A radial mass profile analysis of the lensing cluster MS2137-23

We reanalyze the strong lens modeling of the cluster of galaxies MS2137-23 using a new data set obtained with the ESO VLT. We found the photometric redshifts of the two main arc systems are both at z=1.6. After subtraction of the central cD star light of the HST image we found that only one object lying underneath has the expected properties of the fifth image associated to the tangential arc. We improve the previous lens modelings of the central dark matter distribution of the cluster, using an isothermal model with a core (IS) and the NFW-like model with a cusp. Without the fifth image, the arc properties together with the shear map profile are equally well fit by the and by an IS and a sub-class of generalized-NFW mass profiles having inner slope power index in the range 0.7<alpha<1.2. Adding new constrains provided by the fifth image favors IS profiles that better predict the fifth image properties. A model including cluster galaxy perturbations or the the stellar mass distribution does not change our conclusions but imposes the M/L_I of the cD stellar component is below 10 at a 99% confidence level. Using our new detailed lensing model together with Chandra X-ray data and the cD stellar component we finally discuss intrinsic properties of the gravitational potential. Whereas X-ray and dark matter have a similar shape at various radius, the cD stellar isophotes are twisted by 13 deg. The sub- arc-second azimuthal shift we observe between the radial arc position and the predictions of elliptical models correspond to what is expected from a mass distribution twist. This shift may result from a projection effect of the cD and the cluster halos, thus revealing the triaxiality of the system.Comment: Final version accepted in A&

X-ray observations and mass determinations in the cluster of galaxies Cl0024+17

We present a detailed analysis of the mass distribution in the rich and distant cluster of galaxies Cl0024+17. X-ray data come from both a deep ROSAT/HRI image of the field (Bohringer et al. 1999) and ASCA spectral data. Using a wide field CCD image of the cluster, we optically identify all the faint X-ray sources, whose counts are compatible with deep X-ray number counts. In addition we marginally detect the X-ray counter-part of the gravitational shear perturbation detected by Bonnet et al. (1994) at a 2.5 $\sigma$ level. A careful spectral analysis of ASCA data is also presented. In particular, we extract a low resolution spectrum of the cluster free from the contamination by a nearby point source located 1.2 arcmin from the center. The X-ray temperature deduced from this analysis is $T_X = 5.7 ^{+4.9}_{-2.1}$ keV at the 90% confidence level. The comparison between the mass derived from a standard X-ray analysis and from other methods such as the Virial Theorem or the gravitational lensing effect lead to a mass discrepancy of a factor 1.5 to 3. We discuss all the possible sources of uncertainties in each method of mass determination and give some indications on the way to reduce them. A complementary study of optical data is in progress and may solve the X-ray/optical discrepancy through a better understanding of the dynamics of the cluster.Comment: Revised version, accepted in Astronomy and Astrophysics (Main Journal). Few changes in the discussio

Searching for galaxy clusters using the aperture mass statistics in 50 VLT fields

Application of the aperture mass (Map-) statistics provides a weak lensing method for the detection of cluster-sized dark matter halos. We present a new aperture filter function and maximise the effectiveness of the Map-statistics to detect cluster-sized halos using analytical models. We then use weak lensing mock catalogues generated from ray-tracing through N-body simulations, to analyse the effect of image treatment on the expected number density of halos. Using the Map-statistics, the aperture radius is typically several arcminutes, hence the aperture often lies partly outside a data field, consequently the signal-to-noise ratio of a halo detection decreases. We study these border effects analytically and by using mock catalogues. We find that the expected number density of halos decreases by a factor of two if the size of a field is comparable to the diameter of the aperture used. We finally report on the results of a weak lensing cluster search applying the Map-statistics to 50 randomly selected fields which were observed with FORS1 at the VLT. Altogether the 50 VLT fields cover an area of 0.64 square degrees. The I-band images were taken under excellent seeing conditions (average seeing 0.6 arcsec.) which results in a high number density of galaxies used for the weak lensing analysis (26/sq.arcmin). In five of the VLT fields, we detect a significant Map-signal which coincides with an overdensity of the light distribution. These detections are thus excellent candidates for shear-selected clusters.Comment: 23 pages, 5 tables, 24 figures, published in A&A, Sect. 3.5 and 7 are changed or altered; Fig. 11 is change

