arXiv subjects

A. Refregier

Publications and source records attributed to A. Refregier.

At least 55 records · Page 3Linked to original sources

arXiv2007

The XMM-LSS survey: the Class 1 cluster sample over the initial 5 square degrees and its cosmological modelling

We present a sample of 29 galaxy clusters from the XMM-LSS survey over an area of some 5deg2 out to a redshift of z=1.05. The sample clusters, which represent about half of the X-ray clusters identified in the region, follow well defined X-ray selection criteria and are all spectroscopically confirmed. For all clusters, we provide X-ray luminosities and temperatures as well as masses. The cluster distribution peaks around z=0.3 and T =1.5 keV, half of the objects being groups with a temperature below 2 keV. Our L-T(z) relation points toward self-similar evolution, but does not exclude other physically plausible models. Assuming that cluster scaling laws follow self-similar evolution, our number density estimates up to z=1 are compatible with the predictions of the concordance cosmology and with the findings of previous ROSAT surveys. Our well monitored selection function allowed us to demonstrate that the inclusion of selection effects is essential for the correct determination of the evolution of the L-T relation, which may explain the contradictory results from previous studies. Extensive simulations show that extending the survey area to 10deg2 has the potential to exclude the non-evolution hypothesis, but that constraints on more refined ICM models will probably be limited by the large intrinsic dispersion of the L-T relation. We further demonstrate that increasing the dispersion in the scaling laws increases the number of detectable clusters, hence generating further degeneracy [in addition to sigma8, Omega_m, L(M,z) and T(M,z)] in the cosmological interpretation of the cluster number counts. We provide useful empirical formulae for the cluster mass-flux and mass-count-rate relations as well as a comparison between the XMM-LSS mass sensitivity and that of forthcoming SZ surveys.

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arXiv2007

The XMM-LSS catalogue: X-ray sources and associated optical data. Version I

Following the presentation of the XMM-LSS X-ray source detection package by Pacaud et al., we provide the source lists for the first 5.5 surveyed square degrees. The catalogues pertain to the [0.5-2] and [2-10] keV bands and contain in total 3385 point-like or extended sources above a detection likelihood of 15 in either band. The agreement with deep logN-logS is excellent. The main parameters considered are position, countrate, source extent with associated likelihood values. A set of additional quantities such as astrometric corrections and fluxes are further calculated while errors on the position and countrate are deduced from simulations. We describe the construction of the band-merged catalogue allowing rapid sub-sample selection and easy cross-correlation with external multi-wavelength catalogues. A small optical CFHTLS multi-band subset of objects is associated wich each source along with an X-ray/optical overlay. We make the full X-ray images available in FITS format. The data are available at CDS and, in a more extended form, at the Milan XMM-LSS database.

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arXiv2007

COSMOS : Hubble Space Telescope Observations

The Cosmic Evolution Survey (COSMOS) was initiated with an extensive allocation (590 orbits in Cycles 12-13) using the Hubble Space Telescope (HST) for high resolution imaging. Here we review the characteristics of the HST imaging with the Advanced Camera for Surveys (ACS) and parallel observations with NICMOS and WFPC2. A square field (1.8$\sq$°) has been imaged with single-orbit ACS I-F814W exposures with 50% completeness for sources 0.5\arcsec in diameter at I$_{AB} $ = 26.0 mag. The ACS imaging is a key part of the COSMOS survey, providing very high sensitivity and high resolution (0.09\arcsec FWHM, 0.05\arcsec pixels) imaging and detecting 1.2 million objects to a limiting magnitude of 26.5 (AB). These images yield resolved morphologies for several hundred thousand galaxies. The small HST PSF also provides greatly enhanced sensitivity for weak lensing investigations of the dark matter distribution.

