99 research outputs found
Accretion History of Subhalo Population now and then
In the standard model of structure formation galaxies reside in virialized
dark matter haloes which extend much beyond the observational radius of the
central system. The dark matter halo formation process is hierarchical, small
systems collapse at high redshift and then merge together forming larger ones.
In this work we study the mass assembly history of host haloes at different
observation redshifts and the mass function of accreted satellites (haloes that
merge directly on the main halo progenitor). We show that the satellite mass
function is universal, both independent on the host halo mass and observation
redshift. The satellite mass function also turn out to be universal once only
satellites before or after the host halo formation redshift (time at which the
main halo progenitor assembles half of its final mass) are considered. We show
that the normalizations of these distributions are directly related to the main
halo progenitor mass distributions before and after its formation, while their
slope and the exponential high mass cut-off remain unchanged.Comment: To appear in the proceedings of the "Invisible Universe International
Conference 2009" (6 pages, 3 figures
Weak Lensing Light-Cones in Modified Gravity simulations with and without Massive Neutrinos
We present a novel suite of cosmological N-body simulations called the
DUSTGRAIN-pathfinder, implementing simultaneously the effects of an extension
to General Relativity in the form of gravity and of a non-negligible
fraction of massive neutrinos. We describe the generation of simulated weak
lensing and cluster counts observables within a past light-cone extracted from
these simulations. The simulations have been performed by means of a
combination of the MG-GADGET code and a particle-based implementation of
massive neutrinos, while the light-cones have been generated using the MapSim
pipeline allowing us to compute weak lensing maps through a ray-tracing
algorithm for different values of the source plane redshift. The mock
observables extracted from our simulations will be employed for a series of
papers focussed on understanding and possibly breaking the well-known
observational degeneracy between gravity and massive neutrinos, i.e. the
fact that some specific combinations of the characteristic parameters for these
two phenomena (the scalar amplitude and the total neutrino mass
) may result indistinguishable from the standard
cosmology through several standard observational probes.
In particular, in the present work we show how a tomographic approach to weak
lensing statistics could allow - especially for the next generation of
wide-field surveys - to disentangle some of the models that appear
statistically indistinguishable through standard single-redshift weak lensing
probe.Comment: accepted for publication in MNRAS, added theoretical comparisons to
the simulation measurement
The mass-concentration relation in lensing clusters: the role of statistical biases and selection effects
The relation between mass and concentration of galaxy clusters traces their
formation and evolution. Massive lensing clusters were observed to be
over-concentrated and following a steep scaling in tension with predictions
from the concordance CDM paradigm. We critically revise the relation
in the CLASH, the SGAS, the LOCUSS, and the high-redshift samples of weak
lensing clusters. Measurements of mass and concentration are anti-correlated,
which can bias the observed relation towards steeper values. We corrected for
this bias and compared the measured relation to theoretical predictions
accounting for halo triaxiality, adiabatic contraction of the halo, presence of
a dominant BCG and, mostly, selection effects in the observed sample. The
normalisation, the slope and the scatter of the expected relation are strongly
sample-dependent. For the considered samples, the predicted slope is much
steeper than that of the underlying relation characterising dark-matter only
clusters. We found that the correction for statistical and selection biases in
observed relations mostly solve the tension with the CDM model.Comment: 13 pages, 3 figures; v2: 14 pages, minor changes, in press on MNRA
GLAMER Part II: Multiple Plane Gravitational Lensing
We present an extension to multiple planes of the gravitational lensing code
{\small GLAMER}. The method entails projecting the mass in the observed
light-cone onto a discrete number of lens planes and inverse ray-shooting from
the image to the source plane. The mass on each plane can be represented as
halos, simulation particles, a projected mass map extracted form a numerical
simulation or any combination of these. The image finding is done in a source
oriented fashion, where only regions of interest are iteratively refined on an
initially coarse image plane grid. The calculations are performed in parallel
on shared memory machines. The code is able to handle different types of
analytic halos (NFW, NSIE, power-law, etc.), haloes extracted from numerical
simulations and clusters constructed from semi-analytic models ({\small MOKA}).
