly a uniform number density in z = 0-1.5. We find the AP effect results in tight, unbiased constraints on the density parameter and dark energy equation of state, with 68.3% CL intervals δΩm ~ 0.03 and δw ~ 0.1, and the volume effect leads to much tighter constraints of δΩm ~ 0.007 and δw ~ 0.035. [ABSTRACT FROM AUTHOR]