arXiv subjects

Patrick Simon

Publications and source records attributed to Patrick Simon.

34 records · Page 2Linked to original sources

arXiv2013

Low dimensionality of the diamond surface conductivity

Undoped diamond, a remarkable bulk electrical insulator, exhibits a high surface conductivity in air when the surface is hydrogen-terminated. Although theoretical models have claimed that a two-dimensional hole gas is established as a result of surface energy band bending, no definitive experimental demonstration has been reported so far. Here, we prove the two-dimensional character of the surface conductivity by low temperature characterization of diamond in-plane gated field-effect transistors that enable the lateral confinement of the transistor's drain-source channel to nanometer dimensions. In these devices, we observe Coulomb blockade effects of multiple quantum islands varying in size with the gate voltage. The charging energy and thus the size of these zero-dimensional islands exhibits a gate voltage dependence which is the direct result of the two-dimensional character of the conductive channel formed at hydrogen-terminated diamond surfaces.

cond-mat.mes-hall↗

arXiv2013

Influence of homo-buffer layer on stress control of sputtered (Ba0.45,Sr0.55)TiO3 thin films on Pt-Si

To engineer strain relaxation of sputtered BST thin films on Pt-Si wafers, homo-buffer layer method was applied to eliminate Pt hillock formation. Thin BST homo-buffer layers were deposited at room temperature and subsequently the main BST layer was deposited at 650°C, Pt hillock free BST films were obtained with homo-buffer thickness above 5 nm. Relatively good electrical properties were obtained for BST thin films with 15 and 25 nm homo-buffer layer (T= 30 % at 5V and tan δ= 0.018).

cond-mat.mtrl-sci↗

arXiv2013

Galaxy-galaxy(-galaxy) lensing as a sensitive probe of galaxy evolution

The gravitational lensing effect provides various ways to study the mass environment of galaxies. We investigate how galaxy-galaxy(-galaxy) lensing can be used to test models of galaxy formation and evolution. We consider two semi-analytic galaxy formation models based on the Millennium Run N-body simulation: the Durham model by Bower et al. (2006) and the Garching model by Guo et al. (2011). We generate mock lensing observations for the two models, and then employ Fast Fourier Transform methods to compute second- and third-order aperture statistics in the simulated fields for various galaxy samples. We find that both models predict qualitatively similar aperture signals, but there are large quantitative differences. The Durham model predicts larger amplitudes in general. In both models, red galaxies exhibit stronger aperture signals than blue galaxies. Using these aperture measurements and assuming a linear deterministic bias model, we measure relative bias ratios of red and blue galaxy samples. We find that a linear deterministic bias is insufficient to describe the relative clustering of model galaxies below ten arcmin angular scales. Dividing galaxies into luminosity bins, the aperture signals decrease with decreasing luminosity for brighter galaxies, but increase again for fainter galaxies. This increase is likely an artifact due to too many faint satellite galaxies in massive group and cluster halos predicted by the models. Our study shows that galaxy-galaxy(-galaxy) lensing is a sensitive probe of galaxy evolution.

astro-ph.CO↗

arXiv2013

Towards an understanding of third-order galaxy-galaxy lensing

Third-order galaxy-galaxy lensing (G3L) is a next generation galaxy-galaxy lensing technique that either measures the excess shear about lens pairs or the excess shear-shear correlations about lenses. It is clear that these statistics assess the three-point correlations between galaxy positions and projected matter density. For future applications of these novel statistics, we aim at a more intuitive understanding of G3L to isolate the main features that possibly can be measured. We construct a toy model ("isolated lens model"; ILM) for the distribution of galaxies and associated matter to determine the measured quantities of the two G3L correlation functions and traditional galaxy-galaxy lensing (GGL) in a simplified context. The ILM presumes single lens galaxies to be embedded inside arbitrary matter haloes that, however, are statistically independent ("isolated") from any other halo or lens position. In the ILM, the average mass-to-galaxy number ratio of clusters of any size cannot change. GGL and galaxy clustering alone cannot distinguish an ILM from any more complex scenario. The lens-lens-shear correlator in combination with second-order statistics enables us to detect deviations from a ILM, though. This can be quantified by a difference signal defined in the paper. We demonstrate with the ILM that this correlator picks up the excess matter distribution about galaxy pairs inside clusters. The lens-shear-shear correlator is sensitive to variations among matter haloes. In principle, it could be devised to constrain the ellipticities of haloes, without the need for luminous tracers, or maybe even random halo substructure. [Abridged]