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arXiv2007

Weak Gravitational Lensing with COSMOS: Galaxy Selection and Shape Measurements

With a primary goal of conducting precision weak lensing measurements from space, the COSMOS survey has imaged the largest contiguous area observed by the Hubble Space Telescope (HST) to date using the Advanced Camera for Surveys (ACS). This is the first paper in a series where we describe our strategy for addressing the various technical challenges in the production of weak lensing measurements from the COSMOS data. The COSMOS ACS catalog is constructed from 575 ACS/WFC tiles (1.64 deg^2) and contains a total 1.2x10^6 objects to a limiting magnitude of F814W=26.5. This catalog is made publicly available. The shapes of galaxies have been measured and corrected for the distortion induced by the time varying ACS Point Spread Function and for Charge Transfer Efficiency effects. Next, simulated images are used to derive the shear susceptibility factors that are necessary in order to transform shape measurements into unbiased shear estimators. Finally, for each galaxy, we derive a shape measurement error and utilize this quantity to extract the intrinsic shape noise of the galaxy sample. Interestingly, our results indicate that the intrinsic shape noise varies little with either size, magnitude or redshift. Representing a number density of 66 galaxies per arcmin^2, the final COSMOS weak lensing catalog contains 3.9x10^5 galaxies with accurate shape measurements. The properties of the COSMOS weak lensing catalog described throughout this paper will provide key input numbers for the preparation and design of next-generation wide field space missions.

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arXiv2006

DUNE: The Dark Universe Explorer

Understanding the nature of Dark Matter and Dark Energy is one of the most pressing issues in cosmology and fundamental physics. The purpose of the DUNE (Dark UNiverse Explorer) mission is to study these two cosmological components with high precision, using a space-based weak lensing survey as its primary science driver. Weak lensing provides a measure of the distribution of dark matter in the universe and of the impact of dark energy on the growth of structures. DUNE will also include a complementary supernovae survey to measure the expansion history of the universe, thus giving independent additional constraints on dark energy. The baseline concept consists of a 1.2m telescope with a 0.5 square degree optical CCD camera. It is designed to be fast with reduced risks and costs, and to take advantage of the synergy between ground-based and space observations. Stringent requirements for weak lensing systematics were shown to be achievable with the baseline concept. This will allow DUNE to place strong constraints on cosmological parameters, including the equation of state parameter of the dark energy and its evolution from redshift 0 to 1. DUNE is the subject of an ongoing study led by the French Space Agency (CNES), and is being proposed for ESA's Cosmic Vision programme.

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arXiv2006

The XMM Large Scale Structure survey: The X-ray pipeline and survey selection function

We present the X-ray pipeline developed for the purpose of the cluster search in the XMM-LSS survey. It is based on a two-stage procedure via a dedicated handling of the Poisson nature of the signal: (1) source detection on multi-resolution wavelet filtered images; (2) source analysis by means of a maximum likelihood fit to the photon images. The source detection efficiency and characterisation are studied through extensive Monte-Carlo simulations. This led us to define two samples of extended sources: the C1 class that is uncontaminated, and the less restrictive C2 class that allows for 50% contamination. The resulting predicted selection function is presented and the comparison to the current XMM-LSS confirmed cluster sample shows very good agreement. We arrive at average predicted source densities of about 7 C1 and 12 C2 per deg2, which is higher than any available wide field X-ray survey. We finally notice a substantial deviation of the predicted redshift distribution for our samples from the one obtained using the usual assumption of a flux limited sample.

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arXiv2005

Cosmology with wide-field SZ cluster surveys: Selection and Systematic Effects

The cosmological potential of large-scale structure observations for cosmology have been extensively discussed in the litterature. In particular, it has recently been shown how Sunyaev-Zel'dovich (SZ) cluster surveys can be used to constrain dark energy parameters. In this paper, we study whether selection and systematics effects will limit future wide-field SZ surveys from achieving their cosmological potential. For this purpose, we use a sky simulation and an SZ-cluster detection software presented in Pires et al. (2005), using the future Olimpo, APEX and Planck surveys as a concrete examples. We show that the SZ-cluster selection function and contamination of SZ-cluster catalogues are more complex than is usually assumed. In particular, the simulated field-to-field detected cluster counts is a factor 3 larger than the expected Poisson fluctuations. We also study the impact of missing redshift information and of the uncertainty of the scaling relations for low mass clusters. We quantify, through hypothesis tests, how near-future SZ experiments can be used to discriminate between different structure formation models. Using a maximum likelihood approach, we then study the impact of these systematics on the joint measurement of cosmological models and of cluster scaling relations.

