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We present an optical study of the strong lensing galaxy cluster MS 0440.5$+$0204 at $z=0.19593$, based on CFHT/MegaCam g', r'-photometry and GMOS/Gemini and CFHT/MOS/SIS spectroscopy in a broader area compared to previous works. We have determined new spectroscopic redshifts for the most prominent gravitational arcs surrounding the central galaxy in the cluster. The new redshifts and the information provided by the photometric catalog yield us to perform a detailed weak and strong lensing mass reconstruction of the cluster. The large number of member galaxies and the area covered by our observations allow to estimate more accurately the velocity dispersion and mass of cluster and examine in detail the nature of the cluster and surroundings structures. The dynamical mass is in good agreement with the mass inferred from the lensing analysis and X-ray estimates. About $\sim$68\% of the galaxies are located in the inner $\lesssim$0.86 h$^{-1}_{70}$ Mpc region of the cluster. The galaxy redshift distribution in the inner region of the cluster shows a complex structure with at least three sub-structures along the line-of-sight. Other sub-structures are also identified in the galaxy density map and in the weak lensing mass map. The member galaxies in the North-East overdensity are distributed in a filament between MS 0440.5$+$0204 and ZwCL 0441.1$+$0211 clusters, suggesting that these two structures might be connected. MS 0440$+$0204 appears to be dynamically active, with a cluster core that is likely experiencing a merging process and with other nearby groups at projected distances of $\lesssim$1 h$^{-1}_{70}$ Mpc that could be being accreted by the cluster., Comment: ApJ accepted, 23 pages, 15 figures, 7 tables

The main goal of this work is to compare the results of three dynamical mass estimators to the X-ray hydrostatic values, focussing on massive galaxy clusters at intermediate redshifts $z\sim0.3$. We estimated dynamical masses with the virial theorem, the Jeans equation, and the caustic method using wide-field VIMOS spectroscopy. We investigated the role of colour selection and the impact of substructures on the dynamical estimators. The Jeans and caustic methods give consistent results, whereas the virial theorem leads to masses $\sim15\%$ larger. The Jeans, caustic, and virial masses are respectively $\sim20\%$, $\sim30\%$, and $\sim50\%$ larger than the hydrostatic values. Large scatters of $\gtrsim50\%$ are mainly due to the two outliers RXCJ0014 and RXCJ1347; excluding the latter increases the mass ratios by $\sim10\%$, giving a fractional mass bias significant at $\gtrsim2\sigma$. We found a correlation between the dynamical-to-hydrostatic mass ratio and two substructure indicators, suggesting a bias in the dynamical measurements. The velocity dispersions of blue galaxies are $\sim15\%$ ($\sim25\%$ after removing the substructures) larger than that of the red-sequence galaxies; using the latter leads to dynamical masses $\sim10\%-15\%$ smaller. Discarding the galaxies part of substructures reduces the masses by $\sim15\%$; the effect is larger for the more massive clusters, owing to a higher level of substructures. After the substructure analysis, the dynamical masses are in perfect agreement with the hydrostatic values and the scatters around the mean ratios are divided by $\sim2$.

In this paper we study the cosmic acceleration for five dynamical dark energy models whose equation of state varies with redshift. The cosmological parameters of these models are constrained by performing a MCMC analysis using mainly gas mass fraction, $f_{gas}$, measurements in two samples of galaxy clusters: one reported by Allen et al. (2004), which consists of $42$ points spanning the redshift range $0.05

The hierarchical model of structure formation is a key prediction of the Lambda cold dark matter model, which can be tested by studying the large-scale environment and the substructure content of massive galaxy clusters. We present here a detailed analysis of the clusters RXCJ0225.9-4154, RXCJ0528.9-3927, and RXCJ2308.3-0211, as part of a sample of massive X-ray luminous clusters located at intermediate redshifts. We used a multiwavelength analysis, combining WFI photometric observations, VIMOS spectroscopy, and the X-ray surface brightness maps. We investigated the optical morphology of the clusters, we looked for significant counterparts in the residual X-ray emission, and we ran several tests to assess their dynamical state. We correlated the results to define various substructure features, to study their properties, and to quantify their influence on simple dynamical mass estimators. RXCJ0225 has a bimodal core, and two massive galaxy groups are located in its immediate surroundings; they are aligned in an elongated structure that is also detected in X-rays. RXCJ0528 is located in a poor environment; an X-ray centroid shift and the presence of two central BCGs provide mild evidence for a recent and active dynamical history. RXCJ2308 has complex central dynamics, and it is found at the core of a superstes-cluster. The complexity of the cluster's central dynamics reflects the richness of its large-scale environment: RXCJ0225 and RXCJ2308 present a mass fraction in substructures larger than the typical 0.05-0.15, whereas the isolated cluster RXCJ0528 does not have any major substructures within its virial radius. The largest substructures are found in the cluster outskirts. The optical morphology of the clusters correlates with the orientation of their BCG, and with the position of the main axes of accretion.

