301 research outputs found

### LeMoMaF: Lensed Mock Map Facility

We present the Lensed Mock Map Facility (LeMoMaF), a tool designed to perform
mock weak lensing measurements on numerically simulated chunks of the universe.
Coupling N-body simulations to a semi-analytical model of galaxy formation,
LeMoMaF can create realistic lensed images and mock catalogues of galaxies, at
wavelengths ranging from the UV to the submm. To demonstrate the power of such
a tool we compute predictions of the source-lens clustering effect on the
convergence statistics, and quantify the impact of weak lensing on galaxy
counts in two different filters. We find that the source-lens clustering effect
skews the probability density function of the convergence towards low values,
with an intensity which strongly depends on the redshift distribution of
galaxies. On the other hand, the degree of enhancement or depletion in galaxy
counts due to weak lensing is independent of the source-lens clustering effect.
We discuss the impact on the two-points shear statistics to be measured by
future missions like SNAP and LSST. The source-lens clustering effect would
bias the estimation of sigma_8 from two point statistics by 2% -5%. We conclude
that accurate photometric redshifts for individual galaxies are necessary in
order to quantify and isolate the source-lens clustering effect.Comment: 14 pages, 11 figures, submitted to MNRA

### On the Convergence of the Milky Way and M31 Kinematics from Cosmological Simulations

The kinematics of the Milky Way (MW) and M31, the dominant galaxies in the
Local Group (LG), can be used to estimate the LG total mass. New results on the
M31 proper motion have recently been used to improve that estimate. Those
results are based on kinematic priors that are sometimes guided and evaluated
using cosmological N-body simulations. However, the kinematic properties of
simulated LG analogues could be biased due to the effective power spectrum
truncation induced by the small size of the parent simulation. Here we explore
the dependence of LG kinematics on the simulation box size to argue that
cosmological simulations need a box size on the order of 1 Gpc in order to
claim convergence on the LG kinematic properties. Using a large enough
simulation, we find M31 tangential and radial velocities relative to the MW to
be in the range $v_{\mathrm {tan}}=105^{+94}_{-59}$ km/s and $v_{\mathrm
{rad}}=-108^{+68}_{-81}$ km/s, respectively. This study highlights that LG
kinematics derived from N-body simulations have to be carefully interpreted
taking into account the size of the parent simulation.Comment: 7 pages, 4 figures, Accepted for publication in Ap

### The abundance of Bullet-groups in LCDM

We estimate the expected distribution of displacements between the two
dominant dark matter (DM) peaks (DM-DM displacements) and between DM and
gaseous baryon peak (DM-gas displacements) in dark matter halos with masses
larger than $10^{13}$ Msun/h. We use as a benchmark the observation of SL2S
J08544-0121, which is the lowest mass system ($1.0\times 10^{14}$ Msun/h)
observed so far featuring a bi-modal dark matter distribution with a dislocated
gas component. We find that $(50 \pm 10)$% of the dark matter halos with
circular velocities in the range 300 km/s to 700 km/s (groups) show DM-DM
displacements equal or larger than $186 \pm 30$ kpc/h as observed in SL2S
J08544-0121. For dark matter halos with circular velocities larger than 700
km/s (clusters) this fraction rises to 70 $\pm$ 10%. Using the same simulation
we estimate the DM-gas displacements and find that 0.1 to 1.0% of the groups
should present separations equal or larger than $87\pm 14$kpc/h corresponding
to our observational benchmark; for clusters this fraction rises to (7 $\pm$
3)%, consistent with previous studies of dark matter to baryon separations.
Considering both constraints on the DM-DM and DM-gas displacements we find that
the number density of groups similar to SL2S J08544-0121 is $\sim 6.0\times
10^{-7}$ Mpc$^{-3}$, three times larger than the estimated value for clusters.
These results open up the possibility for a new statistical test of LCDM by
looking for DM-gas displacements in low mass clusters and groups.Comment: 6 pages, 3 figures, accepted for publication in ApJ Letter