Likewise, there are several different options for modeling the source(s) which
can be distributed throughout the light-cone. The distribution of matter in the
light-cone can be either taken from a pre-existing N-body numerical
simulations, from halo catalogs, or are generated from an analytic mass
function. We present several tests of the code and demonstrate some of its
applications such as generating mock images of galaxy and galaxy cluster
lenses.Comment: 14 pages, 10 figures, submitted to MNRA
Accretion of satellites onto central galaxies in clusters: merger mass ratios and orbital parameters
We study the statistical properties of mergers between central and satellite
galaxies in galaxy clusters in the redshift range , using a sample of
dark-matter only cosmological N-body simulations from Le SBARBINE dataset.
Using a spherical overdensity algorithm to identify dark-matter haloes, we
construct halo merger trees for different values of the over-density
. While the virial overdensity definition allows us to probe the
accretion of satellites at the cluster virial radius , higher
overdensities probe satellite mergers in the central region of the cluster,
down to , which can be considered a proxy for the
accretion of satellite galaxies onto central galaxies. We find that the
characteristic merger mass ratio increases for increasing values of :
more than of the mass accreted by central galaxies since
comes from major mergers. The orbits of satellites accreting onto central
galaxies tend to be more tangential and more bound than orbits of haloes
accreting at the virial radius. The obtained distributions of merger mass
ratios and orbital parameters are useful to model the evolution of the
high-mass end of the galaxy scaling relations without resorting to hydrodynamic
cosmological simulations.Comment: accepted by MNRAS (minor comments
Predicting the number of giant arcs expected in the next generation wide-field surveys from space
In this paper we estimate the number of gravitational arcs detectable in a
wide-field survey such as that which will be operated by the Euclid space
mission, assuming a {\Lambda}CDM cosmology. We use the publicly available code
MOKA to obtain realistic deflection angle maps of mock gravitational lenses.
The maps are processed by a ray-tracing code to estimate the strong lensing
cross sections of each lens. Our procedure involves 1) the generation of a
light-cone which is populated with lenses drawn from a theoretical
mass-function; 2) the modeling of each single lens using a triaxial halo with a
NFW (Navarro-Frenk-White) density profile and theoretical concentration-mass
relation, including substructures, 3) the determination of the lensing cross
section as a function of redshift for each lens in the light-cone, 4) the
simulation of mock observations to characterize the redshift distribution of
sources that will be detectable in the Euclid images. We focus on the so-called
giant arcs, i.e. gravitational arcs characterized by large length-to-width
ratios (l/w > 5, 7.5 and 10). We quantify the arc detectability at different
significances above the level of the background. Performing 128 different
realizations of a 15,000 sq. degree survey, we find that the number of arcs
detectable at 1{\sigma} above the local background will be 8912,2914, and 1275
for l/w>5, 7.5 and 10, respectively. The expected arc numbers decrease to 2409,
790, and 346 for a detection limit at 3{\sigma} above the background level.
From our analysis, we find that most of the lenses which contribute to the
lensing optical depth are located at redshifts 0.4<zl<0.7 and that the 50% of
the arcs are images of sources at zs > 3. This is the first step towards the
full characterization of the population of strong lenses that will be observed
by Euclid. [abridged]Comment: replaced to match the accepted version by MNRAS, 12 pag, 10 fig -
more references adde
A look to the inside of haloes: a characterisation of the halo shape as a function of overdensity in the Planck cosmology
In this paper we study the triaxial properties of dark matter haloes of a
wide range of masses extracted from a set of cosmological N-body simulations.
We measure the shape at different distances from the halo centre (characterised
by different overdensity thresholds), both in three and in two dimensions. We
discuss how halo triaxiality increases with mass, redshift and distance from
the halo centre. We also examine how the orientation of the different
ellipsoids are aligned with each other and what is the gradient in internal
shapes for halos with different virial configurations. Our findings highlight
that the internal part of the halo retains memory of the violent formation
process keeping the major axis oriented toward the preferential direction of
the in-falling material while the outer part becomes rounder due to continuous
isotropic merging events. This effect is clearly evident in high mass haloes -
which formed more recently - while it is more blurred in low mass haloes. We
present simple distributions that may be used as priors for various mass
reconstruction algorithms, operating in different wavelengths, in order to
recover a more complex and realistic dark matter distribution of isolated and
relaxed systems.Comment: accepted for publication by MNRAS (15 pag. and 14 fig.