astro-ph.CO↗

arXiv2012

CFHTLenS: The Canada-France-Hawaii Telescope Lensing Survey

We present the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS) that accurately determines a weak gravitational lensing signal from the full 154 square degrees of deep multi-colour data obtained by the CFHT Legacy Survey. Weak gravitational lensing by large-scale structure is widely recognised as one of the most powerful but technically challenging probes of cosmology. We outline the CFHTLenS analysis pipeline, describing how and why every step of the chain from the raw pixel data to the lensing shear and photometric redshift measurement has been revised and improved compared to previous analyses of a subset of the same data. We present a novel method to identify data which contributes a non-negligible contamination to our sample and quantify the required level of calibration for the survey. Through a series of cosmology-insensitive tests we demonstrate the robustness of the resulting cosmic shear signal, presenting a science-ready shear and photometric redshift catalogue for future exploitation.

astro-ph.CO↗

arXiv2012

Retrieving the three-dimensional matter power spectrum and galaxy biasing parameters from lensing tomography

With the availability of galaxy distance indicators in weak lensing surveys, lensing tomography can be harnessed to constrain the three-dimensional (3D) matter power spectrum over a range of redshift and physical scale. By combining galaxy-galaxy lensing and galaxy clustering, this can be extended to probe the 3D galaxy-matter and galaxy-galaxy power spectrum or, alternatively, galaxy biasing parameters. To achieve this aim, this paper introduces and discusses minimum variance estimators and a more general Bayesian approach to statistically invert a set of noisy tomography 2-point correlation functions, measured within a confined opening angle. Both methods are constructed such that they probe deviations of the power spectrum from a fiducial power spectrum, thereby enabling both a direct comparison of theory and data, and in principle the identification of the physical scale and redshift of deviations. By devising a new Monte Carlo technique, we quantify the measurement noise in the correlators for a fiducial survey, and test the performance of the inversion techniques. We conclude that a shear tomography analysis of near future weak lensing surveys promises fruitful insights into the effect of baryons on the nonlinear matter power spectrum at z<~0.3 around k~2 h/Mpc, and into galaxy biasing (z<~0.5). However, a proper treatment of anticipated systematics -- not included in the mock analysis but discussed here -- is likely to reduce the signal-to-noise in the analysis so that a robust assessment of the 3D matter power spectrum probably asks for a survey area of at least 1000 sdeg. [Abridged]

astro-ph.CO↗

arXiv2012

Cosmic Shear Tomography and Efficient Data Compression using COSEBIs

Context. Gravitational lensing is one of the leading tools in understanding the dark side of the Universe. The need for accurate, efficient and effective methods which are able to extract this information along with other cosmological parameters from cosmic shear data is ever growing. COSEBIs, Complete Orthogonal Sets of E-/B-Integrals, is a recently developed statistical measure that encompasses the complete E-/B-mode separable information contained in the shear correlation functions measured on a finite angular range. Aims. The aim of the present work is to test the properties of this newly developed statistics for a higher-dimensional parameter space and to generalize and test it for shear tomography. Methods. We use Fisher analysis to study the effectiveness of COSEBIs. We show our results in terms of figure-of-merit quantities, based on Fisher matrices. Results. We find that a relatively small number of COSEBIs modes is always enough to saturate to the maximum information level. This number is always smaller for 'logarithmic COSEBIs' than for 'linear COSEBIs', and also depends on the number of redshift bins, the number and choice of cosmological parameters, as well as the survey characteristics. Conclusions. COSEBIs provide a very compact way of analyzing cosmic shear data, i.e., all the E-/B-mode separable second-order statistical information in the data is reduced to a small number of COSEBIs modes. Furthermore, with this method the arbitrariness in data binning is no longer an issue since the COSEBIs modes are discrete. Finally, the small number of modes also implies that covariances, and their inverse, are much more conveniently obtainable, e.g., from numerical simulations, than for the shear correlation functions themselves.

