Juhan Kim

GOTPM: A Parallel Hybrid Particle-Mesh Treecode

Abstract We describe a parallel, cosmological N-body code based on a hybrid scheme using the particle-mesh (PM) and Barnes-Hut (BH) oct-tree algorithm. We call the algorithm GOTPM for Grid-of-Oct-Trees-Particle-Mesh. The code is parallelized using the Message Passing Interface (MPI) library and is optimized to run on Beowulf clusters as well as symmetric multi-processors. The gravitational potential is determined on a mesh using a standard PM method with particle forces determined through interpolation. The softened PM force is corrected for short range interactions using a grid of localized BH trees throughout the entire simulation volume in a completely analogous way to P3M methods. This method makes no assumptions about the local density for short range force corrections and so is consistent with the results of the P3M method in the limit that the treecode opening angle parameter, θ→0. The PM method is parallelized using one-dimensional slice domain decomposition. Particles are distributed in slices of equal width to allow mass assignment onto mesh points. The Fourier transforms in the PM method are done in parallel using the MPI implementation of the FFTW package. Parallelization for the tree force corrections is achieved again using one-dimensional slices but the width of each slice is allowed to vary according to the amount of computational work required by the particles within each slice to achieve load balance. The tree force corrections dominate the computational load and so imbalances in the PM density assignment step do not impact the overall load balance and performance significantly. The code performance scales well to 128 processors and is significantly better than competing methods. We present preliminary results from simulations run on different platforms containing up to N=1G particles to verify the code.

Dark matter haloes in modified gravity and dark energy: interaction rate, small- and large-scale alignment

We study the properties of dark matter haloes in a wide range of modified gravity models, namely,

FLY-BY ENCOUNTERS BETWEEN DARK MATTER HALOS IN COSMOLOGICAL SIMULATIONS

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The Top-two Biggest Simulations ever Performed for the Study of the Large-scale Structures of the Universe

, DGP, and interacting dark energy models. We study the effects of modified gravity and dark energy on the internal properties of haloes, such as the spin and the structural parameters. We find that

Diffuse Dark and Bright Objects in the Hubble Deep Field

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Comparison between the Pair Fractions of Dark Matter Halos and Galaxies in Cosmological Simulations

gravity enhance the median value of the Bullock spin parameter, but could not detect such effects for DGP and coupled dark energy.

Alignments of interacting haloes in the Horizon run 4 simulation

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A Test of Correspondence Model with the HorizonRun 4 Simulation

also yields a lower median sphericity and oblateness, while coupled dark energy has the opposite effect. However, these effects are very small. We then study the interaction rate of haloes in different gravity, and find that only strongly coupled dark energy models enhance the interaction rate. We then quantify the enhancement of the alignment of the spins of interacting halo pairs by modified gravity. Finally, we study the alignment of the major axes of haloes with the large-scale structures. The alignment of the spins of interacting pairs of haloes in DGP and coupled dark energy models show no discrepancy with GR, while

A New Hydrodynamic Simulation Using Unstructured Moving Meshes

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A Study of Halo-Galaxy Correspondence from the Horizon Run 4

shows a weaker alignment. Strongly coupled dark energy shows a stronger alignment of the halo shape with the large-scale structures.