Adami et al. (2010) have detected several cluster candidates at z>0.5 as part of a systematic search for clusters in the Canada France Hawaii Telescope Legacy Survey, based on photometric redshifts. We focus here on two of them, located in the D3 field: D3-6 and D3-43. We have obtained spectroscopy with Gemini/GMOS and measured redshifts for 23 and 14 galaxies in the two structures. These redshifts were combined with those available in the literature. A dynamical and a weak lensing analysis were also performed, together with the study of X-ray Chandra archive data. Cluster D3-6 is found to be a single structure of 8 spectroscopically confirmed members at an average redshift z=0.607, with a velocity dispersion of 423 km/s. It appears to be a relatively low mass cluster. D3-43-S3 has 46 spectroscopically confirmed members at an average redshift z=0.739. It can be decomposed into two main substructures, having a velocity dispersion of about 600 and 350 km/s. An explanation to the fact that D3-43-S3 is detected through weak lensing (only marginally, at the ~3sigma level) but not in X-rays could be that the two substructures are just beginning to merge more or less along the line of sight. We also show that D3-6 and D3-43-S3 have similar global galaxy luminosity functions, stellar mass functions, and star formation rate (SFR) distributions. The only differences are that D3-6 exhibits a lack of faint early type galaxies, a deficit of extremely high stellar mass galaxies compared to D3-43-S3, and an excess of very high SFR galaxies. This study shows the power of techniques based on photometric redshifts to detect low to moderately massive structures, even at z~0.75.
We analyse the structures of all the clusters in the DAFT/FADA survey for which XMM-Newton and/or a sufficient number of galaxy redshifts in the cluster range is available, with the aim of detecting substructures and evidence for merging events. These properties are discussed in the framework of standard cold dark matter cosmology.XMM-Newton data were available for 32 clusters, for which we derive the X-ray luminosity and a global X-ray temperature for 25 of them. For 23 clusters we were able to fit the X-ray emissivity with a beta-model and subtract it to detect substructures in the X-ray gas. A dynamical analysis based on the SG method was applied to the clusters having at least 15 spectroscopic galaxy redshifts in the cluster range: 18 X-ray clusters and 11 clusters with no X-ray data. Only major substructures will be detected. Ten substructures were detected both in X-rays and by the SG method. Most of the substructures detected both in X-rays and with the SG method are probably at their first cluster pericentre approach and are relatively recent infalls. We also find hints of a decreasing X-ray gas density profile core radius with redshift. The percentage of mass included in substructures was found to be roughly constant with redshift with values of 5-15%, in agreement both with the general CDM framework and with the results of numerical simulations. Galaxies in substructures show the same general behaviour as regular cluster galaxies; however, in substructures, there is a deficiency of both late type and old stellar population galaxies. Late type galaxies with recent bursts of star formation seem to be missing in the substructures close to the bottom of the host cluster potential well. However, our sample would need to be increased to allow a more robust analysis.
(Abridged) Based on XMM-Newton, Chandra and SDSS data, we investigate the baryon distribution in groups and clusters and its use as a cosmological constraint. For this, we considered a sample of 123 systems, with total masses in the mass range M500 = ~ 10^13 - 4 x 10^15 h_70^-1 Msun. The gas masses and total masses are derived from X-ray data under the assumption of hydrostatic equilibrium and spherical symmetry. The stellar masses are based on SDSS-DR8 data. For the 37 systems out of 123 that had both optical and X-ray data available, we investigated the gas, stellar and total baryon mass fractions inside r2500 and r500, and the differential gas mass fraction within the spherical annulus between r2500 and r500, as a function of total mass. For the other objects, we investigated the gas mass fraction only. We find that the gas mass fraction inside r2500 and r500 depends on the total mass. However, the differential gas mass fraction does not show any dependence on total mass for systems with M500 > 10^14 Msun. We find that the total baryonic content increases with cluster mass. This led us to investigate the contribution of the ICL to the total baryon budget for lower mass systems, but we find that it cannot account for the difference observed. The gas mass fraction dependence on total mass observed for groups and clusters could be due to the difficulty of low-mass systems to retain gas inside the inner region. Due to their shallower potential well, non-thermal processes are more effective in expelling the gas from their central regions outwards. Since the differential gas mass fraction is nearly constant it provides better constraints for cosmology. Using our total f_b estimates, our results imply 0.17 < Omega_m < 0.55.
Cluster properties do not seem to be changing significantly during their mature evolution phase, for example they do not seem to show strong dynamical evolution at least up to z~0.5, their galaxy red sequence is already in place at least up to z$sim$1.2, and their diffuse light content remains stable up to z~0.8. The question is now to know if cluster properties can evolve more significantly at redshifts notably higher than 1. We propose here to see how the properties of the intracluster light (ICL) evolve with redshift by detecting and analysing the ICL in the X-ray cluster CL J1449+0856 at z=2.07 (discovered by Gobat et al. 2011), based on deep HST NICMOS H band exposures.We used the same wavelet-based method as that applied to 10 clusters between z=0.4 and 0.8 by Guennou et al. (2012). We detect three diffuse light sources with respective total magnitudes of H=24.8, 25.5, and 25.9, plus a more compact object with a magnitude H=25.3. We discuss the significance of our detections and show that they are robust. The three sources of diffuse light indicate an elongation along a north-east south-west axis, similar to that of the distribution of the central galaxies and to the X-ray elongation. This strongly suggests a history of merging events along this direction. While Guennou et al. (2012) found a roughly constant amount of diffuse light for clusters between z~0 and 0.8, we put in evidence at least a 1.5 magnitude increase between z~0.8 and 2. If we assume that the amount of diffuse light is directly linked to the infall activity on the cluster, this implies that CL J1449+0856 is still undergoing strong merging events.
