84 research outputs found
Lensed: a code for the forward reconstruction of lenses and sources from strong lensing observations
Robust modelling of strong lensing systems is fundamental to exploit the
information they contain about the distribution of matter in galaxies and
clusters. In this work, we present Lensed, a new code which performs forward
parametric modelling of strong lenses. Lensed takes advantage of a massively
parallel ray-tracing kernel to perform the necessary calculations on a modern
graphics processing unit (GPU). This makes the precise rendering of the
background lensed sources much faster, and allows the simultaneous optimisation
of tens of parameters for the selected model. With a single run, the code is
able to obtain the full posterior probability distribution for the lens light,
the mass distribution and the background source at the same time. Lensed is
first tested on mock images which reproduce realistic space-based observations
of lensing systems. In this way, we show that it is able to recover unbiased
estimates of the lens parameters, even when the sources do not follow exactly
the assumed model. Then, we apply it to a subsample of the SLACS lenses, in
order to demonstrate its use on real data. The results generally agree with the
literature, and highlight the flexibility and robustness of the algorithm.Comment: v2: major revision; accepted by MNRAS; lens reconstruction code
available at http://glenco.github.io/lensed
Zooming into the Cosmic Horseshoe: new insights on the lens profile and the source shape
The gravitational lens SDSS J1148+1930, also known as the Cosmic Horseshoe,
is one of the biggest and of the most detailed Einstein rings ever observed. We
use the forward reconstruction method implemented in the lens fitting code
Lensed to investigate with great detail the properties of the lens and of the
background source. We model the lens with different mass distributions,
focusing in particular on the determination of the slope of the dark matter
component. The inherent degeneracy between the lens slope and the source size
can be broken when we can isolate separate components of each lensed image, as
in this case. For an elliptical power law model, , the
results favour a flatter-than-isothermal slope with a maximum-likelihood value
t = 0.08. Instead, when we consider the contribution of the baryonic matter
separately, the maximum-likelihood value of the slope of the dark matter
component is t = 0.31 or t = 0.44, depending on the assumed Initial Mass
Function. We discuss the origin of this result by analysing in detail how the
images and the sources change when the slope t changes. We also demonstrate
that these slope values at the Einstein radius are not inconsistent with recent
forecast from the theory of structure formation in the LambdaCDM model.Comment: 13 pages, 9 figures, accepted for publication in MNRA
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
Testing future weak lensing surveys through simulations of observations
Weak lensing experiments such as the future ESA-accepted mission Euclid aim to measure cosmological parameters with unprecedented accuracy. It is important to assess the precision that can be obtained in these measurements by applying analysis software on mock images that contain many sources of noise present in the real data. In this Thesis, we show a method to perform simulations of observations, that produce realistic images of the sky according to characteristics of the instrument and of the survey. We then use these images to test the performances of the Euclid mission. In particular, we concentrate on the precision of the photometric redshift measurements, which are key data to perform cosmic shear tomography. We calculate the fraction of the total observed sample that must be discarded to reach the required level of precision, that is equal to 0.05(1+z) for a galaxy with measured redshift z, with different ancillary ground-based observations. The results highlight the importance of u-band observations, especially to discriminate between low (z < 0.5) and high (z ~ 3) redshifts, and the need for good observing sites, with seeing FWHM < 1. arcsec. We then construct an optimal filter to detect galaxy clusters through photometric catalogues of galaxies, and we test it on the COSMOS field, obtaining 27 lensing-confirmed detections. Applying this algorithm on mock Euclid data, we verify the possibility to detect clusters with mass above 10^14.2 solar masses with a low rate of false detections
AMICO galaxy clusters in KiDS-DR3: weak-lensing mass calibration
We present the mass calibration for galaxy clusters detected with the AMICO
code in KiDS DR3 data. The cluster sample comprises 7000 objects and
covers the redshift range 0.1 < < 0.6. We perform a weak lensing stacked
analysis by binning the clusters according to redshift and two different mass
proxies provided by AMICO, namely the amplitude (measure of galaxy
abundance through an optimal filter) and the richness (sum of
membership probabilities in a consistent radial and magnitude range across
redshift). For each bin, we model the data as a truncated NFW profile plus a
2-halo term, taking into account uncertainties related to concentration and
miscentring. From the retrieved estimates of the mean halo masses, we construct
the - and the - relations. The relations extend
over more than one order of magnitude in mass, down to at = 0.2 (0.5), with small evolution in redshift.
