## Publications (10)49.69 Total impact

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**ABSTRACT:**We study various measures of weak lensing distortions in future surveys, taking into account the noise arising from the finite survey size and the intrinsic ellipticity of galaxies. We also consider a realistic redshift distribution of the sources, as expected for the SNAP mission. We focus on the low order moments and the full distribution function (pdf) of the aperture-mass $\Map$ and of the smoothed shear component $\gammais$. We also propose new unbiased estimators for low-order cumulants which have less scatter than the usual estimators of non-Gaussianity based on the moments themselves. Then, using an analytical model which has already been seen to provide a good description of weak gravitational lensing through comparison against numerical simulations, we study the statistical measures which can be extracted from future surveys like the SNAP experiment. We recover the fact that at small angular scales ($1'<\theta_s<10'$) the variance can be extracted with a few percent level accuracy. Non-Gaussianity can also be measured from the skewness of the aperture-mass (at a 10% level) while the shear kurtosis is more noisy and cannot be easily measured beyond 6'. On the other hand, we find that the pdf of the estimator associated with the aperture-mass can be distinguished both from the Gaussian and the Edgeworth expansion and could provide useful constraints, while this appears to be difficult to realize with the shear component. Finally, we investigate various survey strategies and the possibility to perform a redshift binning of the sample. Comment: 16 pages, final version published in MNRASMonthly Notices of the Royal Astronomical Society 02/2004; · 5.52 Impact Factor - [show abstract] [hide abstract]

**ABSTRACT:**In this paper we describe the predictions for the smoothed weak lensing shear and aperture-mass of two simple analytical models of the density field: the minimal tree-model and the stellar model. Both models give identical results for the statistics of the 3-d density contrast smoothed over spherical cells and only differ by the detailed angular dependence of the many-body density correlations. We have shown in previous work that they also yield almost identical results for the pdf of the smoothed convergence, $\kappa_s$. We find that both models give rather close results for both the shear and the positive tail of the aperture-mass. However, we note that at small angular scales ($\theta_s \la 2'$) the tail of the pdf $\cP(\Map)$ for negative $\Map$ shows a strong variation between the two models and the stellar model actually breaks down for $\theta_s \la 0.4'$ and $\Map<0$. This shows that the statistics of the aperture-mass provides a very precise probe of the detailed structure of the density field, as it is sensitive to both the amplitude and the detailed angular behaviour of the many-body correlations. On the other hand, the minimal tree-model shows good agreement with numerical simulations over all scales and redshifts of interest, while both models provide a good description of the pdf $\cP(\gamma_{is})$ of the smoothed shear components. Therefore, the shear and the aperture-mass provide robust and complimentary tools to measure the cosmological parameters as well as the detailed statistical properties of the density field. Comment: 19 pages, minor changes, matches final version published in MNRASMonthly Notices of the Royal Astronomical Society 09/2003; · 5.52 Impact Factor - [show abstract] [hide abstract]

**ABSTRACT:**Weak lensing convergence can be used directly to map and probe the dark mass distribution in the universe. Building on earlier studies, we recall how the statistics of the convergence field are related to the statistics of the underlying mass distribution, in particular to the many-body density correlations. We describe two model-independent approximations which provide two simple methods to compute the probability distribution function, pdf, of the convergence. We apply one of these to the case where the density field can be described by a log-normal pdf. Next, we discuss two hierarchical models for the high-order correlations which allow one to perform exact calculations and evaluate the previous approximations in such specific cases. Finally, we apply these methods to a very simple model for the evolution of the density field from linear to highly non-linear scales. Comparisons with the results obtained from numerical simulations, obtained from a number of different realizations, show excellent agreement with our theoretical predictions. We have probed various angular scales in the numerical work and considered sources at 14 different redshifts in each of two different cosmological scenarios, an open cosmology and a flat cosmology with non-zero cosmological constant. Our simulation technique employs computations of the full 3-d shear matrices along the line of sight from the source redshift to the observer and is complementary to more popular ray-tracing algorithms. Our results therefore provide a valuable cross-check for such complementary simulation techniques, as well as for our simple analytical model, from the linear to the highly non-linear regime. Comment: 20 pages, final version published in MNRASMonthly Notices of the Royal Astronomical Society 04/2003; · 5.52 Impact Factor - [show abstract] [hide abstract]

