19 research outputs found
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 Îr. 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 Îr 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 Îr are good indicators of a halo's dynamical state, showing strong positive correlations with recent mass accretion and merging activity, but we argue that Îr provides a more robust and better defined measure of dynamical state for use in cosmological N-body simulations at zâ 0. We find that ÎrâČ 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 InnovaciĂłn (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 on the spectral index of the linear matter
power spectrum using cosmological -body simulations of scale-free
models (i.e. ). 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
is allowed to vary while preserving the asymptotic form at large
radii, we obtain preferred central slopes for haloes in each of our models.
There is a strong correlation between and , such that
becomes shallower as becomes steeper. However, if we normalise our mass
profiles by , 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
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 population of subhaloes in an environment similar
to our own. We find that above a certain mass cut, 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
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
SubHaloes going Notts: The SubHalo-Finder Comparison Project
We present a detailed comparison of the substructure properties of a single
Milky Way sized dark matter halo from the Aquarius suite at five different
resolutions, as identified by a variety of different (sub-)halo finders for
simulations of cosmic structure formation. These finders span a wide range of
techniques and methodologies to extract and quantify substructures within a
larger non-homogeneous background density (e.g. a host halo). This includes
real-space, phase-space, velocity-space and time- space based finders, as well
as finders employing a Voronoi tessellation, friends-of-friends techniques, or
refined meshes as the starting point for locating substructure.A common
post-processing pipeline was used to uniformly analyse the particle lists
provided by each finder. We extract quantitative and comparable measures for
the subhaloes, primarily focusing on mass and the peak of the rotation curve
for this particular study. We find that all of the finders agree extremely well
on the presence and location of substructure and even for properties relating
to the inner part part of the subhalo (e.g. the maximum value of the rotation
curve). For properties that rely on particles near the outer edge of the
subhalo the agreement is at around the 20 per cent level. We find that basic
properties (mass, maximum circular velocity) of a subhalo can be reliably
recovered if the subhalo contains more than 100 particles although its presence
can be reliably inferred for a lower particle number limit of 20. We finally
note that the logarithmic slope of the subhalo cumulative number count is
remarkably consistent and <1 for all the finders that reached high resolution.
If correct, this would indicate that the larger and more massive, respectively,
substructures are the most dynamically interesting and that higher levels of
the (sub-)subhalo hierarchy become progressively less important.Comment: 16 pages, 7 figures, 2 tables, Accepted for MNRA
Haloes gone MAD: The Halo-Finder Comparison Project
[abridged] We present a detailed comparison of fundamental dark matter halo
properties retrieved by a substantial number of different halo finders. These
codes span a wide range of techniques including friends-of-friends (FOF),
spherical-overdensity (SO) and phase-space based algorithms. We further
introduce a robust (and publicly available) suite of test scenarios that allows
halo finder developers to compare the performance of their codes against those
presented here. This set includes mock haloes containing various levels and
distributions of substructure at a range of resolutions as well as a
cosmological simulation of the large-scale structure of the universe. All the
halo finding codes tested could successfully recover the spatial location of
our mock haloes. They further returned lists of particles (potentially)
belonging to the object that led to coinciding values for the maximum of the
circular velocity profile and the radius where it is reached. All the finders
based in configuration space struggled to recover substructure that was located
close to the centre of the host halo and the radial dependence of the mass
recovered varies from finder to finder. Those finders based in phase space
could resolve central substructure although they found difficulties in
accurately recovering its properties. Via a resolution study we found that most
of the finders could not reliably recover substructure containing fewer than
30-40 particles. However, also here the phase space finders excelled by
resolving substructure down to 10-20 particles. By comparing the halo finders
using a high resolution cosmological volume we found that they agree remarkably
well on fundamental properties of astrophysical significance (e.g. mass,
position, velocity, and peak of the rotation curve).Comment: 27 interesting pages, 20 beautiful figures, and 4 informative tables
accepted for publication in MNRAS. The high-resolution version of the paper
as well as all the test cases and analysis can be found at the web site
http://popia.ft.uam.es/HaloesGoingMA
CLARA's view on the escape fraction of Lyman-Alpha photons in high redshift galaxies
Using CLARA (Code for Lyman Alpha Radiation Analysis) we constrain the escape
fraction of Lyman-Alpha radiation in galaxies in the redshift range 5<z<7,
based on the MareNostrum High-z Universe, a SPH cosmological simulation with
more than 2 billion particles. We approximate Lyman-Alpha Emitters (LAEs) as
dusty gaseous slabs with Lyman-Alpha radiation sources homogeneously mixed in
the gas. Escape fractions for such a configuration and for different gas and
dust contents are calculated using our newly developed radiative transfer code
CLARA. The results are applied to the MareNostrum High-z Universe numerical
galaxies. The model shows a weak redshift evolution and good agreement with
estimations of the escape fraction as a function of reddening from observations
at z \sim 2.2 and z \sim 3. We extend the slab model by including additional
dust in a clumpy component in order to reproduce the UV con- tinuum luminosity
function and UV colours at redshifts z>~5. The LAE Luminosity Function (LF)
based on the extended clumpy model reproduces broadly the bright end of the LF
derived from observations at z \sim 5 and z \sim 6. At z \sim 7 our model
over-predicts the LF by roughly a factor of four, presumably because the
effects of the neutral intergalactic medium are not taken into account. The
remaining tension between the observed and simulated faint end of the LF, both
in the UV-continuum and Lyman-Alpha at redshifts z \sim 5 and z \sim 6 points
towards an overabundance of simulated LAEs hosted in haloes of masses
1.0x10^10h-1Msol < Mh < 4.0x10^10h-1Msol. Given the difficulties in explaining
the observed overabundance by dust absorption, a probable origin of the
mismatch are the high star formation rates in the simulated haloes around the
quoted mass range. A more efficient supernova feedback should be able to
regulate the star formation process in the shallow potential wells of these
haloes.Comment: 17 pages, 9 figures. Accepted for publication in MNRA