The 11 clusters presented in the sample are all bright X-ray clusters. Some of them also present specific optical properties or are already known as strong gravitational lenses. We review in this section the properties of the clusters, mostly in the optical. Their X-ray properties will be presented in a companion paper (Arnaud et al., in prep.).
This very rich cluster has been studied for many years, since the identification of a high fraction of red galaxies in its population (Koo 1981). Included in the CNOC cluster sample, its spectroscopic survey was presented in Ellingson et al. (1998) with more than 180 objects observed spectroscopically. The resulting velocity dispersion km s-1 is a high value consistent with its galaxy richness (Borgani et al. 1999). MS 0015.9+1609 is one of the brightest and most distant X-ray cluster included in the EMSS sample (Gioia & Luppino 1994). It is also part of the highly luminous X-ray clusters identified in the MACS sample at redshift higher than 0.5 (Ebeling et al. 2007) and is identified as MACS J0018.5+1626.
The weak-lensing properties were described by Smail et al. (1995) and then by Clowe et al. (2000). The authors found a rather low signal and therefore a total mass not consistent with the optical velocity dispersion of the galaxies. More recently, Hoekstra (2007) re-analysed a large sample of clusters observed in good seeing conditions at the CFHT and found for MS 0015.9+1609 a total mass described by a SIS with km s-1 or by a NFW profile with . Note that despite the high mass value of the cluster, no strong-lensing features were detected in HST images (Sand et al. 2005). More recently and thanks to a detailed analysis of HST/ACS images, Zitrin et al. (2011) identified three systems of multiple images, but they have not yet been confirmed spectroscopically. They were used to provide a lensing model of the mass distribution in the centre of the cluster.
There is no dominant central galaxy in this cluster but a chain of bright ellipticals, giving a significant elongation in the galaxy distribution. This elongation was confirmed in the weak-lensing map provided by Zitrin et al. (2011) on the central area of the cluster. In our wide-field map, the ellipticity of the mass distribution does not appear clearly (Fig. A.1). We suspect that the bright star close to the cluster centre prevents a correct study of the cluster mass map obtained from weak-lensing reconstruction.
MS 0015.9+1609 is embedded in a large-scale structure of the size of a supercluster, identified spectroscopically by Connolly et al. (1996). At least three clusters lie within less than 30 Mpc form each other, and a long and massive filamentary structure crosses the cluster in the same direction as the galaxy elongation (Tanaka et al. 2007, 2009). The weak-lensing reconstruction we presented only focused on the central area around the cluster, but we checked that most of the structures spectroscopically identified by Tanaka et al. (2007) were also visible in our global mass map. This may be the case for the south-west elongation seen in the mass map displayed in Fig. A.1. Further work is in progress to better quantify these correlations.
This cluster is the most X-ray luminous cluster in the EMSS catalogue (Gioia & Luppino 1994) and is also part of the CNOC sample. Intensive spectroscopic follow-up of the galaxies provided more than 100 spectra of cluster members (Ellingson et al. 1998) and a line-of-sight velocity dispersion of km s-1 (Borgani et al. 1999). Weak-lensing masses measured by Clowe et al. (2000) are roughly compatible with this value as well as those obtained by Hoekstra et al. (2012). Our own measurements are higher by 50%, but they remain compatible within the uncertainties (Foëx et al. 2012). The cluster is also identified as MACS J0454.1–0300.
A few thin and elongated features were suspected to be strong-lensing candidates by Luppino et al. (1999) and were later spectroscopically confirmed by Borys et al. (2004). Interestingly, a SCUBA detection of an extended source in the cluster centre led to the identification of an ERO pair, triple imaged (Chapman et al. 2002; Takata et al. 2003; Berciano Alba et al. 2010). These features point towards the bright central galaxy as the centre of the mass distribution. The observed elongation of the weak-lensing mass reconstruction (Fig. A.2) is well correlated with the global elongation of the light distribution in the SE/NW direction. This is also true for the orientation of the BCG. The latest strong-lens model presented by Zitrin et al. (2011) indicates that the central mass distribution is highly elliptical, with an orientation that matches the SE/NW elongation of the cluster at large scale.
This cluster is part of the NORAS sample (Northern ROSAT all-sky galaxy cluster survey), a purely X-ray selected sample (Böhringer et al. 2000). It was included in the EXCPRES sample because of its high X-ray luminosity. It is an optically bright cluster identified in the SDSS-DR6 ([WHL2009] J085612.7+375615, Wen et al. 2009), with a redshift measurement of the BCG at z = 0.411. The cluster displays a well-defined and regular luminous over-density dominated by a bright and extended cD galaxy (Fig. A.3). The mass map also presents a very regular aspect around its centre and provides a coherent picture of a relaxed cluster.