We present a new framework were we simultaneously fit strong lensing (SL) and dynamical data. The SL analysis is based on LENSTOOL, and the dynamical analysis uses MAMPOSSt code, which we have integrated into LENSTOOL. After describing the implementation of this new tool, we apply it on the galaxy group SL2S\,J02140-0535 ($z_{\rm spec}=0.44$), which we have already studied in the past. We use new VLT/FORS2 spectroscopy of multiple images and group members, as well as shallow X-ray data from \xmm. We confirm that the observed lensing features in SL2S\,J02140-0535 belong to different background sources. One of this sources is located at $z_{\rm spec}$ = 1.017 $\pm$ 0.001, whereas the other source is located at $z_{\rm spec}$ = 1.628 $\pm$ 0.001. With the analysis of our new and our previously reported spectroscopic data, we find 24 secure members for SL2S\,J02140-0535. Both data sets are well reproduced by a single NFW mass profile: the dark matter halo coincides with the peak of the light distribution, with scale radius, concentration, and mass equal to $r_s$ =$82^{+44}_{-17}$ kpc , $c_{200}$ = $10.0^{+1.7}_{-2.5}$, and $M_{200}$ = $1.0^{+0.5}_{-0.2}$ $\times$ 10$^{14}$M$_{\odot}$ respectively. These parameters are better constrained when we fit simultaneously SL and dynamical information. The mass contours of our best model agrees with the direction defined by the luminosity contours and the X-ray emission of SL2S\,J02140-0535. The simultaneous fit lowers the error in the mass estimate by 0.34 dex, when compared to the SL model, and in 0.15 dex when compared to the dynamical method.The combination of SL and dynamics tools yields a more accurate probe of the mass profile of SL2S\,J02140-0535 up to $r_{200}$. However, there is tension between the best elliptical SL model and the best spherical dynamical model., Comment: Accepted by Astronomy and Astrophysics. A version with better resolution in the images is available at: https://goo.gl/nzypP3

Our work is based on the stacked weak-lensing analysis of a sample of 80 strong-lensing galaxy groups. Our main results are the following: (i) the lensing signal does not allow us to firmly reject a simple singular isothermal sphere mass distribution compared to the expected NFW mass profile; (ii) we obtain an average concentration $c_{200}=8.6_{-1.3}^{+2.1}$ that is much higher than the expected value from numerical simulations for the corresponding average mass $M_{200}=0.73_{-0.10}^{+0.11}\times10^{14}\mathrm{M_{\odot}}$; (iii) the combination of our results with those at larger mass scales gives a mass-concentration relation $c(M)$ over nearly two decades in mass, with a slope in disagreement with predictions from numerical simulations using unbiased populations of dark matter haloes; (iv) our combined $c(M)$ relation matches results from simulations using only haloes with a large strong-lensing cross section, i.e. elongated with a major axis close to the line of sight; (v) for the simplest case of prolate haloes, we estimate with a toy model a lower limit on the minor:major axis ratio $a/c=0.5$ for the average SARCS galaxy group. Our analysis based on galaxy groups confirmed the results obtained at larger mass scales: strong lenses present apparently too large concentrations, which can be explained by triaxial haloes preferentially oriented with the line of sight. Because more massive systems already have large lensing cross sections, they do not require a large elongation along the line of sight, contrary to less massive galaxy groups. Therefore, it is natural to observe larger lensing (projected) concentrations for such systems, resulting in an overall mass-concentration relation steeper than that of non-lensing haloes.