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arXiv2005

Prospects for Dark Energy Evolution: a Frequentist Multi-Probe Approach

A major quest in cosmology is the understanding of the nature of dark energy. It is now well known that a combination of cosmological probes is required to break the underlying degeneracies on cosmological parameters. In this paper, we present a method, based on a frequentist approach, to combine probes without any prior constraints, taking full account of the correlations in the parameters. As an application, a combination of current SNIa and CMB data with an evolving dark energy component is first compared to other analyses. We emphasise the consequences of the implementation of the dark energy perturbations on the result for a time varying equation of state. The impact of future weak lensing surveys on the measurement of dark energy evolution is then studied in combination with future measurements of the cosmic microwave background and type Ia supernovae. We present the combined results for future mid-term and long-term surveys and confirm that the combination with weak lensing is very powerful in breaking parameter degeneracies. A second generation of experiment is however required to achieve a 0.1 error on the parameters describing the evolution of dark energy.

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arXiv2004

The XMM-LSS survey. Survey design and first results

We have designed a medium deep large area X-ray survey with XMM - the XMM Large Scale Structure survey, XMM-LSS - with the scope of extending the cosmological tests attempted using ROSAT cluster samples to two redshift bins between 0<z<1 while maintaining the precision of earlier studies. Two main goals have constrained the survey design: the evolutionary study of the cluster-cluster correlation function and of the cluster number density. The results are promising and, so far, in accordance with our predictions as to the survey sensitivity and cluster number density. The feasibility of the programme is demonstrated and further X-ray coverage is awaited in order to proceed with a truly significant statistical analysis. (Abridged)

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arXiv2004

Supernova / Acceleration Probe: A Satellite Experiment to Study the Nature of the Dark Energy

The Supernova / Acceleration Probe (SNAP) is a proposed space-based experiment designed to study the dark energy and alternative explanations of the acceleration of the Universe's expansion by performing a series of complementary systematics-controlled measurements. We describe a self-consistent reference mission design for building a Type Ia supernova Hubble diagram and for performing a wide-area weak gravitational lensing study. A 2-m wide-field telescope feeds a focal plane consisting of a 0.7 square-degree imager tiled with equal areas of optical CCDs and near infrared sensors, and a high-efficiency low-resolution integral field spectrograph. The SNAP mission will obtain high-signal-to-noise calibrated light-curves and spectra for several thousand supernovae at redshifts between z=0.1 and 1.7. A wide-field survey covering one thousand square degrees resolves ~100 galaxies per square arcminute. If we assume we live in a cosmological-constant-dominated Universe, the matter density, dark energy density, and flatness of space can all be measured with SNAP supernova and weak-lensing measurements to a systematics-limited accuracy of 1%. For a flat universe, the density-to-pressure ratio of dark energy can be similarly measured to 5% for the present value w0 and ~0.1 for the time variation w'. The large survey area, depth, spatial resolution, time-sampling, and nine-band optical to NIR photometry will support additional independent and/or complementary dark-energy measurement approaches as well as a broad range of auxiliary science programs. (Abridged)