This cluster is also known as Abell 851 or Cl 0939+4713. It is the only Abell cluster of our sample. High-resolution HST images of the centre revealed a large population of blue galaxies and many merging galaxies (Dressler et al. 1994). Seitz et al. (1996) used this deep HST/WFPC2 image to identify a few lensed objects, but no highly magnified gravitational arcs were detected. X-ray observations of A851, first with ROSAT and more recently with XMM-Newton, showed a very perturbed distribution with pronounced substructures and evidence for a dynamically young cluster (De Filippis et al. 2003). Tentative 2D spectro-imaging led to the identification of a hot region between the two main sub-clusters, a characteristic of a major merger in an early phase.
The galaxy distribution is complex, with a high galaxy density in the central area. It can be separated into two clumps that trace the cluster interaction and are coherent with the gas distribution. Several bright galaxies dominate the light distribution and are more concentrated in the south-west extension of the cluster (Fig. A.4). In contrast, the weak-lensing mass map is surprisingly regular, with only one main structure, but elongated along the direction of the interaction. The separation between the two X-ray peaks is 50′′, well below the resolution of the mass map. With the present data there is therefore no chance to obtain a more detailed view of the mass distribution at a scale where the physical processes of the cluster merger could be identified. Deeper imaging is necessary to proceed in this analysis. Because of the high evidence for merging processes, this cluster was later removed from the EXCPRES sample, but as optical data were obtained in good conditions, we kept it in our sample.
The cluster was initially identified by its X-ray extended emission in the NORAS sample (Böhringer et al. 2000), and it was later re-detected in the 400d ROSAT sample (Burenin et al. 2007). Nothing was really known on the optical properties of this cluster, which displays a bright galaxy in its centre and a rather loose distribution of cluster members. Another bright galaxy is located 2.3′ south-west, with similar properties. It is centred on a secondary peak in the X-ray gas distribution and the mass map is centred in between these two galaxies. But the bi-modality of the cluster is more visible in the galaxy distribution than in the mass map (Fig. A.5), which is limited by its spatial resolution. We suspect that this cluster results from the merging of two sub-clusters, and all conclusions regarding RXC J1003.0+3254 in the global analysis of the sample must be taken with caution.
This cluster belongs to the Bright SHARC survey (Romer et al. 2000) and was included in the WARPS II catalogue (the Wide Angle ROSAT Pointed Survey, Horner et al. 2008). The cluster was observed with XMM-Newton and analysed by Arnaud et al. (2002), who found a regular X-ray emission with a spherical morphology. They also claimed that no cooling flow or central gas concentration is present in this cluster, which is consistent with the cooling time being longer than the age of the Universe at this redshift. With its redshift z = 0.612, RX J1120.1+4318 is the most distant cluster of the EXCPRES sample. The light distribution of cluster members shows an east-west elongation, which was also been measured in the Chandra X-ray map (Maughan et al. 2008). But the ellipticity is rather low and does not attenuate the regular morphology of the cluster, which is clearly in a relaxed phase. Unfortunately, the lensing signal in RX J1120.1+4318 is barely detected, at less than 3σ (Fig. A.6). This makes it difficult to draw any conclusions on the mass distribution in the cluster. Even the shift between the mass peak and the light peak cannot be considered significant.
This cluster is one of the brightest clusters of the REFLEX sample (Böhringer et al. 2004). It belongs to the MACS sample (Ebeling et al. 2010) as MACS J1206.2–0847 and is part of the CLASH sample (Postman et al. 2012). It displays a bright and spectacular arc system, initially spectroscopically observed by Sand et al. (2004) and confirmed more recently by Ebeling et al. (2009) at a redshift z = 1.036. A detailed analysis of the central mass distribution was made both with strong-lensing and X-ray data, giving a discrepancy of a factor 2 between the two mass estimates. But the X-ray distribution of the gas shows some signs of merging processes in the centre, which could explain this discrepancy. Similar trends have already been noticed in other clusters such as A1689 (Limousin et al. 2007).
The weak-lensing mass distribution is clearly peaked, with a regular shape and a central concentration that fits the luminous mass as well as the X-ray mass (Fig. A.7). Note that the central galaxy is also a bright radio source with a steep spectrum (Ebeling et al. 2010).
Umetsu et al. (2012) recently comprehensively analysed this cluster by combining weak and strong lensing derived from wide-field Subaru imaging and HST observations. Their morphological analysis of both the reconstructed mass map and light distribution revealed the presence of a large-scale structure around RXC J1206.2–0848. The orientation of this structure matches the position angle of the BCG and that of the cluster light distribution and projected mass map. The ellipticity they derived for the latter is somehow higher than ours, but we obtain consistent results for the light distribution. The overall shape of RXC J1206.2–0848 indicates that light follows mass up to the large scales of the cosmic web.