astro-ph.CO↗

arXiv2011

The bispectrum covariance beyond Gaussianity: A log-normal approach

To investigate and specify the statistical properties of cosmological fields with particular attention to possible non-Gaussian features, accurate formulae for the bispectrum and the bispectrum covariance are required. The bispectrum is the lowest-order statistic providing an estimate for non-Gaussianities of a distribution, and the bispectrum covariance depicts the errors of the bispectrum measurement and their correlation on different scales. Currently, there do exist fitting formulae for the bispectrum and an analytical expression for the bispectrum covariance, but the former is not very accurate and the latter contains several intricate terms and only one of them can be readily evaluated from the power spectrum of the studied field. Neglecting all higher-order terms results in the Gaussian approximation of the bispectrum covariance. We study the range of validity of this Gaussian approximation for two-dimensional non-Gaussian random fields. For this purpose, we simulate Gaussian and non-Gaussian random fields, the latter represented by log-normal fields and obtained directly from the former by a simple transformation. From the simulated fields, we calculate the power spectra, the bispectra, and the covariance from the sample variance of the bispectra, for different degrees of non-Gaussianity α, which is equivalent to the skewness on a given angular scale θg. We find that the Gaussian approximation provides a good approximation for α<0.6 and a reasonably accurate approximation for α< 1, both on scales >8θg. Using results from cosmic shear simulations, we estimate that the cosmic shear convergence fields are described by α<0.7 at θg~4". We therefore conclude that the Gaussian approximation for the bispectrum covariance is likely to be applicable in ongoing and future cosmic shear studies.

astro-ph.CO↗

arXiv2010

Evidence for the accelerated expansion of the Universe from weak lensing tomography with COSMOS

We present a tomographic cosmological weak lensing analysis of the HST COSMOS Survey. Applying our lensing-optimized data reduction, principal component interpolation for the ACS PSF, and improved modelling of charge-transfer inefficiency, we measure a lensing signal which is consistent with pure gravitational modes and no significant shape systematics. We carefully estimate the statistical uncertainty from simulated COSMOS-like fields obtained from ray-tracing through the Millennium Simulation. We test our pipeline on simulated space-based data, recalibrate non-linear power spectrum corrections using the ray-tracing, employ photometric redshifts to reduce potential contamination by intrinsic galaxy alignments, and marginalize over systematic uncertainties. We find that the lensing signal scales with redshift as expected from General Relativity for a concordance LCDM cosmology, including the full cross-correlations between different redshift bins. For a flat LCDM cosmology, we measure sigma_8(Omega_m/0.3)^0.51=0.75+-0.08 from lensing, in perfect agreement with WMAP-5, yielding joint constraints Omega_m=0.266+0.025-0.023, sigma_8=0.802+0.028-0.029 (all 68% conf.). Dropping the assumption of flatness and using HST Key Project and BBN priors only, we find a negative deceleration parameter q_0 at 94.3% conf. from the tomographic lensing analysis, providing independent evidence for the accelerated expansion of the Universe. For a flat wCDM cosmology and prior w in [-2,0], we obtain w<-0.41 (90% conf.). Our dark energy constraints are still relatively weak solely due to the limited area of COSMOS. However, they provide an important demonstration for the usefulness of tomographic weak lensing measurements from space. (abridged)

