45 research outputs found
Weak lensing detection of intra-cluster filaments with ground based data
According to the current standard model of Cosmology, matter in the Universe
arranges itself along a network of filamentary structure. These filaments
connect the main nodes of this so-called 'Cosmic Web', which are clusters of
galaxies. Although its large-scale distribution is clearly characterized by
numerical simulations, constraining the dark matter content of the cosmic web
in reality turns out to be difficult. The natural method of choice is
gravitational lensing. However, the direct detection and mapping of the elusive
filament signal is challenging and in this work we present two
methods,specifically tailored to achieve this task. A linear matched filter
aims at the detection of the smooth mass component of filaments and is
optimized to perform a shear decomposition that follows the anisotropic
component of the lensing signal. Filaments clearly inherit this property due to
their morphology. At the same time, the contamination arising from the central
massive cluster is controlled in a natural way. The filament 1 {\sigma}
detection is of about {\kappa} ~ 0.01-0.005 according to the filter's template
width and length, enabling the detection of structures out of reach with other
approaches. The second, complementary method seeks to detect the clumpy
component of filaments. The detection is determined by the number density of
sub-clump identifications in an area enclosing the potential filament, as it
was found within the observed field with the filter approach. We test both
methods against Mock observations based on realistic N-Body simulations of
filamentary structure and prove the feasibility of detecting filaments with
ground-based data.Comment: 9 pages, 7 figures. Submitted to A&A. Comments very welcom
Multi-color detection of gravitational arcs
Strong gravitational lensing provides fundamental insights into the
understanding of the dark matter distribution in massive galaxies, galaxy
clusters and the background cosmology. Despite their importance, the number of
gravitational arcs discovered so far is small. The urge for more complete,
large samples and unbiased methods of selecting candidates is rising. A number
of methods for the automatic detection of arcs have been proposed in the
literature, but large amounts of spurious detections retrieved by these methods
forces observers to visually inspect thousands of candidates per square degree
in order to clean the samples. This approach is largely subjective and requires
a huge amount of eye-ball checking, especially considering the actual and
upcoming wide field surveys, which will cover thousands of square degrees. In
this paper we study the statistical properties of colours of gravitational arcs
detected in the 37 deg^2 of the CARS survey. We have found that most of them
lie in a relatively small region of the (g'-r',r'-i') colour-colour diagram. To
explain this property, we provide a model which includes the lensing optical
depth expected in a LCDM cosmology that, in combination with the sources'
redshift distribution of a given survey, in our case CARS, peaks for sources at
redshift z~1. By further modelling the colours derived from the SED of the
galaxies dominating the population at that redshift, the model well reproduces
the observed colours. By taking advantage of the colour selection suggested by
both data and model, we show that this multi-band filtering returns a sample
83% complete and a contamination reduced by a factor of ~6.5 with respect to
the single-band arcfinder sample. New arc candidates are also proposed.Comment: 13 pages, 7 figures, 4 tables; title modified, text extended, figures
improved, error estimate improve
Camouflaged galactic CMB foregrounds: total and polarized contributions of the kinetic Sunyaev Zeldovich effect
We consider the role of the galactic kinetic Sunyaev Zeldovich (SZ) effect as
a CMB foreground. While the galactic thermal Sunyaev Zeldovich effect has
previously been studied and discarded as a potential CMB foreground, we find
that the kinetic SZ effect is dominant in the galactic case. We analyse the
detectability of the kinetic SZ effect by means of an optimally matched filter
technique applied to a simulation of an ideal observation. We obtain no
detection, getting a S/N ratio of 0.1, thereby demonstrating that the kinetic
SZ effect can also safely be ignored as a CMB foreground. However we provide
maps of the expected signal for inclusion in future high precision data
processing. Furthermore, we rule out the significant contamination of the
polarised CMB signal by second scattering of galactic kinetic Sunyaev-Zeldovich
photons, since we show that the scattering of the CMB quadrupole photons by
galactic electrons is a stronger effect than the Sunyaev Zeldovich second
scattering, and has already been shown to produce no significant polarised
contamination. We confirm the latter assessment also by means of an optimally
matched filter.Comment: 9 pages, 6 figures, 2 tables. submitte
AMICO: optimised detection of galaxy clusters in photometric surveys
We present AMICO (Adaptive Matched Identifier of Clustered Objects), a new
algorithm for the detection of galaxy clusters in photometric surveys. AMICO is
based on the Optimal Filtering technique, which allows to maximise the
signal-to-noise ratio of the clusters. In this work we focus on the new
iterative approach to the extraction of cluster candidates from the map
produced by the filter. In particular, we provide a definition of membership
probability for the galaxies close to any cluster candidate, which allows us to
remove its imprint from the map, allowing the detection of smaller structures.
As demonstrated in our tests, this method allows the deblending of close-by and
aligned structures in more than of the cases for objects at radial
distance equal to or redshift distance equal to , being the typical uncertainty of photometric redshifts.
Running AMICO on mocks derived from N-body simulations and semi-analytical
modelling of the galaxy evolution, we obtain a consistent mass-amplitude
relation through the redshift range , with a logarithmic slope
and a logarithmic scatter . The fraction of false
detections is steeply decreasing with S/N, and negligible at S/N > 5.Comment: 18 pages, accepted for publication in MNRA
AMICO galaxy clusters in KiDS-DR3: sample properties and selection function
We present the first catalogue of galaxy cluster candidates derived from the
third data release of the Kilo Degree Survey (KiDS-DR3). The sample of clusters
has been produced using the Adaptive Matched Identifier of Clustered Objects
(AMICO) algorithm. In this analysis AMICO takes advantage of the luminosity and
spatial distribution of galaxies only, not considering colours. In this way, we
prevent any selection effect related to the presence or absence of the
red-sequence in the clusters. The catalogue contains 7988 candidate galaxy
clusters in the redshift range 0.13.5 with a purity
approaching 95% over the entire redshift range. In addition to the catalogue of
galaxy clusters we also provide a catalogue of galaxies with their
probabilistic association to galaxy clusters. We quantify the sample purity,
completeness and the uncertainties of the detection properties, such as
richness, redshift, and position, by means of mock galaxy catalogues derived
directly from the data. This preserves their statistical properties including
photo-z uncertainties, unknown absorption across the survey, missing data,
spatial correlation of galaxies and galaxy clusters. Being based on the real
data, such mock catalogues do not have to rely on the assumptions on which
numerical simulations and semi-analytic models are based on. This paper is the
first of a series of papers in which we discuss the details and physical
properties of the sample presented in this work.Comment: 16 pages, 14 figures, 3 tables, submitted to MNRA