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arXiv2003

Power Spectrum Normalisation and the Non-Gaussian Halo Model

The normalisation of the matter power spectrum, sigma_8, is an essential ingredient to predict the phenomenology of the low redshift universe. It has been measured using several methods, such as X-ray cluster counts, weak lensing and the cosmic microwave background, which have yielded values ranging from 0.7 to 1.0. While these differences could be due to systematic effects, they could also be due to physics beyond the standard LambdaCDM model. An obvious possibility is the presence of non-Gaussian initial fluctuations in the density field. To study the impact of non-Gaussianity on each of these methods, we use a generalised halo model to compute cluster counts and the non-linear power spectrum for non-Gaussian models. Assuming scale invariance, the upper-limits on non-Gaussianity from the WMAP CMB experiment correspond to roughly a 4% shift in sigma_8 as measured from cluster counts and about 2% shift through weak lensing. This is not enough to account for the current internal and mutual discrepancies between the different methods, unless non-Gaussianity is strongly scale dependent. A comparison between future X-ray surveys with a two fold improvement in cluster mass calibration and future cosmic shear surveys with 400 deg^2 will be required to constrain non-Gaussianity on small scales with a precision matching that of the current CMB constraints on larger scales. Our results argue for the presence of systematics in the current cluster and cosmic shear surveys, or to non-standard physics other than non-Gaussianity.

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arXiv2002

Cosmological Parameters, Dark Energy and Large Scale Structure

We review the current status of cosmological parameters, dark energy and large-scale structure, from a theoretical and observational perspective. We first present the basic cosmological parameters and discuss how they are measured with different observational techniques. We then describe the recent evidence for dark energy from Type Ia supernovae. Dynamical models of the dark energy, quintessence, are then described, as well as how they relate to theories of gravity and particle physics. The basic theory of structure formation via gravitational instability is then reviewed. Finally, we describe new observational probes of the large-structure of the universe, and how they constrain cosmological parameters.

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arXiv2002

Cosmic Shear and Power Spectrum Normalization with the Hubble Space Telescope

Weak lensing by large-scale structure provides a direct measurement of matter fluctuations in the universe. We report a measurement of this `cosmic shear' based on 271 WFPC2 archival images from the Hubble Space Telescope Medium Deep Survey (MDS). Our measurement method and treatment of systematic effects were discussed in an earlier paper. We measure the shear variance on scales ranging from 0.7' to 1.4', with a detection significance greater than 3.8. This allows us to measure the normalization of the matter power spectrum to be sigma8 = (0.94 +/- 0.10 +/- 0.14) (0.3/Omega_m)^0.44 (0.21/Gamma)^0.15, in a LCDM universe. The first 1sigma error includes statistical errors only, while the latter also includes (gaussian) cosmic variance and the uncertainty in the galaxy redshift distribution. Our results are consistent with earlier cosmic shear measurements from the ground and from space. We compare our cosmic shear results and those from other groups to the normalization from cluster abundance and galaxy surveys. We find that the combination of four recent cosmic shear measurements are somewhat inconsistent with the recent normalization using these methods, and discuss possible explanations for the discrepancy.

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arXiv2002

The XMM Large Scale Structure Survey: Scientific Motivation and First Observations

Thanks to its unrivalled sensitivity and large field of view, XMM potentially occupies a leading position as a survey instrument. We present cosmological arguments in favour of a medium-sensitivity, large-scale structure survey with XMM, using galaxy clusters as tracers of the cosmic network. We show how this has motivated the definition of a concrete survey, the XMM Large-Scale Structure Survey (XMM-LSS), which will cover 64 square degrees with a sensitivity about 1000 times better than that of the ROSAT All-Sky Survey. We present our predictions for cluster counts based on the Press-Schechter formalism and detailed X-ray image simulations, and show how they agree with the cluster statistics from recent ROSAT cluster surveys. We also present the extensive multi-wavelength follow-up associated with XMM-LSS, as well as the first observations from the programme.

