arXiv subjects

L. van Waerbeke

Publications and source records attributed to L. van Waerbeke.

45 records · Page 3Linked to original sources

arXiv2000

Cosmic Shear with the CFHT

We present results of our cosmic shear survey in progress at the Canada- France-Hawaii Telescope (CFHT). We analysed 1.7 sq. degrees of data, out of which we measured a significant correlation of galaxy shape-orientation over several arcmin scale. We present measurements of the variance of the shear $<γ^2>$ and of the correlation functions $ $, $ $. The signal is consistent with gravitational lensing by large scale structures predictions. The level of residual systematics after PSF correction is small compared to our measured signal. We outline several evolutions of our work by using additional data covering 9 sq. degrees.

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arXiv2000

Cosmic Shear with the VLT

We report on the detection of cosmic shear on angular scales of 1.3-6.5 arcmin using 45 independent empty fields observed with the Very Large Telescope (VLT). This result confirms previous measurements obtained with the CFH Telescope at the same angular scales. We present the data analysis and the first preliminary results.

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arXiv2000

A Numerical Study of Cosmic Shear Statistics

We explore the stability of the variance and skewness of the cosmic gravitational convergence field, using two different approaches: first we simulate a whole MEGACAM survey (100 sq. degrees). The reconstructed mass map, obtained from a shear map, shows that the state-of-the-art data analysis methods can measure weak-lensing statistics at angular scales ranging from 2.5' to 25'. We looked also at the influence of a varying signal-to-noise ratio over the shear map (due to local variations of source density) on the mass reconstruction, by means of Monte-Carlo simulation. The effect at small scales can easily be corrected-for in most of the relevant cases. These results enhance the confidence in the capability of future large surveys to measure accurately cosmologically interesting quantities.

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arXiv2000

CMB B-polarization to map the Large-scale Structures of the Universe

We explore the possibility of using the B-type polarization of the CMB to map the large-scale structures of the Universe taking advantage of the lens effects on the CMB polarization. The functional relation between the B component with the primordial CMB polarization and the line-of-sight mass distribution is explicited. Noting that a sizeable fraction (at least 40%) of the dark halo population which is responsible of this effect can also be detected in galaxy weak lensing survey, we present statistical quantities that should exhibit a strong sensitivity to this overlapping. We stress that it would be a sound test of the gravitational instability picture, independent on many systematic effects that may hamper lensing detection in CMB or galaxy survey alone. Moreover we estimate the intrinsic cosmic variance of the amplitude of this effect to be less than 8% for a 100, deg^2 survey with a 10' CMB beam. Its measurement would then provide us with an original mean for constraining the cosmological parameters, more particularly, as it turns out, the cosmological constant Lambda.

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arXiv1999

Mass-detection of a matter concentration projected near the cluster Abell 1942: Dark clump or high-redshift cluster?

A weak-lensing analysis of wide-field $V$- and $I$-band images centered on the cluster Abell 1942 has uncovered a mass concentration $\sim 7$ arcminutes South of the cluster center. A statistical analysis shows that the detections are highly significant. No strong concentration of bright galaxies is seen at the position of the mass concentration, though a slight galaxy number overdensity and a weak extended X-ray source are present about 1' away from its center. From the spatial dependence of the tangential alignment around the center of the mass concentration, we inferred a lower bound on the mass inside a sphere of radius $0.5 h^{-1}$\ts Mpc of $1\times 10^{14}h^{-1}M_\odot$, much higher than crude mass estimates based on X-ray data. No firm conclusion can be inferred about the nature of the clump. If it were a high-redshift cluster, the weak X-ray flux would indicate that it had an untypically low X-ray luminosity for its mass; if the X-ray emission were physically unrelated to the mass concentration, this conclusion would be even stronger. The search for massive halos by weak lensing enables us for the first time to select halos based on their mass properties only and to detect new types of objects, e.g., dark halos. The mass concentration in the field of A1942 may be the first example of such a halo.

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arXiv1998

Dark Matter and Gravitational Lensing

The last decade has shown a considerable development of gravitational lensing for cosmology because it probes the amount and the nature of dark matter, and provides information on the density parameter $Ω$, the cosmological constant $Λ$ and the Hubble constant $H_{o}$. Therefore, gravitational lensing can constrain the cosmological scenario which gave birth to the Universe as it appears today. The ongoing programs and future projects which are developing now all over the world show that gravitational lensing is considered as a major cosmological tool for the coming years as well. In this review, we summarize some of the most recent advances in the fields relevant for the dark matter issue. We will focus on the microlensing, the arc(let)s and the weak lensing studies. The possibility to check the existence of a non-zero $Λ$ is presented elsewhere.

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arXiv1997

Weak Lensing Statistics as a Probe of Omega and Power Spectrum

The possibility of detecting weak lensing effects from deep wide field imaging surveys has open new means of probing the large-scale structures of the Universe and measuring cosmological parameters. In this paper we present a systematic study of the expected dependence of the low order moments of the filtered gravitational local convergence with the power spectrum of the density fluctuations and with the cosmological parameters Omega_0 and Lambda. The results show a significant dependence with all these parameters. Though we note that this degeneracy could be partially raised by considering two populations of sources, at different redshifts, computing the third moment is more promising since it is expected, in the quasi-linear regime and for Gaussian initial conditions, to be only $Ω_0$ dependent (with a slight degeneracy with Lambda) when it is correctly expressed in terms of the second moment. More precisely we show that the variance of the convergence varies approximately as P(k) Omega_0^{1.5} z_s^{1.5}, whereas the skewness varies as Omega_0^{-0.8} z_s^{-1.35}, where P(k) is the projected power spectrum and z_s the redshift of sources. Thus, used jointly they can provide both P(k) and Omega_0. However, the dependence with the redshift of sources is large and could be a major concern for a practical implementation. We have estimated the errors expected for these parameters in realistic scenario and sketched what would be the observational requirements for doing such measurements. A more detailed study of an observational strategy is left for a second paper.

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arXiv1997

Detection of shear due to weak lensing by large-scale structure

We present evidence for a coherent shear signal in a field containing the z=1.2 radio-source PKS1508-05. Since there were no intervening mass concentrations known before targeting this field, we interpret this signal as due to weak lensing by large-scale structure. This result is the outcome of a re-analysis of the observations of Fort et al. (1996) in the fields of three high redshift QSO/radio-sources. Several tests of the robustness of the signal were performed: subdivision of the field and tests using simulated data with randomized ellipticity orientation; using the pixel-to-pixel autocorrelation function method to measure the shear; using the correlation function of the galaxy ellipticities with bootstrap resampling to estimate errors and to find the typical angular separation that dominates the signal. We find that the field of PKS1508-05 contains a robust shear signal that is coherent over the 2' by 2' field. The signal is detected at a high level of significance (99.6%) using the tests described above. The amplitude of the shear on a scale of 1' is about 3%, which is consistent with theoretical expectations from weak lensing by large-scale structure.

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arXiv1996

Recent Advances in Gravitational Lensing

The gravitational lensing effect is one of the most promising tools for cosmology. Indeed it probes directly the total mass distribution in large-scale structures and can as well provide valuable informations on the values of the density parameter Omega and the cosmological constant lambda. In this review we summarize the most spectacular observational and theoretical advances obtained in this field during the last five years.

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