astro-ph.CO↗

arXiv2009

First lensing measurements of SZ-discovered clusters

We present the first lensing mass measurements of Sunyaev-Zel'dovich (SZ) selected clusters. Using optical imaging from the Southern Cosmology Survey (SCS), we present weak lensing masses for three clusters selected by their SZ emission in the South Pole Telescope survey (SPT). We confirm that the SZ selection procedure is successful in detecting mass concentrations. We also study the weak lensing signals from 38 optically-selected clusters in ~8 square degrees of the SCS survey. We fit Navarro, Frenk and White (NFW) profiles and find that the SZ clusters have amongst the largest masses, as high as 5x10^14 Msun. Using the best fit masses for all the clusters, we analytically calculate the expected SZ integrated Y parameter, which we find to be consistent with the SPT observations.

astro-ph.CO↗

arXiv2009

Unfolding the matter distribution using 3-D weak gravitational lensing

Combining redshift and galaxy shape information offers new exciting ways of exploiting the gravitational lensing effect for studying the large scales of the cosmos. One application is the three-dimensional reconstruction of the matter density distribution which is explored in this paper. We give a generalisation of an already known minimum-variance estimator of the 3-D matter density distribution that facilitates the combination of thin redshift slices of sources with samples of broad redshift distributions for an optimal reconstruction. We show how, in principle, intrinsic alignments of source ellipticities or shear/intrinsic alignment correlations can be accommodated, albeit these effects are not the focus of this paper. We describe an efficient and fast way to implement the estimator on a contemporary desktop computer. Analytic estimates for the noise and biases in the reconstruction are given. The bias -- a spread and shift of structures in radial direction -- can be expressed in terms of a radial PSF, comprising the limitations of the reconstruction due to source shot-noise and the unavoidably broad lensing kernel. We conclude that a 3-D mass-density reconstruction on galaxy cluster scales is feasible but, for foreseeable surveys, a map with a S/N>~3 threshold is limited to structures with M200>~10^14 Msol/h, or 7x10^14 Msol/h, at low to moderate redshifts (z=0.1 or 0.6). However, we find that a heavily smoothed full-sky map of the very large-scale density field may also be possible as the S/N of reconstructed modes increases towards larger scales. Future improvements of the method can be obtained by including higher-order lensing information (flexion) which could also be implemented into our algorithm. [ABRIDGED]

astro-ph.CO↗

arXiv2009

Locating the Baryon Acoustic Peak

Forthcoming photometric redshift surveys should provide an accurate probe of the acoustic peak in the two-point galaxy correlation function, in the form of angular clustering of galaxies within a given shell in redshift space. We investigate the form of the anticipated signal, quantifying the distortions that arise due to projection effects, and in particular explore the validity of applying the Limber approximation. A single-integral prescription is presented, which provides an alternative to Limber's equation, and produces a significantly improved prediction in the regime of interest. The position of the acoustic peak within the angular correlation function relates to the angular diameter distance to the far side of the redshift bin. Thicker redshift bins therefore shift comoving features towards smaller angular scales. As a result, the value of the photometric redshift error acquires a greater significance, particularly at lower redshifts. In order to recover the dark energy equation of state to a level of 1%, we find the total redshift dispersion must be determined to within Δσ_z ~ 10^-3, which may prove challenging to achieve in practice.

astro-ph.CO↗

arXiv2004

The galaxy-dark matter bias in the Garching-Bonn Deep Survey

We measured the bias and correlation factor of galaxies with respect to the dark matter using the aperture statistics including the aperture mass from weak gravitational lensing. The analysis was performed for three galaxy samples selected by R-band magnitudes; the median redshifts of the samples were $\ave{z}=0.34,0.49$ and 0.65, respectively. The brightest sample has the strongest peak in redshift and can therefore be most accurately represented by a single redshift. The data used is the GaBoDS, and the COMBO-17 survey for an accurate estimate of the redshift distribution of the galaxies. Assuming the currently favoured $Λ\rm CDM$ model as cosmology, we obtained values for the linear stochastic galaxy-dark matter bias on angular scales $1^\prime\leθ\le20^\prime$. Averaging the measurements for the bias over the range $4^\prime\le θ_{\rm ap}\le 18^\prime$, weighting with the cosmic variance error, yields $b=0.89(5), 0.79(5), 0.89(5)$, respectively ($1σ$). As for the correlation factor $r$ we see, as Hoekstra et al. (2002), a slight increase to $r=1$ in the last angular bin from an almost constant value on smaller scales; the weighted average here over the same range as before is $r=0.8(1), 0.8(1), 0.5(1)$, respectively (abridged).

