Using a statistical sample of dark matter haloes drawn from a suite of cosmological N-body simulations of the Cold Dark Matter (CDM) model, we quantify the impact of a simulated halo's mass accretion ...and merging history on two commonly used measures of its dynamical state, the virial ratio eta and the centre of mass offset Delta r. Quantifying this relationship is important because the degree to which a halo is dynamically equilibrated will influence the reliability with which we can measure characteristic equilibrium properties of the structure and kinematics of a population of haloes. We begin by verifying that a halo's formation redshift zform correlates with its virial mass Mvir and we show that the fraction of its recently accreted mass and the likelihood of it having experienced a recent major merger increases with increasing Mvir and decreasing zform. We then show that both eta and Delta r increase with increasing Mvir and decreasing zform, which implies that massive recently formed haloes are more likely to be dynamically unrelaxed than their less massive and older counterparts. Our analysis shows that both eta and Delta r are good indicators of a halo's dynamical state, showing strong positive correlations with recent mass accretion and merging activity, but we argue that Delta r provides a more robust and better defined measure of dynamical state for use in cosmological N-body simulations at z~0. We find that Delta r < 0.04 is sufficient to pick out dynamically relaxed haloes at z=0. Finally, we assess our results in the context of previous studies, and consider their observational implications.
Ahf: Amiga's Halo Finder Knollmann, Steffen R; Knebe, Alexander
arXiv.org,
04/2009
Paper, Journal Article
Odprti dostop
Cosmological simulations are the key tool for investigating the different processes involved in the formation of the universe from small initial density perturbations to galaxies and clusters of ...galaxies observed today. The identification and analysis of bound objects, halos, is one of the most important steps in drawing useful physical information from simulations. In the advent of larger and larger simulations, a reliable and parallel halo finder, able to cope with the ever-increasing data files, is a must. In this work we present the freely available MPI parallel halo finder AHF. We provide a description of the algorithm and the strategy followed to handle large simulation data. We also describe the parameters a user may choose in order to influence the process of halo finding, as well as pointing out which parameters are crucial to ensure untainted results from the parallel approach. Furthermore, we demonstrate the ability of AHF to scale to high resolution simulations.
We use a set of high-resolution simulations of scale-free Einstein-de Sitter cosmologies to investigate the logarithmic slope of the phase-space density profile \(Q(r) = \rho(r)/\sigma^3(r)\) of dark ...matter (DM) haloes. The initial conditions for the simulations are determined by a power law power spectrum of the form \(P(k) \propto k^n\). We compute the Q(r) profiles using the radial, tangential and full velocity dispersion, and the velocity anisotropy parameter, \(\beta(r)\). We express Q(r) as a single power-law \(Q(r) \propto r^\alpha\) and derive a median slope \(\alpha\) in each simulation and for each definition of Q. Our main findings are: 1. The various Q(r) profiles follow a power law to a good approximation. 2. The slopes depend on the concentration parameter c of the DM haloes, where for \(c \gtrsim 10\) the slopes steepen with rising concentration and for \(c \lesssim 10\) the trend flattens and even turns around. 3. The asymptotic value of \(\beta\) as \(r\to R_{\mathrm{vir}}\) increases with the value of c. 4. In accordance with Zait et al. 2007 \(\alpha_{\mathrm{rad}}\) becomes more negative as the asymptotic value of \(\beta\) at the virial radius increases. 5. This introduces a weak dependence of the \(Q(r)\) slopes on the slope of the power spectrum.
We explore the dependence of the central logarithmic slope of dark matter halo density profiles \(\alpha\) on the spectral index \(n\) of the linear matter power spectrum \(P(k)\) using cosmological ...\(N\)-body simulations of scale-free models (i.e. \(P(k) \propto k^n\)). For each of our simulations we identify samples of well resolved haloes in dynamical equilibrium and we analyse their mass profiles. By parameterising the mass profile using a ``generalised'' Navarro, Frenk & White profile in which the central logarithmic slope \(\alpha\) is allowed to vary while preserving the \(r^{-3}\) asymptotic form at large radii, we obtain preferred central slopes for haloes in each of our models. There is a strong correlation between \(\alpha\) and \(n\), such that \(\alpha\) becomes shallower as \(n\) becomes steeper. However, if we normalise our mass profiles by \(r_{-2}\), the radius at which the logarithmic slope of the density profile is -2, we find that these differences are no longer present. We conclude that there is no evidence for convergence to a unique central asymptotic slope, at least on the scales that we can resolve.
We use two simulations performed within the Constrained Local UniversE Simulation (CLUES) project to study both the shape and radial alignment of (the dark matter component of) subhaloes; one of the ...simulations is a dark matter only model while the other run includes all the relevant gas physics and star formation recipes. We find that the involvement of gas physics does not have a statistically significant effect on either property -- at least not for the most massive subhaloes considered in this study. However, we observe in both simulations including and excluding gasdynamics a (pronounced) evolution of the dark matter shapes of subhaloes as well as of the radial alignment signal since infall time. Further, this evolution is different when positioned in the central and outer regions of the host halo today; while subhaloes tend to become more aspherical in the central 50% of their host's virial radius, the radial alignment weakens in the central regime while strengthening in the outer parts. We confirm that this is due to tidal torquing and the fact that subhaloes at pericentre move too fast for the alignment signal to respond.
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