**ABSTRACT:**Weak gravitational lensing surveys are rapidly becoming important tools to probe directly the mass density fluctuations in the universe and its background dynamics. Earlier studies have shown that it is possible to model the statistics of the convergence field on small angular scales by suitably modeling the statistics of the underlying density field in the highly non-linear regime. We extend such methods to model the complete probability distribution function, PDF, of the shear as a function of smoothing angle. Our model relies on a simple hierarchical Ansatz for the behaviour of the higher-order correlations in the density field. We compare our predictions with the results of numerical simulations and find excellent agreement for different cosmological scenarios. Our method provides a new way to study the evolution of non-Gaussianity in gravitational clustering and should help to break the degeneracies in parameter estimation based on analysis of the power spectrum alone. Comment: 14 pages, final version published in MNRASMonthly Notices of the Royal Astronomical Society 03/2003; · 5.52 Impact Factor - [show abstract] [hide abstract]

**ABSTRACT:**We present new results on the gravitational lensing shear and magnification power spectra obtained from numerical simulations of a flat cosmology with a cosmological constant. These results are of considerable interest since both the shear and the magnification are observables. We find that the power spectrum in the convergence behaves as expected, but the magnification develops a shot-noise spectrum due to the effects of discrete, massive clusters and symptomatic of moderate lensing beyond the weak-lensing regime. We find that this behaviour can be suppressed by "clipping" of the largest projected clusters. Our results are compared with predictions from a Halo Model-inspired functional fit for the non-linear evolution of the matter field and show excellent agreement. We also study the higher-order moments of the convergence field and find a new scaling relationship with redshift. In particular, the statistic $S_3$ is found to vary as $z_s^{-2.00\pm 0.08}$ (where $z_s$ is the source redshift) for the cosmology studied, which makes corrections for different median redshifts in different observational surveys particularly simple to apply. Comment: Revised in accordance with referee's report; now accepted by MNRASMonthly Notices of the Royal Astronomical Society 12/2002; · 5.52 Impact Factor - [show abstract] [hide abstract]

**ABSTRACT:**Significant adjustments to the values of the cosmological parameters estimated from high-redshift Type Ia supernovae data are reported, almost an order of magnitude greater than previously found. They arise from the effects of weak gravitational lensing on observations of high-redshift sources. The lensing statistics used have been obtained from computations of the three-dimensional shear in a range of cosmological N-body simulations, from which it is estimated that cosmologies with an underlying deceleration parameter may be interpreted as having q0=−0.55 (appropriate to the currently popular cosmology with density parameter ΩM=0.3 and vacuum energy density parameter ΩΛ=0.7). In addition, the standard deviation expected from weak lensing for the peak magnitudes of Type Ia supernovae at redshifts of z=1 is expected to be approximately 0.078 mag, and 0.185 mag at redshift z=2. This latter value is greater than the accepted intrinsic dispersion of 0.17 mag. Consequently, the effects of weak lensing in observations of high-redshift sources must be taken properly into account.Monthly Notices of the Royal Astronomical Society 09/2000; 318(1):195 - 202. · 5.52 Impact Factor - [show abstract] [hide abstract]