astro-ph↗

arXiv2004

Time evolution of the stochastic linear bias of interacting galaxies on linear scales

We extend the time dependent bias model of Tegmark & Peebles (1998) to predict the large-scale evolution of the stochastic linear bias of different galaxy populations with respect to both the dark matter and each other. The resulting model equations contain a general expression, coined the "interaction term", accounting for the destruction or production of galaxies. This term may be used to model couplings between different populations that lead to an increase or decrease of the number of a galaxies belonging to a population, e.g. passive evolution or merging processes. This is explored in detail using a toy model. In particular, it is shown that the presence of such a coupling may change the evolution of the bias parameter compared to an interaction-free evolution. We argue that the observation of the evolution of the large-scale bias and galaxy number density with wide-field surveys may be used to infer fundamental interaction parameters between galaxy populations, possibly giving an insight in their formation and evolution.

astro-ph↗

arXiv2004

The covariance of cosmic shear correlation functions and cosmological parameter estimates using redshift information

Cosmological weak lensing by the large scale structure of the Universe, cosmic shear, is coming of age as a powerful probe of the parameters describing the cosmological model and matter power spectrum. It complements CMB studies, by breaking degeneracies and providing a cross-check. An important measure of the cosmic shear signal are the shear correlation functions; these can be directly calculated from data, and compared with theoretical expectations for different cosmological models and matter power spectra. We present a Monte Carlo method to quickly simulate mock cosmic shear surveys. One application of this method is in the determination of the full covariance matrix for the correlation functions; this includes redshift binning and is applicable to arbitrary survey geometries. Terms arising from shot noise and cosmic variance (dominant on small and large scales respectively) are accounted for naturally. As an illustration of the use of such covariance matrices, we consider to what degree confidence regions on parameters are tightened when redshift binning is employed. The parameters considered are those commonly discussed in cosmic shear analyses - the matter density parameter, dark energy density parameter (classical cosmological constant), power spectrum normalisation and shape parameter. We incorporate our covariance matrices into a likelihood treatment, and also use the Fisher formalism to explore a larger region of parameter space (abridged).

astro-ph↗

arXiv2000

Signatures in a Giant Radio Galaxy of a Cosmological Shock Wave at Intersecting Filaments of Galaxies

Sensitive images of low-level, Mpc-sized radio cocoons offer new opportunities to probe large scale intergalactic gas flows outside clusters of galaxies. New radio images of high surface brightness sensitivity at strategically chosen wavelengths of the giant radio galaxy NGC 315 (Mack et al. 1997,1998) reveal significant asymmetries and particularities in the morphology, radio spectrum and polarization of the ejected radio plasma. We argue that the combination of these signatures provides a sensitive probe of an environmental shock wave. Analysis of optical redshifts in NGC 315 vicinity confirms its location to be near, or at a site of large-scale flow collisions in the 100 Mpc sized Pisces-Perseus Supercluster region. NGC 315 resides at the intersection of several galaxy filaments, and its radio plasma serves there as a `weather station' (Burns 1998) probing the flow of the elusive and previously invisible IGM gas. If our interpretation is correct, this is the first indication for a shock wave in flows caused by the cosmological large scale structure formation, which is located in a filament of galaxies. The possibility that the putative shock wave is a source of gamma-rays and ultra high energy cosmic rays is briefly discussed.

astro-ph↗