The logarithmic slope is for the -mass relation, and
for the -mass relation, consistent with previous estimations on mock
catalogues and coherent with the different nature of the two observables.Comment: 19 pages, 16 figures, accepted by MNRA
CoMaLit -- VI. Intrinsic scatter in stacked relations. The weak lensing AMICO galaxy clusters in KiDS-DR3
Unbiased and precise mass calibration of galaxy clusters is crucial to fully
exploit galaxy clusters as cosmological probes. Stacking of weak lensing signal
allows us to measure observable-mass relations down to less massive halos halos
without extrapolation. We propose a Bayesian inference method to constrain the
intrinsic scatter of the mass proxy in stacked analyses. The scatter of the
stacked data is rescaled with respect to the individual scatter based on the
number of binned clusters. We apply this method to the galaxy clusters detected
with the AMICO (Adaptive Matched Identifier of Clustered Objects) algorithm in
the third data release of the Kilo-Degree Survey. The results confirm the
optical richness as a low scatter mass proxy. Based on the optical richness and
the calibrated weak lensing mass-richness relation, mass of individual objects
down to ~10^13 solar masses can be estimated with a precision of ~20 per cent.Comment: 12 pages, 6 figures; in press on MNRA
SEAGLE--II: Constraints on feedback models in galaxy formation from massive early type strong lens galaxies
We use nine different galaxy formation scenarios in ten cosmological
simulation boxes from the EAGLE suite of {\Lambda}CDM hydrodynamical
simulations to assess the impact of feedback mechanisms in galaxy formation and
compare these to observed strong gravitational lenses. To compare observations
with simulations, we create strong lenses with >
with the appropriate resolution and noise level, and model them with an
elliptical power-law mass model to constrain their total mass density slope. We
also obtain the mass-size relation of the simulated lens-galaxy sample. We find
significant variation in the total mass density slope at the Einstein radius
and in the projected stellar mass-size relation, mainly due to different
implementations of stellar and AGN feedback. We find that for lens selected
galaxies, models with either too weak or too strong stellar and/or AGN feedback
fail to explain the distribution of observed mass-density slopes, with the
counter-intuitive trend that increasing the feedback steepens the mass density
slope around the Einstein radius ( 3-10 kpc). Models in which stellar
feedback becomes inefficient at high gas densities, or weaker AGN feedback with
a higher duty cycle, produce strong lenses with total mass density slopes close
to isothermal (i.e. -d log({\rho})/d log(r) 2.0) and slope
distributions statistically agreeing with observed strong lens galaxies in
SLACS and BELLS. Agreement is only slightly worse with the more heterogeneous
SL2S lens galaxy sample. Observations of strong-lens selected galaxies thus
appear to favor models with relatively weak feedback in massive galaxies.Comment: re-submitted to MNRAS, bug fixed, conclusions unchanged, updated
appendices and references, 23 pages, 10 Figures, 6 Table
AMICO galaxy clusters in KiDS-DR3: galaxy population properties and their redshift dependence
A catalogue of galaxy clusters was obtained in an area of 414 sq deg up to a
redshift from the Data Release 3 of the Kilo-Degree Survey
(KiDS-DR3), using the Adaptive Matched Identifier of Clustered Objects (AMICO)
algorithm. The catalogue and the calibration of the richness-mass relation were
presented in two companion papers. Here we describe the selection of the
cluster central galaxy and the classification of blue and red cluster members,
and analyze the main cluster properties, such as the red/blue fraction, cluster
mass, brightness and stellar mass of the central galaxy, and their dependence
on redshift and cluster richness. We use the Illustris-TNG simulation, which
represents the state-of-the-art cosmological simulation of galaxy formation, as
a benchmark for the interpretation of the results. A good agreement with
simulations is found at low redshifts (), while at higher redshifts
the simulations indicate a lower fraction of blue galaxies than what found in
the KiDS-AMICO catalogue: we argue that this may be due to an underestimate of
star-forming galaxies in the simulations. The selection of clusters with a
larger magnitude difference between the two brightest central galaxies, which
may indicate a more relaxed cluster dynamical status, improves the agreement
between the observed and simulated cluster mass and stellar mass of the central
galaxy. We also find that at a given cluster mass the stellar mass of blue
central galaxies is lower than that of the red ones.Comment: 14 pages, 16 figures, accepted for publication on MNRA
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