We analyzed the Einstein radius, $\theta_E$, in our sample of SL2S galaxy groups, and compared it with $R_A$ (the distance from the arcs to the center of the lens), using three different approaches: 1.- the velocity dispersion obtained from weak lensing assuming a Singular Isothermal Sphere profile ($\theta_{E,I}$), 2.- a strong lensing analytical method ($\theta_{E,II}$) combined with a velocity dispersion-concentration relation derived from numerical simulations designed to mimic our group sample, 3.- strong lensing modeling ($\theta_{E,III}$) of eleven groups (with four new models presented in this work) using HST and CFHT images. Finally, $R_A$ was analyzed as a function of redshift $z$ to investigate possible correlations with L, N, and the richness-to-luminosity ratio (N/L). We found a correlation between $\theta_{E}$ and $R_A$, but with large scatter. We estimate $\theta_{E,I}$ = (2.2 $\pm$ 0.9) + (0.7 $\pm$ 0.2)$R_A$, $\theta_{E,II}$ = (0.4 $\pm$ 1.5) + (1.1 $\pm$ 0.4)$R_A$, and $\theta_{E,III}$ = (0.4 $\pm$ 1.5) + (0.9 $\pm$ 0.3)$R_A$ for each method respectively. We found a weak evidence of anti-correlation between $R_A$ and $z$, with Log$R_A$ = (0.58$\pm$0.06) - (0.04$\pm$0.1)$z$, suggesting a possible evolution of the Einstein radius with $z$, as reported previously by other authors. Our results also show that $R_A$ is correlated with L and N (more luminous and richer groups have greater $R_A$), and a possible correlation between $R_A$ and the N/L ratio. Our analysis indicates that $R_A$ is correlated with $\theta_E$ in our sample, making $R_A$ useful to characterize properties like L and N (and possible N/L) in galaxy groups. Additionally, we present evidence suggesting that the Einstein radius evolves with $z$., Comment: Accepted for publication in Astronomy & Astrophysics. Typos corrected

We estimate the expected distribution of displacements between the two dominant dark matter (DM) peaks (DM-DM displacements) and between DM and gaseous baryon peak (DM-gas displacements) in dark matter halos with masses larger than $10^{13}$ Msun/h. We use as a benchmark the observation of SL2S J08544-0121, which is the lowest mass system ($1.0\times 10^{14}$ Msun/h) observed so far featuring a bi-modal dark matter distribution with a dislocated gas component. We find that $(50 \pm 10)$% of the dark matter halos with circular velocities in the range 300 km/s to 700 km/s (groups) show DM-DM displacements equal or larger than $186 \pm 30$ kpc/h as observed in SL2S J08544-0121. For dark matter halos with circular velocities larger than 700 km/s (clusters) this fraction rises to 70 $\pm$ 10%. Using the same simulation we estimate the DM-gas displacements and find that 0.1 to 1.0% of the groups should present separations equal or larger than $87\pm 14$kpc/h corresponding to our observational benchmark; for clusters this fraction rises to (7 $\pm$ 3)%, consistent with previous studies of dark matter to baryon separations. Considering both constraints on the DM-DM and DM-gas displacements we find that the number density of groups similar to SL2S J08544-0121 is $\sim 6.0\times 10^{-7}$ Mpc$^{-3}$, three times larger than the estimated value for clusters. These results open up the possibility for a new statistical test of LCDM by looking for DM-gas displacements in low mass clusters and groups., Comment: 6 pages, 3 figures, accepted for publication in ApJ Letters

We report on the X-ray observation of a strong lensing selected group, SL2S J08544-0121, with a total mass of $2.4 \pm 0.6 \times 10^{14}$ $\rm{M_\odot}$ which revealed a separation of $124\pm20$ kpc between the X-ray emitting collisional gas and the collisionless galaxies and dark matter (DM), traced by strong lensing. This source allows to put an order of magnitude estimate to the upper limit to the interaction cross section of DM of 10 cm$^2$ g$^{-1}$. It is the lowest mass object found to date showing a DM-baryons separation and it reveals that the detection of bullet-like objects is not rare and confined to mergers of massive objects opening the possibility of a statistical detection of DM-baryons separation with future surveys., Comment: 5 pages, 3 figures. Accepted for publication in MNRAS Letters. Typos corrected