Cluster Cores, Gravitational Lensing, and Cosmology

Many multiply--imaged quasars have been found over the years, but none so far with image separation in excess of 8\arcsec. The absence of such large splittings has been used as a test of cosmological models: the standard Cold Dark Matter model has been excluded on the basis that it predicts far too many large--separation double images. These studies assume that the lensing structure has the mass profile of a singular isothermal sphere. However, such large splittings would be produced by very massive systems such as clusters of galaxies, for which other gravitational lensing data suggest less singular mass profiles. Here we analyze two cases of mass profiles for lenses: an isothermal sphere with a finite core radius (density $\rho \propto (r^2+r_{core}^2)^{-1})$, and a Hernquist profile ($\rho \propto r^{-1}(r+a)^{-3}$). We find that small core radii $r_{core} \sim 30 h^{-1}$ kpc, as suggested by the cluster data, or large a \gsim 300 h^{-1} kpc, as needed for compatibility with gravitational distortion data, would reduce the number of large--angle splittings by an order of magnitude or more. Thus, it appears that these tests are sensitive both to the cosmological model (number density of lenses) and to the inner lens structure, which is unlikely to depend sensitively on the cosmology, making it difficult to test the cosmological models by large--separation quasar lensing until we reliably know the structure of the lenses themselves.Comment: 17 pages, uuencoded compressed tarred postscript file including text and 1 figure. To appear in January 20, 1996 issue of ApJ Letter

Evolution of hierarchical clustering in the CFHTLS-Wide since z~1

We present measurements of higher order clustering of galaxies from the latest release of the Canada-France-Hawaii-Telescope Legacy Survey (CFHTLS) Wide. We construct a volume-limited sample of galaxies that contains more than one million galaxies in the redshift range 0.2<z<1 distributed over the four independent fields of the CFHTLS. We use a counts in cells technique to measure the variance and the hierarchical moments S_n = /^(n-1) (3<n<5) as a function of redshift and angular scale.The robustness of our measurements if thoroughly tested, and the field-to-field scatter is in very good agreement with analytical predictions. At small scales, corresponding to the highly non-linear regime, we find a suggestion that the hierarchical moments increase with redshift. At large scales, corresponding to the weakly non-linear regime, measurements are fully consistent with perturbation theory predictions for standard LambdaCDM cosmology with a simple linear bias.Comment: 17 pages, 11 figures, submitted to MNRA

Ringing effects reduction by improved deconvolution algorithm Application to A370 CFHT image of gravitational arcs

We develop a self-consistent automatic procedure to restore informations from astronomical observations. It relies on both a new deconvolution algorithm called LBCA (Lower Bound Constraint Algorithm) and the use of the Wiener filter. In order to explore its scientific potential for strong and weak gravitational lensing, we process a CFHT image of the galaxies cluster Abell 370 which exhibits spectacular strong gravitational lensing effects. A high quality restoration is here of particular interest to map the dark matter within the cluster. We show that the LBCA turns out specially efficient to reduce ringing effects introduced by classical deconvolution algorithms in images with a high background. The method allows us to make a blind detection of the radial arc and to recover morphological properties similar to thoseobserved from HST data. We also show that the Wiener filter is suitable to stop the iterative process before noise amplification, using only the unrestored data.Comment: A&A in press 9 pages 9 figure

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.Comment: Sumitted to A&A Main Journal. 15 pages, 11 figures. 75 Kb gzipped tar file. Figures with images not included, but available on ftp.iap.fr /pub/from_users/mellier/A1942: a1942darkclump.ps.gz (2.1 Mb