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 $\sigma_8$-$\Omega_{\rm m}$-plane to be marginally consistent with results from $Planck$ for a simple five parameter $\Lambda$CDM model. For the projected parameter $S_8 \equiv \sigma_8(\Omega_{\rm m}/0.3)^{0.5}$ we obtain a best-fitting value of $S_8 = 0.768_{-0.039}^{+0.045}$. 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 $\Sigma m_\nu < 4.53 \, {\rm eV}$ at 95 per cent credibility, while a Bayesian model comparison does not favour any model extension beyond a simple five parameter $\Lambda$CDM model. Combining the shear likelihood with $Planck$ breaks the $\sigma_8$-$\Omega_{\rm m}$-degeneracy and yields $\sigma_8=0.818 \pm 0.013$ and $\Omega_{\rm m} = 0.300 \pm 0.011$ which is fully consistent with results from $Planck$ 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 Abschützung der gravitativen Scherung anhand von Momenten der Flächenhelligkeit kleiner und verrauschter Galaxienbilder. Wir zeigen, in welcher Weise KSB auf einschränkenden mathematischen Annahmen beruht, die das Verhält- nis von gefalteter zu ungefalteter Elliptizität, die Form der Punktbildfunktion des Teleskops ebenso wie die Beziehung zwischen Elliptizität und Scherung betreffen und von denen keine in Wirklichkeit erfüllt ist. Wir schlagen Verbesserungen des ursprünglichen KSB-Verfahrens vor und zeigen, dass diese Erweiterungen Fehlein- schätzungen in Scherungsmessungen deutlich reduzieren. Darüber hinaus diskutieren wir die Unmäglichkeit, die Annahmen über die Form der Punkbildfunktion im Rah- men von KSB abzuschwächen. Aus diesem Grund entwickeln wir eine neuartige Meth- ode für Messungen des schwachen Gravitationslinseneffekts, DEIMOS, die auf einer mathematisch exakten Entfaltung der Momente der scheinbaren Flächenhelligkeit von der Punktbildfunktion beruht. Wir weisen durch eine Reihe spezailisierter Tests die Genauigkeit und die Stärke dieser neuen Methode nach und zeigen anhand der Daten des GREAT08-Wettbewerbs, wie konkurrenzfähig diese Methode ist. Darüber hinaus stellen wir eine mögliche Anwendung von Scherungsmessungen auf die Untersuchung der Eigenschaften von Galaxienhaufen dar. Sie beruht auf linearen Filtertechniken und schätzt die innere Steigung des Dichteprofils in Halos aus dunkler Materie ab. Wir finden, dass unter idealisierten Bedingungen die Genauigkeit der Abschätzung bei 15 % liegt, wenn die Konzentration c des Halos bekannt ist, und bei 30% falls nicht. Wenn die Signale vieler Halos überlagert werden können, sollten ihre Dichteprofile daher durch den vorgeschlagenen linearen Filter gut bestimmt sein. Gegenüber Analysen des starken Gravitationslinseneffekts hat diese Methode den Vorteil, unempfindlich gegenüber Substrukturen in den Galaxienhaufen zu sein

Meccanica quantistica in sistemi di riferimento non inerziali.

Il concetto di trasformazione tra sistemi di riferimento è 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, verrà 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&amp;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&amp;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&amp;Q activities concerning early product architecture designs is proposed. Product architecture alternatives, test schedules and cost are connected through the quantification of T&amp;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