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arXiv2001

Detection of Cosmic Shear with the HST Survey Strip

Weak lensing by large-scale structure provides a unique method to directly measure matter fluctuations in the universe, and has recently been detected from the ground. Here, we report the first detection of this `cosmic shear' based on space-based images. The detection was derived from the Hubble Space Telescope (HST) Survey Strip (or Groth Strip), a 4' by 42' set of 28 contiguous WFPC2 pointings with I<27. The small size of the HST Point-Spread Function (PSF) affords both a lower statistical noise, and a much weaker sensitivity to systematic effects, a crucial limiting factor of cosmic shear measurements. Our method and treatment of systematic effects were discussed in an earlier paper (Rhodes, Refregier & Groth 2000). We measure an rms shear of 1.8% on the WFPC2 chip scale (1.27'), in agreement with the predictions of cluster-normalized CDM models. Using a Maximum Likelihood (ML) analysis, we show that our detection is significant at the 99.5% confidence level (CL), and measure the normalization of the matter power spectrum to be sigma8*Omega_m^(0.48) = 0.51 (+0.14,-0.17), in a LambdaCDM universe. These 68% CL errors include (Gaussian) cosmic variance, systematic effects and the uncertainty in the redshift distribution of the background galaxies. Our result is consistent with earlier lensing measurements from the ground, and with the normalization derived from cluster abundance. We discuss how our measurement can be improved with the analysis of a large number of independent WFPC2 fields.

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arXiv2000

Extragalactic Foregrounds of the Cosmic Microwave Background: Prospects for the MAP Mission

(Abridged) While the major contribution to the Cosmic Microwave Background (CMB) anisotropies are the sought-after primordial fluctuations produced at the surface of last scattering, other effects produce secondary fluctuations at lower redshifts. Here, we study the extragalactic foregrounds of the CMB in the context of the upcoming MAP mission. We first survey the major extragalactic foregrounds and show that discrete sources, the Sunyaev-Zel'dovich (SZ) effect, and gravitational lensing are the most dominant ones for MAP. We then show that MAP will detect (>5 sigma) about 46 discrete sources and 10 SZ clusters directly with 94 GHz fluxes above 2 Jy. The mean SZ fluxes of fainter clusters can be probed by cross-correlating MAP with cluster positions extracted from existing catalogs. For instance, a MAP-XBACs cross-correlation will be sensitive to clusters with S(94GHz)>200mJy, and will thus provide a test of their virialization state and a measurement of their gas fraction. Finally, we consider probing the hot gas on supercluster scales by cross-correlating the CMB with galaxy catalogs. Assuming that galaxies trace the gas, we show that a cross-correlation between MAP and the APM catalog should yield a marginal detection, or at least a four-fold improvement on the COBE upper limits for the rms Compton y-parameter.

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arXiv2000

The Power Spectrum of the Sunyaev-Zel'dovich Effect

(Abridged) The hot gas in the IGM produces anisotropies in the Cosmic Microwave Background (CMB) through the thermal Sunyaev-Zel'dovich (SZ) effect. The SZ effect is a powerful probe of large-scale structure in the universe and must be carefully subtracted from measurements of the primary CMB anisotropies. We use moving-mesh hydrodynamical simulations to study the 3-dimensional statistics of the gas, and compute the mean comptonization parameter and the angular power spectrum of the SZ fluctuations, for different cosmologies. We compare these results with predictions using the Press-Schechter formalism. We find that the two methods agree approximately, but differ in details. We discuss this discrepancy, and show that resolution limits the reliability of our results to the 200<l<2000 range. For cluster- normalized CDM models, the SZ power spectrum is comparable to the primordial power spectrum around l=2000. We show that groups and filaments (kT<5 keV) contribute about 50% of the SZ power spectrum at l=500. About half of the SZ power spectrum on these scales is produced at redshifts z<0.1, and can thus be detected and removed using existing catalogs of galaxies and X-ray clusters. We discuss the implications of these results for the future MAP and Planck Surveyor missions.

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arXiv1999

Overview of Secondary Anisotropies of the CMB

While the major contribution to the Cosmic Microwave Background (CMB) anisotropies are the sought-after primordial fluctuations produced at the surface of last scattering, other effects produce secondary fluctuations at lower redshifts. These secondary fluctuations must be carefully accounted for, in order to isolate primordial fluctuations. In addition, they are interesting in their own right, since they provide a wealth of information on the geometry and local properties of the universe. Here, I survey the different sources of secondary anisotropies and extragalactic foregrounds of the CMB. I show their relative importance on the multipole-frequency plane. I discuss in particular their impact in the future CMB missions MAP and Planck Surveyor.

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