203 research outputs found
How well can we determine cluster mass profiles from weak lensing?
Weak gravitational lensing provides a direct way to study the mass
distribution of clusters of galaxies at large radii. Unfortunately, large scale
structure along the line of sight also contributes to the lensing signal, and
consequently affects the measurements. We quantify the effect of distant
uncorrelated large scale structure on the inferred mass profile of clusters as
measured from weak lensing. We consider NFW profiles, and find that large scale
structure is a major source of uncertainty for most practical situations, when
a model, with the mass M_200 and the concentration parameter c as free
parameters, is fit to the observations. We find that the best constraints are
found for clusters at intermediate redshifts (z~0.3). For a cluster at z=0.3,
optimal results are obtained when the lensing signal is measured out to 10-15
arcminutes. Measurements at larger radii do not improve the accuracy with which
the profile can be determined, contrary to what is expected when the
contribution from large scale structure is ignored. The true uncertainties in
M_200 and the concentration parameter c are ~2 times larger than when distant
large scale structure is not included in the error budget.Comment: submitted to MNRA
Large scale bias and stochasticity of halos and dark matter
On large scales galaxies and their halos are usually assumed to trace the
dark matter with a constant bias and dark matter is assumed to trace the linear
density field. We test these assumption using several large N-body simulations
with 384^3-1024^3 particles and box sizes between 100-1000h/Mpc, which can both
resolve the small galactic size halos and sample the large scale fluctuations.
We explore the average halo bias relation as a function of halo mass and show
that existing fitting formulae overestimate the halo bias by up to 20% in the
regime just below the nonlinear mass. We propose a new expression that fits our
simulations well. We find that the halo bias is nearly constant, b~0.65-0.7,
for masses below one tenth of the nonlinear mass. We explore next the relation
between the initial and final dark matter in individual Fourier modes and show
that there are significant fluctuations in their ratio, ranging from 10% rms at
k~0.03h/Mpc to 50% rms at k~0.1h/Mpc. We argue that these large fluctuations
are caused by perturbative effects beyond the linear theory, which are
dominated by long wavelength modes with large random fluctuations. Similar or
larger fluctuations exist between halos and dark matter and between halos of
different mass. While these fluctuations are small compared to the sampling
variance, they are significant for attempts to determine the bias by relating
directly the maps of galaxies and dark matter or the maps of different galaxy
populations, which would otherwise be immune to sampling variance.Comment: 9 pages, 8 figures, matches accepted version in MNRA
Halo stochasticity in global clustering analysis
In the present work we study the statistics of haloes, which in the halo
model determines the distribution of galaxies. Haloes are known to be biased
tracer of dark matter, and at large scales it is usually assumed there is no
intrinsic stochasticity between the two fields. Following the work of Seljak &
Warren (2004), we explore how correct this assumption is and, moving a step
further, we try to qualify the nature of stochasticity. We use Principal
Component Analysis applied to the outputs of a cosmological N-body simulation
to: (1) explore the behaviour of stochasticity in the correlation between
haloes of different masses; (2) explore the behaviour of stochasticity in the
correlation between haloes and dark matter. We show results obtained using a
catalogue with 2.1 million haloes, from a PMFAST simulation with box size of
1000h^{-1}Mpc. In the relation between different populations of haloes we find
that stochasticity is not-negligible even at large scales. In agreement with
the conclusions of Tegmark & Bromley (1999) who studied the correlations of
different galaxy populations, we found that the shot-noise subtracted
stochasticity is qualitatively different from `enhanced' shot noise and,
specifically, it is dominated by a single stochastic eigenvalue. We call this
the `minimally stochastic' scenario, as opposed to shot noise which is
`maximally stochastic'. In the correlation between haloes and dark matter, we
find that stochasticity is minimized, as expected, near the dark matter peak (k
~ 0.02 h Mpc^{-1} for a LambdaCDM cosmology) and, even at large scales, it is
of the order of 15 per cent above the shot noise. Moreover, we find that the
reconstruction of the dark matter distribution is improved when we use
eigenvectors as tracers of the bias. [Abridged]Comment: 9 pages, 12 figures. Submitted to MNRA
Optimal capture of non-Gaussianity in weak lensing surveys: power spectrum, bispectrum and halo counts
We compare the efficiency of weak lensing-selected galaxy clusters counts and
of the weak lensing bispectrum at capturing non-Gaussian features in the dark
matter distribution. We use the halo model to compute the weak lensing power
spectrum, the bispectrum and the expected number of detected clusters, and
derive constraints on cosmological parameters for a large, low systematic weak
lensing survey, by focusing on the - plane and on the dark
energy equation of state. We separate the power spectrum into the resolved and
the unresolved parts of the data, the resolved part being defined as detected
clusters, and the unresolved part as the rest of the field. We consider four
kinds of clusters counts, taking into account different amount of information :
signal-to-noise ratio peak counts; counts as a function of clusters' mass;
counts as a function of clusters' redshift; and counts as a function of
clusters' mass and redshift. We show that when combined with the power
spectrum, those four kinds of counts provide similar constraints, thus allowing
one to perform the most direct counts, signal-to-noise peaks counts, and get
percent level constraints on cosmological parameters. We show that the weak
lensing bispectrum gives constraints comparable to those given by the power
spectrum and captures non-Gaussian features as well as clusters counts, its
combination with the power spectrum giving errors on cosmological parameters
that are similar to, if not marginally smaller than, those obtained when
combining the power spectrum with cluster counts. We finally note that in order
to reach its potential, the weak lensing bispectrum must be computed using all
triangle configurations, as equilateral triangles alone do not provide useful
information.Comment: Matches ApJ-accepted versio
Weak Lensing by Galaxies in Groups and Clusters: I.--Theoretical Expectations
Galaxy-galaxy lensing is rapidly becoming one of the most promising means to
accurately measure the average relation between galaxy properties and halo
mass. In order to obtain a signal of sufficient signal-to-noise, one needs to
stack many lens galaxies according to their property of interest, such as
luminosity or stellar mass. Since such a stack consists of both central and
satellite galaxies, which contribute very different lensing signals, the
resulting shear measurements can be difficult to interpret. In the past,
galaxy-galaxy lensing studies have either completely ignored this problem, have
applied rough isolation criteria in an attempt to preferentially select
`central' galaxies, or have tried to model the contribution of satellites
explicitely. However, if one is able to {\it a priori} split the galaxy
population in central and satellite galaxies, one can measure their lensing
signals separately. This not only allows a much cleaner measurement of the
relation between halo mass and their galaxy populations, but also allows a
direct measurement of the sub-halo masses around satellite galaxies. In this
paper, we use a realistic mock galaxy redshift survey to show that galaxy
groups, properly selected from large galaxy surveys, can be used to accurately
split the galaxy population in centrals and satellites. Stacking the resulting
centrals according to their group mass, estimated from the total group
luminosity, allows a remarkably accurate recovery of the masses and density
profiles of their host haloes. In addition, stacking the corresponding
satellite galaxies according to their projected distance from the group center
yields a lensing signal that can be used to accurate measure the masses of both
sub-haloes and host haloes. (Abridged)Comment: 16 pages, 10 figures, Accepted for publication in MNRA
Spring migration of Black-tailed Godwits in Iberia 2015:Mission Report Sado, Tejo and Extremadura
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