### The Coarse Geometry of Merger Trees in \Lambda CDM

We introduce the contour process to describe the geometrical properties of
merger trees. The contour process produces a one-dimensional object, the
contour walk, which is a translation of the merger tree. We portray the contour
walk through its length and action. The length is proportional to to the number
of progenitors in the tree, and the action can be interpreted as a proxy of the
mean length of a branch in a merger tree.
We obtain the contour walk for merger trees extracted from the public
database of the Millennium Run and also for merger trees constructed with a
public Monte-Carlo code which implements a Markovian algorithm. The trees
correspond to halos of final masses between 10^{11} h^{-1} M_sol and 10^{14}
h^{-1} M_sol. We study how the length and action of the walks evolve with the
mass of the final halo. In all the cases, except for the action measured from
Markovian trees, we find a transitional scale around 3 \times 10^{12} h^{-1}
M_sol. As a general trend the length and action measured from the Markovian
trees show a large scatter in comparison with the case of the Millennium Run
trees.Comment: 7 pages, 5 figures, submitted to MNRA

### Characterizing SL2S galaxy groups using the Einstein radius

We analyzed the Einstein radius, $\theta_E$, in our sample of SL2S galaxy
groups, and compared it with $R_A$ (the distance from the arcs to the center of
the lens), using three different approaches: 1.- the velocity dispersion
obtained from weak lensing assuming a Singular Isothermal Sphere profile
($\theta_{E,I}$), 2.- a strong lensing analytical method ($\theta_{E,II}$)
combined with a velocity dispersion-concentration relation derived from
numerical simulations designed to mimic our group sample, 3.- strong lensing
modeling ($\theta_{E,III}$) of eleven groups (with four new models presented in
this work) using HST and CFHT images. Finally, $R_A$ was analyzed as a function
of redshift $z$ to investigate possible correlations with L, N, and the
richness-to-luminosity ratio (N/L). We found a correlation between $\theta_{E}$
and $R_A$, but with large scatter. We estimate $\theta_{E,I}$ = (2.2 $\pm$ 0.9)
+ (0.7 $\pm$ 0.2)$R_A$, $\theta_{E,II}$ = (0.4 $\pm$ 1.5) + (1.1 $\pm$
0.4)$R_A$, and $\theta_{E,III}$ = (0.4 $\pm$ 1.5) + (0.9 $\pm$ 0.3)$R_A$ for
each method respectively. We found a weak evidence of anti-correlation between
$R_A$ and $z$, with Log$R_A$ = (0.58$\pm$0.06) - (0.04$\pm$0.1)$z$, suggesting
a possible evolution of the Einstein radius with $z$, as reported previously by
other authors. Our results also show that $R_A$ is correlated with L and N
(more luminous and richer groups have greater $R_A$), and a possible
correlation between $R_A$ and the N/L ratio. Our analysis indicates that $R_A$
is correlated with $\theta_E$ in our sample, making $R_A$ useful to
characterize properties like L and N (and possible N/L) in galaxy groups.
Additionally, we present evidence suggesting that the Einstein radius evolves
with $z$.Comment: Accepted for publication in Astronomy & Astrophysics. Typos correcte

### Halo based reconstruction of the cosmic mass density field

We present the implementation of a halo based method for the reconstruction
of the cosmic mass density field. The method employs the mass density
distribution of dark matter haloes and its environments computed from
cosmological N-body simulations and convolves it with a halo catalog to
reconstruct the dark matter density field determined by the distribution of
haloes. We applied the method to the group catalog of Yang etal (2007) built
from the SDSS Data Release 4. As result we obtain reconstructions of the cosmic
mass density field that are independent on any explicit assumption of bias. We
describe in detail the implementation of the method, present a detailed
characterization of the reconstructed density field (mean mass density
distribution, correlation function and counts in cells) and the results of the
classification of large scale environments (filaments, voids, peaks and sheets)
in our reconstruction. Applications of the method include morphological studies
of the galaxy population on large scales and the realization of constrained
simulations.Comment: Accepted for publication in MNRA