We present the weak lensing and optical analysis of the SL2S-ARCS (SARCS) sample of strong lens candidates. The sample is based on the Strong Lensing Legacy Survey (SL2S), a systematic search of strong lensing systems in the photometric Canada-France-Hawaii Telescope Legacy Survey (CFHTLS). The SARCS sample focuses on arc-like features and is designed to contain mostly galaxy groups. We briefly present the weak lensing methodology that we use to estimate the mass of the SARCS objects. Among 126 candidates, we obtain a weak lensing detection for 89 objects with velocity dispersions of the Singular Isothermal Sphere mass model ranging from 350 to 1000 km/s with an average value of 600km/s, corresponding to a rich galaxy group (or poor cluster). From the galaxies belonging to the bright end of the group's red sequence (M_i<-21), we derive the optical properties of the SARCS candidates. We obtain typical richnesses of N=5-15 galaxies and optical luminosities of L=0.5-1.5e+12 Lsol (within a radius of 0.5 Mpc). We use these galaxies to compute luminosity density maps, from which a morphological classification reveals that a large fraction of the sample are groups with a complex light distribution, either elliptical or multimodal, suggesting that these objects are dynamically young structures. We finally combine the lensing and optical analyses to draw a sample of 80 most secure group candidates, i.e. weak lensing detection and over-density at the lens position in the luminosity map, to remove false detections and galaxy-scale systems from the initial sample. We use this reduced sample to probe the optical scaling relations in combination with a sample of massive galaxy clusters. We detect the expected correlations over the probed range in mass with a typical scatter of 25% in the SIS velocity dispersion at a given richness or luminosity, making these scaling laws interesting mass proxies

We present VLT spectroscopic observations of 7 discovered galaxy groups between 0.3

The total mass of clusters of galaxies is a key parameter to study massive halos. It relates to numerous gravitational and baryonic processes at play in the framework of large scale structure formation, thus rendering its determination important but challenging. From a sample of the 11 X-ray bright clusters selected from the excpres sample, we investigate the optical and X-ray properties of clusters with respect to their total mass derived from weak gravitational lensing. From multi-color wide field imaging obtained with MegaCam at CFHT, we derive the shear profile of each individual cluster of galaxies. We perform a careful investigation of all systematic sources related to the weak lensing mass determination. The weak lensing masses are then compared to the X-ray masses obtained from the analysis of XMM observations and assuming hydrostatic equilibrium. We find a good agreement between the two mass proxies although a few outliers with either perturbed morphology or poor quality data prevent to derive robust mass estimates. The weak lensing mass is also correlated with the optical richness and the total optical luminosity, as well as with the X-ray luminosity, to provide scaling relations within the redshift range 0.4

[abridged] We analyze a sample of 9 massive clusters at 0.49.5). We find that the red sequence is already in place at z~0.5 and it is mainly composed of very massive (log(M/Msol)>11.3) early-type galaxies. These massive galaxies seem to be already formed when they enter the cluster, probably in infalling groups, since the fraction remains constant with the cluster radius. Their presence in the cluster center could be explained by a segregation effect reflecting an early assembly history. Any evolution that takes place in the galaxy cluster population occurs therefore at lower masses (10.3

The existence of strong lensing systems with Einstein radii (Re) covering the full mass spectrum, from ~1-2" (produced by galaxy scale dark matter haloes) to >10" (produced by galaxy cluster scale haloes) have long been predicted. Many lenses with Re around 1-2" and above 10" have been reported but very few in between. In this article, we present a sample of 13 strong lensing systems with Re in the range 3"- 8", i.e. systems produced by galaxy group scale dark matter haloes, spanning a redshift range from 0.3 to 0.8. This opens a new window of exploration in the mass spectrum, around 10^{13}- 10^{14} M_{sun}, which is a crucial range for understanding the transition between galaxies and galaxy clusters. Our analysis is based on multi-colour CFHTLS images complemented with HST imaging and ground based spectroscopy. Large scale properties are derived from both the light distribution of the elliptical galaxies group members and weak lensing of the faint background galaxy population. On small scales, the strong lensing analysis yields Einstein radii between 2.5" and 8". On larger scales, the strong lenses coincide with the peak of the light distribution, suggesting that mass is traced by light. Most of the luminosity maps have complicated shapes, indicating that these intermediate mass structures are dynamically young. Fitting the reduced shear with a Singular Isothermal Sphere, we find sigma ~ 500 km/s and an upper limit of ~900 km/s for the whole sample. The mass to light ratio for the sample is found to be M/L_i ~ 250 (solar units, corrected for evolution), with an upper limit of 500. This can be compared to mass to light ratios of small groups (with sigma ~ 300 km/s and galaxy clusters with sigma > 1000 km/s, thus bridging the gap between these mass scales., Comment: A&A Accepted. Draft with Appendix images can be found at http://www.dark-cosmology.dk/~marceau/groups_sl2s.pdf