The dynamical state of dark matter haloes in cosmological simulations - I. Correlations with mass assembly history

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©2011 RAS © 2011 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.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 η and the centre of mass offset Δr. 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 increase with increasing Mvir and decreasing Zform. We then show that both η and Δr 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 η and Δr are good indicators of a halo's dynamical state, showing strong positive correlations with recent mass accretion and merging activity, but we argue that Δr provides a more robust and better defined measure of dynamical state for use in cosmological N-body simulations at z≃ 0. We find that Δr≲ 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 implicationsAK is supported by the Spanish Ministerio de Ciencia e Innovación (MICINN) in Spain through the Ramon y Cajal programme as well as the grants AYA 2009-13875-C03-02, AYA2009-12792-C03-03, CSD2009- 00064 and CAMS2009/ESP-1496. He acknowledges support by the MICINN under the Consolider-Ingenio, SyeC project CSD- 2007-0005

Dark Matter Halo Profiles in Scale-Free Cosmologies

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.Comment: 9 pages, 4 figures. Accepted for publication in MNRA

The luminosities of backsplash galaxies in constrained simulations of the Local Group

We study the differences and similarities in the luminosities of bound, infalling and the so-called backsplash (Gill et al. 2005) galaxies of the Milky Way and M31 using a hydrodynamical simulation performed within the Constrained Local UniversE Simulation (CLUES) project. The simulation models the formation of the Local Group within a self-consistent cosmological framework. We find that even though backsplash galaxies passed through the virial radius of their host halo and hence may have lost a (significant) fraction of their mass, their stellar populations are hardly affected. This leaves us with comparable luminosity functions for infalling and backsplash galaxies and hence little hope to decipher their past (and different) formation and evolutionary histories by luminosity measurements alone. Nevertheless, due to the tidal stripping of dark matter we find that the mass-to-light ratios have changed when comparing the various populations against each other: they are highest for the infalling galaxies and lowest for the bound satellites with the backsplash galaxies in-between.Comment: 9 pages, 10 figures, 1 table, accepted for publication in MNRA

On the starting redshift for cosmological simulations: Focusing on halo properties

We systematically study the effects of varying the starting redshift z_i for cosmological simulations in the highly non-linear regime. Our primary focus lies with the (individual) properties of dark matter halos -- namely the mass, spin, triaxiality, and concentration -- where we find that even substantial variations in z_i leave only a small imprint, at least for the probed mass range M \in [10^{10}, 10^{13}] Msun/h and when investigated at redshift z=0. We further compare simulations started by using the standard Zel'dovich approximation to runs based upon initial conditions produced with second order Lagrangian perturbation theory. Here we observe the same phenomenon, i.e. that differences in the studied (internal) properties of dark matter haloes are practically undetectable. These findings are (for the probed mass range) in agreement with other work in the literature. We therefore conclude that the commonly used technique for setting up cosmological simulations leads to stable results at redshift z=0 for the mass, the spin parameter, the triaxiality, and the concentration of dark matter haloes.Comment: 10 pages, 9 figures, accepted for publication in Ap

The impact of baryonic physics on the shape and radial alignment of substructures in cosmological dark matter haloes

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.Comment: 10 pages, 8 figures, 2 tables, accepted for publication in MNRAS, replaced with proof-corrected version (minor typos

Constrained simulations of the Local Group: on the radial distribution of substructures

We examine the properties of satellites found in high resolution simulations of the local group. We use constrained simulations designed to reproduce the main dynamical features that characterize the local neighborhood, i.e. within tens of Mpc around the Local Group (LG). Specifically, a LG-like object is found located within the 'correct' dynamical environment and consisting of three main objects which are associated with the Milky Way, M31 and M33. By running two simulations of this LG from identical initial conditions - one with and one without baryons modeled hydrodynamically - we can quantify the effect of gas physics on the $z=0$ population of subhaloes in an environment similar to our own. We find that above a certain mass cut, $M_{\rm sub} > 2\times10^{8}h^{-1} M_{\odot}$ subhaloes in hydrodynamic simulations are more radially concentrated than those in simulations with out gas. This is caused by the collapse of baryons into stars that typically sit in the central regions of subhaloes, making them denser. The increased central density of such a subhalo, results in less mass loss due to tidal stripping than the same subhalo simulated with only dark matter. The increased mass in hydrodynamic subhaloes with respect to dark matter ones, causes dynamical friction to be more effective, dragging the subhalo towards the centre of the host. This results in these subhaloes being effectively more radially concentrated then their dark matter counterparts.Comment: 12 pages, 9 figure

Cosmic variance of the local Hubble flow in large-scale cosmological simulations

The increasing precision in the determination of the Hubble parameter has reached a per cent level at which large-scale cosmic flows induced by inhomogeneities of the matter distribution become non-negligible. Here, we use large-scale cosmological N-body simulations to study statistical properties of the local Hubble parameter as measured by local observers. We show that the distribution of the local Hubble parameter depends not only on the scale of inhomogeneities, but also on how one defines the positions of observers in the cosmic web and what reference frame is used. Observers located in random dark matter haloes measure on average lower expansion rates than those at random positions in space or in the centres of cosmic voids, and this effect is stronger from the halo rest frames compared to the cosmic microwave background (CMB) rest frame. We compare the predictions for the local Hubble parameter with observational constraints based on Type Ia supernova (SNIa) and CMB observations. Due to cosmic variance, for observers located in random haloes we show that the Hubble constant determined from nearby SNIa may differ from that measured from the CMB by ±0.8 per cent at 1σ statistical significance. This scatter is too small to significantly alleviate a recently claimed discrepancy between current measurements assuming a flat Λ cold dark matter (ΛCDM) model. However, for observers located in the centres of the largest voids permitted by the standard ΛCDM model, we find that Hubble constant measurements from SNIa would be biased high by 5 per cent, rendering this tension non-existent in this extreme case

Ahf: Amiga's Halo Finder

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.Comment: 18 pages, 18 figures. Accepted for publication in ApJ