### Unbiased clustering estimates with the DESI fibre assignment

The Emission Line Galaxy survey made by the Dark Energy Spectroscopic
Instrument (DESI) survey will be created from five passes of the instrument on
the sky. On each pass, the constrained mobility of the ends of the fibres in
the DESI focal plane means that the angular-distribution of targets that can be
observed is limited. Thus, the clustering of samples constructed using a
limited number of passes will be strongly affected by missing targets. In two
recent papers, we showed how the effect of missing galaxies can be corrected
when calculating the correlation function using a weighting scheme for pairs.
Using mock galaxy catalogues we now show that this method provides an unbiased
estimator of the true correlation function for the DESI survey after any number
of passes. We use multiple mocks to determine the expected errors given one to
four passes, compared to an idealised survey observing an equivalent number of
randomly selected targets. On BAO scales, we find that the error is a factor 2
worse after one pass, but that after three or more passes, the errors are very
similar. Thus we find that the fibre assignment strategy enforced by the design
of DESI will not affect the cosmological measurements to be made by the survey,
and can be removed as a potential risk for this experiment.Comment: 11 pages, 8 figure

### Filaments in observed and mock galaxy catalogues

Context. The main feature of the spatial large-scale galaxy distribution is
an intricate network of galaxy filaments. Although many attempts have been made
to quantify this network, there is no unique and satisfactory recipe for that
yet. Aims. The present paper compares the filaments in the real data and in the
numerical models, to see if our best models reproduce statistically the
filamentary network of galaxies. Methods. We apply an object point process with
interactions (the Bisous process) to trace and describe the filamentary network
both in the observed samples (the 2dFGRS catalogue) and in the numerical models
that have been prepared to mimic the data.We compare the networks. Results. We
find that the properties of filaments in numerical models (mock samples) have a
large variance. A few mock samples display filaments that resemble the observed
filaments, but usually the model filaments are much shorter and do not form an
extended network. Conclusions. We conclude that although we can build numerical
models that are similar to observations in many respects, they may fail yet to
explain the filamentary structure seen in the data. The Bisous-built filaments
are a good test for such a structure.Comment: 13 pages, accepted for publication in Astronomy and Astrophysic

### A Dynamical Classification of the Cosmic Web

A dynamical classification of the cosmic web is proposed. The large scale
environment is classified into four web types: voids, sheets, filaments and
knots. The classification is based on the evaluation of the deformation tensor,
i.e. the Hessian of the gravitational potential, on a grid. The classification
is based on counting the number of eigenvalues above a certain threshold,
lambda_th at each grid point, where the case of zero, one, two or three such
eigenvalues corresponds to void, sheet, filament or a knot grid point. The
collection of neighboring grid points, friends-of-friends, of the same web
attribute constitutes voids, sheets, filaments and knots as web objects.
A simple dynamical consideration suggests that lambda_th should be
approximately unity, upon an appropriate scaling of the deformation tensor. The
algorithm has been applied and tested against a suite of (dark matter only)
cosmological N-body simulations. In particular, the dependence of the volume
and mass filling fractions on lambda_th and on the resolution has been
calculated for the four web types. Also, the percolation properties of voids
and filaments have been studied.
Our main findings are: (a) Already at lambda_th = 0.1 the resulting web
classification reproduces the visual impression of the cosmic web. (b) Between
0.2 < lambda_th < 0.4, a system of percolated voids coexists with a net of
interconected filaments. This suggests a reasonable choice for lambda_th as the
parameter that defines the cosmic web. (c) The dynamical nature of the
suggested classification provides a robust framework for incorporating
environmental information into galaxy formation models, and in particular the
semi-analytical ones.Comment: 11 pages, 6 figures, submitted to MNRA

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