As part of the EMMS sample (Gioia & Luppino 1994), this cluster was also observed in the optical, but no significant strong-lensing feature was detected (Luppino et al. 1999). The luminosity distribution is complex, with a southern extension possibly related to the main cluster. However, neither the mass distribution nor the X-ray gas shows a similar trend. Both are regular and centred in the BCG, embedded in a bright and extended envelope. This means that firm conclusions are difficult to draw because of the low S/N ratio of the mass map (Fig. A.8). The second over-density of galaxies could also be due to some contamination along the line of sight. Deeper and multi-colour images are necessary to confirm the reality of an in-falling substructure on the main cluster.
This is the brightest cluster of the REFLEX sample (Böhringer et al. 2004) and is part of the CLASH sample (Postman et al. 2012). It presents the spectacular strong-lensing system detected by Schindler et al. (1995). It was also identified as a cluster with a strong central cooling flow (Allen 2000), feeding a powerful radio source (Pointecouteau et al. 2001). A detailed combined analysis of the strong- and weak-lensing effects (Bradač et al. 2005a,b) led to a very accurate view of the dynamical status of the cluster in the inner regions: the cluster presents a mass concentration centred on the BCG with some extension to the SW and much evidence of sub-cluster merging. But RX J1347.5–1145 is definitely not a major merger. After some controversy, the different mass estimates seemed to converge, especially those measured close to the centre using the strong-lensing features (Halkola et al. 2008; Bradač et al. 2008). But a factor of 2 remains between the X-ray and the weak-lensing masses at large radius (Fischer & Tyson 1997; Kling et al. 2005; Gitti et al. 2007). In this context, our mass map confirms the previous results and does not bring new evidence on the mass distribution (Fig. A.9). It was mostly used to check and validate our weak-lensing procedure before applying it to other clusters. Similar results were published on RX J1347.5–1145 by Hoekstra et al. (2012) with the same CFHT data. Fortunately, they obtained very similar mass measures.
Several studies (Lu et al. 2010; Verdugo et al. 2012) revealed that RX J1347.5–1145 is embedded in a large-scale structure, extending up to 20 Mpc in the NE–SW direction. Our luminosity map confirms the existence of several over-densities on a large scale, aligned along this direction. The main orientation of the cluster light distribution also follows the same direction. Like RXC J1206.2–0848, this cluster supports the picture of the cosmic web where massive clusters are fed by filaments whose orientation matches the global morphology of the central node.
As part of the EMSS cluster sample (Gioia & Luppino 1994), the cluster was rapidly identified as a strong lens with a nice gravitational arc located around a radio galaxy that is not the brightest cluster galaxy (Luppino et al. 1999). No spectroscopic redshift is currently available for the arc, although its lensed nature is not in doubt (Sand et al. 2005). MS 1621.5+2640 is also part of the CNOC sample and was spectroscopically observed with more than 100 cluster redshifts available (Ellingson et al. 1997). The velocity dispersion is low ( km s-1, Borgani et al. 1999). More recently, Hoekstra (2007) reported a very accurate weak-lensing analysis, and his results agree well with the dynamical mass estimate. It is also consistent with the X-ray mass obtained with ROSAT (Hicks et al. 2006). With our weak-lensing mass reconstruction, we find a mass distribution rather elongated and coherent with the light distribution. The large shift between the mass and light peaks is more probably an artefact than real (Fig. A.10).
This cluster is part of the NORAS sample (Böhringer et al. 2000) and also belongs to the MACS sample (MACS J2228.5+2036, Ebeling et al. 2007). Because it is at low galactic latitude, very few optical observations are available. Our weak-lensing reconstruction is rather uncertain (Fig. A.11) and possibly flawed because of the large number of bright stars in the field of view. However, in addition to its strong X-ray emission, this cluster was detected for its SZ signal, allowing one of the first combined analyses between the X-ray and the SZ signals (Pointecouteau et al. 2002; Jia et al. 2008). Both confirm that the cluster is quite massive and dynamically perturbed. The weak-lensing map shows a poor signal close to the cluster centre but suggests that the cluster has an elongated shape. This is also valid for the complex light distribution. The most convincing feature is a galaxy clump located in the south-west direction, detected on the mass map with higher significance than the main cluster. It is associated with a galaxy excess centred on a bright elliptical galaxy with similar magnitude as the cluster BCG. We suspect that this clump is at a similar redshift as RX J2228.5+2036 and may be the cause of a future major merger with RX J2228.5+2036. Surprisingly, there is no X-ray counter-part to this clump.
Up: 15′ × 15′ inset of the cluster field extracted from the full r′ MegaCam image. The thick red contours show the mass distribution derived from the 2D weak-lensing analysis. The contour levels are linearly spaced in σ of the mass reconstruction, starting at 2σ. The thin blue contours come from the X-ray map obtained with XMM-Newton. The image was filtered with wavelets and the contours are scaled logarithmically. Bottom left: same mass isocontours overlaid on the galaxy luminosity distribution where cluster members are selected within the cluster red sequence and mr< 23. Bottom right: true colour image of the cluster centre from g′, r′, i′ combination. The field of view is 3′ × 3′.
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