1,653 research outputs found
A study of the sensitivity of shape measurements to the input parameters of weak lensing image simulations
Improvements in the accuracy of shape measurements are essential to exploit
the statistical power of planned imaging surveys that aim to constrain
cosmological parameters using weak lensing by large-scale structure. Although a
range of tests can be performed using the measurements, the performance of the
algorithm can only be quantified using simulated images. This yields, however,
only meaningful results if the simulated images resemble the real observations
sufficiently well. In this paper we explore the sensitivity of the
multiplicative bias to the input parameters of Euclid-like image simulations.We
find that algorithms will need to account for the local density of sources. In
particular the impact of galaxies below the detection limit warrants further
study, because magnification changes their number density, resulting in
correlations between the lensing signal and multiplicative bias. Although
achieving sub-percent accuracy will require further study, we estimate that
sufficient archival Hubble Space Telescope data are available to create
realistic populations of galaxies.Comment: 18 pages, accepted for publications in MNRA
On the Probability Distributions of Ellipticity
In this paper we derive an exact full expression for the 2D probability
distribution of the ellipticity of an object measured from data, only assuming
Gaussian noise in pixel values. This is a generalisation of the probability
distribution for the ratio of single random variables, that is well-known, to
the multivariate case. This expression is derived within the context of the
measurement of weak gravitational lensing from noisy galaxy images. We find
that the third flattening, or epsilon-ellipticity, has a biased maximum
likelihood but an unbiased mean; and that the third eccentricity, or normalised
polarisation chi, has both a biased maximum likelihood and a biased mean. The
very fact that the bias in the ellipticity is itself a function of the
ellipticity requires an accurate knowledge of the intrinsic ellipticity
distribution of the galaxies in order to properly calibrate shear measurements.
We use this expression to explore strategies for calibration of biases caused
by measurement processes in weak gravitational lensing. We find that upcoming
weak lensing surveys like KiDS or DES require calibration fields of order of
several square degrees and 1.2 magnitude deeper than the wide survey in order
to correct for the noise bias. Future surveys like Euclid will require
calibration fields of order 40 square degree and several magnitude deeper than
the wide survey. We also investigate the use of the Stokes parameters to
estimate the shear as an alternative to the ellipticity. We find that they can
provide unbiased shear estimates at the cost of a very large variance in the
measurement. The python code used to compute the distributions presented in the
paper and to perform the numerical calculations are available on request.Comment: 24 pages, 18 figures, 2 Tables. Accepted for publication in Monthly
Notices of the Royal Astronomical Society Main Journa
A direct measurement of tomographic lensing power spectra from CFHTLenS
We measure the weak gravitational lensing shear power spectra and their
cross-power in two photometric redshift bins from the Canada-France-Hawaii
Telescope Lensing Survey (CFHTLenS). The measurements are performed directly in
multipole space in terms of adjustable band powers. For the extraction of the
band powers from the data we have implemented and extended a quadratic
estimator, a maximum likelihood method that allows us to readily take into
account irregular survey geometries, masks, and varying sampling densities. We
find the 68 per cent credible intervals in the --plane to be marginally consistent with results from for a simple
five parameter CDM model. For the projected parameter we obtain a best-fitting value of . This constraint is consistent with results from other
CFHTLenS studies as well as the Dark Energy Survey. Our most conservative
model, including modifications to the power spectrum due to baryon feedback and
marginalization over photometric redshift errors, yields an upper limit on the
total mass of three degenerate massive neutrinos of at 95 per cent credibility, while a Bayesian model comparison does
not favour any model extension beyond a simple five parameter CDM
model. Combining the shear likelihood with breaks the
--degeneracy and yields
and which is fully consistent with results
from alone.Comment: 19 pages, 12 figures, 7 tables. Accepted for publication in MNRAS.