**ABSTRACT:**We present the results of weak gravitational lensing statistics in four different cosmological $N$-body simulations. The data has been generated using an algorithm for the three-dimensional shear, which makes use of a variable softening facility for the $N$-body particle masses, and enables a physical interpretation for the large-scale structure to be made. Working in three-dimensions also allows the correct use of the appropriate angular diameter distances. Our results are presented on the basis of the filled beam approximation in view of the variable particle softening scheme in our algorithm. The importance of the smoothness of matter in the universe for the weak lensing results is discussed in some detail. The low density cosmology with a cosmological constant appears to give the broadest distributions for all the statistics computed for sources at high redshifts. In particular, the range in magnification values for this cosmology has implications for the determination of the cosmological parameters from high-redshift Type Ia Supernov\ae. The possibility of determining the density parameter from the non-Gaussianity in the probability distribution for the convergence is discussed. Comment: 25 pages, LaTeX, 30 figuresMonthly Notices of the Royal Astronomical Society 02/2000; · 5.52 Impact Factor - [show abstract] [hide abstract]

**ABSTRACT:**The effects of differences between the local and global values of the Hubble parameter on the cosmologies consistent with studies of high-redshift Type Ia supernov\ae are discussed. It is found that with a local Hubble parameter around 10 per cent higher than the global value then open cosmological models (such as $\Omega_{M} = 0.3, \Omega_{\Lambda}=0$) are prefered and if the local value is around 20 per cent higher then standard cosmological models ($\Omega_{M} = 1, \Omega_{\Lambda}=0$) can be recovered. Even in the case where the Hubble parameter ratio is 1, low $\Omega_M$ open cosmologies with $\Omega_{\Lambda} = 0$ are not rejected at the 95 per cent confidence level.07/1999; - [show abstract] [hide abstract]

**ABSTRACT:**We investigate the effects of weak gravitational lensing in the standard Cold Dark Matter cosmology, using an algorithm which evaluates the shear in three dimensions. The algorithm has the advantage of variable softening for the particles, and our method allows the appropriate angular diameter distances to be applied to every evaluation location within each three-dimensional simulation box. We investigate the importance of shear in the distance-redshift relation, and find it to be very small. We also establish clearly defined values for the smoothness parameter in the relation, finding its value to be at least 0.88 at all redshifts in our simulations. From our results, obtained by linking the simulation boxes back to source redshifts of 4, we are able to observe the formation of structure in terms of the computed shear, and also note that the major contributions to the shear come from a very broad range of redshifts. We show the probability distributions for the magnification, source ellipticity and convergence, and also describe the relationships amongst these quantities for a range of source redshifts. We find a broad range of magnifications and ellipticities; for sources at a redshift of 4, 97{1/2}% of all lines of sight show magnifications up to 1.3 and ellipticities up to 0.195. There is clear evidence that the magnification is not linear in the convergence, as might be expected for weak lensing, but contains contributions from higher order terms in both the convergence and the shear. Comment: 14 pages, LaTeX, 15 figures includedMonthly Notices of the Royal Astronomical Society 01/1999; · 5.52 Impact Factor - [show abstract] [hide abstract]

**ABSTRACT:**We have developed a new three-dimensional algorithm, based on the standard P$^3$M method, for computing deflections due to weak gravitational lensing. We compare the results of this method with those of the two-dimensional planar approach, and rigorously outline the conditions under which the two approaches are equivalent. Our new algorithm uses a Fast Fourier Transform convolution method for speed, and has a variable softening feature to provide a realistic interpretation of the large-scale structure in a simulation. The output values of the code are compared with those from the Ewald summation method, which we describe and develop in detail. With an optimal choice of the high frequency filtering in the Fourier convolution, the maximum errors, when using only a single particle, are about 7 per cent, with an rms error less than 2 per cent. For ensembles of particles, used in typical $N$-body simulations, the rms errors are typically 0.3 per cent. We describe how the output from the algorithm can be used to generate distributions of magnification, source ellipticity, shear and convergence for large-scale structure. Comment: 22 pages, latex, 11 figuresMonthly Notices of the Royal Astronomical Society 10/1998; · 5.52 Impact Factor

#### Publication Stats

82 | Citations | |

49.69 | Total Impact Points | |