Characterizing dark interactions with the halo mass accretion history and structural properties
We study the halo mass accretion history (MAH) and its correlation with the
internal structural properties in coupled dark energy cosmologies (cDE). To
accurately predict all the non-linear effects caused by dark interactions, we
use the COupled Dark Energy Cosmological Simulations (CoDECS). We measure the
halo concentration at z=0 and the number of substructures above a mass
resolution threshold for each halo. Tracing the halo merging history trees back
in time, following the mass of the main halo, we develope a MAH model that
accurately reproduces the halo growth in term of M_{200} in the {\Lambda}CDM
Universe; we then compare the MAH in different cosmological scenarios. For cDE
models with a weak constant coupling, our MAH model can reproduce the
simulation results, within 10% of accuracy, by suitably rescaling the
normalization of the linear matter power spectrum at z=0, {\sigma}_8. However,
this is not the case for more complex scenarios, like the "bouncing" cDE model,
for which the numerical analysis shows a rapid growth of haloes at high
redshifts, that cannot be reproduced by simply rescaling the value of
{\sigma}_8. Moreover, at fixed value of {\sigma}_8, cold dark matter (CDM)
haloes in these cDE scenarios tend to be more concentrated and have a larger
amount of substructures with respect to {\Lambda}CDM predictions. Finally, we
present an accurate model that relates the halo concentration to the time at
which it assembles half or 4% of its mass. Combining this with our MAH model,
we show how halo concentrations change while varying only {\sigma}_8 in a
{\Lambda}CDM Universe, at fixed halo mass.Comment: 18 pages, 14 figures, accepted for publication in MNRA
The mass accretion rate of galaxy clusters: a measurable quantity
We explore the possibility of measuring the mass accretion rate (MAR) of
galaxy clusters from their mass profiles beyond the virial radius . We
derive the accretion rate from the mass of a spherical shell whose inner radius
is , whose thickness changes with redshift, and whose infall velocity
is assumed to be equal to the mean infall velocity of the spherical shells of
dark matter halos extracted from -body simulations. This approximation is
rather crude in hierarchical clustering scenarios where both smooth accretion
and aggregation of smaller dark matter halos contribute to the mass accretion
of clusters.Nevertheless, in the redshift range , our prescription
returns an average MAR within of the average rate derived from the
merger trees of dark matter halos extracted from -body simulations. The MAR
of galaxy clusters has been the topic of numerous detailed numerical and
theoretical investigations, but so far it has remained inaccessible to
measurements in the real universe. Since the measurement of the mass profile of
clusters beyond their virial radius can be performed with the caustic technique
applied to dense redshift surveys of the cluster outer regions, our result
suggests that measuring the mean MAR of a sample of galaxy clusters is actually
feasible. We thus provide a new potential observational test of the
cosmological and structure formation models.Comment: 11 pages, 7 figures, 5 tables, minor text modifications to match the
published version, typos correcte
Mass and Concentration estimates from Weak and Strong Gravitational Lensing: a Systematic Study
We study how well halo properties of galaxy clusters, like mass and
concentration, are recovered using lensing data. In order to generate a large
sample of systems at different redshifts we use the code MOKA. We measure halo
mass and concentration using weak lensing data alone (WL), fitting to an NFW
profile the reduced tangential shear profile, or by combining weak and strong
lensing data, by adding information about the size of the Einstein radius
(WL+SL). For different redshifts, we measure the mass and the concentration
biases and find that these are mainly caused by the random orientation of the
halo ellipsoid with respect to the line-of-sight. Since our simulations account
for the presence of a bright central galaxy, we perform mass and concentration
measurements using a generalized NFW profile which allows for a free inner
slope. This reduces both the mass and the concentration biases. We discuss how
the mass function and the concentration mass relation change when using WL and
WL+SL estimates. We investigate how selection effects impact the measured
concentration-mass relation showing that strong lens clusters may have a
concentration 20-30% higher than the average, at fixed mass, considering also
the particular case of strong lensing selected samples of relaxed clusters.
Finally, we notice that selecting a sample of relaxed galaxy clusters, as is
done in some cluster surveys, explain the concentration-mass relation biases.Comment: (1) DIFA-UniBO, (2) INAF-OABo, (3) INFN-BO, (4) JPL-Pasadena 18
pages, 19 figures - accepted for publication by MNRAS, two figures added for
comparison with SGAS-SDSS and LoCuSS cluster
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