Minor corrections and updates with respect to previous versio
The Canadian Cluster Comparison Project: detailed study of systematics and updated weak lensing masses
Masses of clusters of galaxies from weak gravitational lensing analyses of
ever larger samples are increasingly used as the reference to which baryonic
scaling relations are compared. In this paper we revisit the analysis of a
sample of 50 clusters studied as part of the Canadian Cluster Comparison
Project. We examine the key sources of systematic error in cluster masses. We
quantify the robustness of our shape measurements and calibrate our algorithm
empirically using extensive image simulations. The source redshift distribution
is revised using the latest state-of-the-art photometric redshift catalogs that
include new deep near-infrared observations. Nonetheless we find that the
uncertainty in the determination of photometric redshifts is the largest source
of systematic error for our mass estimates. We use our updated masses to
determine b, the bias in the hydrostatic mass, for the clusters detected by
Planck. Our results suggest 1-b=0.76+-0.05(stat)}+-0.06(syst)}, which does not
resolve the tension with the measurements from the primary cosmic microwave
background.Comment: resubmitted to MNRAS after review by refere
On shear and flexion measurements and properties of dark matter halos
In dieser Arbeit untersuchen wir die Kaiser-Squires-Broadhurst- Methode (KSB) zur AbschĂŒtzung der gravitativen Scherung anhand von Momenten der FlĂ€chenhelligkeit kleiner und verrauschter Galaxienbilder. Wir zeigen, in welcher Weise KSB auf einschrĂ€nkenden mathematischen Annahmen beruht, die das VerhĂ€lt- nis von gefalteter zu ungefalteter ElliptizitĂ€t, die Form der Punktbildfunktion des Teleskops ebenso wie die Beziehung zwischen ElliptizitĂ€t und Scherung betreffen und von denen keine in Wirklichkeit erfĂŒllt ist. Wir schlagen Verbesserungen des ursprĂŒnglichen KSB-Verfahrens vor und zeigen, dass diese Erweiterungen Fehlein- schĂ€tzungen in Scherungsmessungen deutlich reduzieren. DarĂŒber hinaus diskutieren wir die UnmĂ€glichkeit, die Annahmen ĂŒber die Form der Punkbildfunktion im Rah- men von KSB abzuschwĂ€chen. Aus diesem Grund entwickeln wir eine neuartige Meth- ode fĂŒr Messungen des schwachen Gravitationslinseneffekts, DEIMOS, die auf einer mathematisch exakten Entfaltung der Momente der scheinbaren FlĂ€chenhelligkeit von der Punktbildfunktion beruht. Wir weisen durch eine Reihe spezailisierter Tests die Genauigkeit und die StĂ€rke dieser neuen Methode nach und zeigen anhand der Daten des GREAT08-Wettbewerbs, wie konkurrenzfĂ€hig diese Methode ist. DarĂŒber hinaus stellen wir eine mögliche Anwendung von Scherungsmessungen auf die Untersuchung der Eigenschaften von Galaxienhaufen dar. Sie beruht auf linearen Filtertechniken und schĂ€tzt die innere Steigung des Dichteprofils in Halos aus dunkler Materie ab. Wir finden, dass unter idealisierten Bedingungen die Genauigkeit der AbschĂ€tzung bei 15 % liegt, wenn die Konzentration c des Halos bekannt ist, und bei 30% falls nicht. Wenn die Signale vieler Halos ĂŒberlagert werden können, sollten ihre Dichteprofile daher durch den vorgeschlagenen linearen Filter gut bestimmt sein. GegenĂŒber Analysen des starken Gravitationslinseneffekts hat diese Methode den Vorteil, unempfindlich gegenĂŒber Substrukturen in den Galaxienhaufen zu sein
Meccanica quantistica in sistemi di riferimento non inerziali.
Il concetto di trasformazione tra sistemi di riferimento Ăš racchiuso nell'idea di simmetria. Definiti, nel primo capitolo, i concetti di simmetria fisica e algebrica, si osserva come in meccanica quantistica ogni simmetria algebrica possa essere estesa ad una simmetria fisica. Usando alcuni risultati della teoria delle rappresentazioni, brevemente esposti nel secondo capitolo, verrĂ affrontato l'argomento principale del lavoro di tesi: la ricerca di una descrizione quanto-meccanica della fisica in sistemi di riferimento non inerziali. Nel terzo capitolo si propone un'analisi della generalizzazione del gruppo delle rototraslazioni: un gruppo di dimensione infinita ad un parametro, chiamato gruppo lineare euclideo. Trovati i generatori del gruppo ad un parametro si costruiscono gli operatori unitari associati: essi, agendo sull'hamiltoniana, danno luogo a termini analoghi alle forze fittizie nell'equazione di Schroedinger. Viene infine proposta una discussione dei termini aggiuntivi che appaiono nell'hamiltoniana e della loro relazione con la presenza di un potenziale gravitazionale.
Dall'analisi risulta che il prinipio di equivalenza, illustrato nell'introduzione, viene generalmente violato in meccanica quantistica.ope
X-ray variability with WFXT: AGNs, transients and more
The Wide Field X-ray Telescope (WFXT) is a proposed mission with a high
survey speed, due to the combination of large field of view (FOV) and effective
area, i.e. grasp, and sharp PSF across the whole FOV. These characteristics
make it suitable to detect a large number of variable and transient X-ray
sources during its operating lifetime. Here we present estimates of the WFXT
capabilities in the time domain, allowing to study the variability of thousand
of AGNs with significant detail, as well as to constrain the rates and
properties of hundreds of distant, faint and/or rare objects such as X-ray
Flashes/faint GRBs, Tidal Disruption Events, ULXs, Type-I bursts etc. The
planned WFXT extragalactic surveys will thus allow to trace variable and
transient X-ray populations over large cosmological volumes.Comment: Proceedings of "The Wide Field X-ray Telescope Workshop", held in
Bologna, Italy, Nov. 25-26 2009 (arXiv:1010.5889). To appear in Memorie della
Societ\`a Astronomica Italiana 2010 - Minor corrections to text
Design for test and qualification through activity-based modelling in product architecture design
Test and qualification (T&Q) phases take a significant portion of the time to market for critical products in the space industry, especially when introducing new technologies. Since T&Q are treated as standard procedures, they tend to be independent of the architectural design phases and kept away from design decisions. However, when introducing new technologies, qualification procedures may differ from those established in regular design scenarios, and the estimation of qualification costs and duration is problematic. There is a lack of design for qualification methods capable of modelling these activities in early phases and use those models to support the architecture design of products with affordable test and qualification phases. In this article, a computer-assisted, model-based design method to model T&Q activities concerning early product architecture designs is proposed. Product architecture alternatives, test schedules and cost are connected through the quantification of T&Q drivers and driver rates. The design method is demonstrated using a case study about electric propulsion for satellites. The method is applicable for design situations where the choice of technology has a strong dependence on the qualification procedure
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