In order to enlarge publicly available optical cluster catalogs, in particular at high redshift, we have performed a systematic search for clusters of galaxies in the CFHTLS. We used the Le Phare photometric redshifts for the galaxies detected with magnitude limits of i=25 and 23 for the Deep and Wide fields respectively. We then constructed galaxy density maps in photometric redshift bins of 0.1 based on an adaptive kernel technique and detected structures with SExtractor. In order to assess the validity of our cluster detection rates, we applied a similar procedure to galaxies in Millennium simulations. We measured the correlation function of our cluster candidates. We analyzed large scale properties and substructures by applying a minimal spanning tree algorithm both to our data and to the Millennium simulations. We have detected 1200 candidate clusters with various masses (minimal masses between 1.0 10$^{13}$ and 5.5 10$^{13}$ and mean masses between 1.3 10$^{14}$ and 12.6 10$^{14}$ M$_odot$), thus notably increasing the number of known high redshift cluster candidates. We found a correlation function for these objects comparable to that obtained for high redshift cluster surveys. We also show that the CFHTLS deep survey is able to trace the large scale structure of the universe up to z$geq$1. Our detections are fully consistent with those made in various CFHTLS analyses with other methods. We now need accurate mass determinations of these structures to constrain cosmological parameters.
We compare the distribution of optically and Halpha (Ha) selected galaxies in the Southern half of the nearby Abell 85 (A85) cluster with the recently discovered X-ray filament (XRF). We search for galaxies where star formation (SF) may have been triggered by interactions with intracluster gas or tidal pressure due to the cluster potential when entering the cluster. Our analysis is based on images obtained with CFHT MegaPrime/MegaCam (1x1 deg2 field) in four bands (ugri) and ESO 2.2mWFI (38x36 field) in a narrow band filter corresponding to the redshifted Halpha (Ha) line and in a broad R-band filter. The LFs are estimated by statistically subtracting a reference field. Background contamination is minimized by cutting out galaxies redder than the observed red sequence in the g-i vs. i colour-magnitude diagram. The galaxy distribution shows a significantly flattened cluster, whose principal axis is slightly offset from the XRF. The analysis of the broad band LFs shows that the filament region is well populated. The filament is also independently detected as a gravitationally bound structure by the Serna & Gerbal hierarchical method. 101 galaxies are detected in Ha, among which 23 have spectroscopic redshifts in the cluster, 2 have spectroscopic redshifts higher than the cluster and 58 have photometric redshifts that tend to indicate that they are background objects.The 23 galaxies with spectroscopic redshifts in the cluster are mostly concentrated in the South part of the cluster and along the filament. We find a number of galaxies showing evidence for SF in the XRF, and all our results are consistent with the previous hypothesis that the XRF in A85 is a gravitationally bound structure made of groups falling on to the main cluster.
We discuss the different physical processes that are important to understand the thermal X-ray emission and absorption spectra of the diffuse gas in clusters of galaxies and the warm-hot intergalactic medium. The ionisation balance, line and continuum emission and absorption properties are reviewed and several practical examples are given that illustrate the most important diagnostic features in the X-ray spectra.
An excess over the extrapolation to the extreme ultraviolet and soft X-ray ranges of the thermal emission from the hot intracluster medium has been detected in a number of clusters of galaxies. We briefly present each of the satellites (EUVE, ROSAT PSPC and BeppoSAX, and presently XMM-Newton, Chandra and Suzaku) and their corresponding instrumental issues, which are responsible for the fact that this soft excess remains controversial in a number of cases. We then review the evidence for this soft X-ray excess and discuss the possible mechanisms (thermal and non-thermal) which could be responsible for this emission.
We investigate the LF in the very relaxed cluster Abell 496. Our analysis is based on deep images obtained at CFHT with MegaPrime/MegaCam in four bands (ugri) covering a 1x1 deg2 region, which is centered on the cluster Abell 496 and extends to near its virial radius. The LFs are estimated by statistically subtracting a reference field taken as the mean of the 4 Deep fields of the CFHTLS survey. Background contamination is minimized by cutting out galaxies redder than the observed Red Sequence in the g-i versus i colour-magnitude diagram. In Abell 496, the global LFs show a faint-end slope alpha=-1.55+/-0.06 and vary little with observing band. Without colour cuts, the LFs are much noisier but not significantly steeper. The faint-end slopes show a statistically significant steepening from alpha=-1.4+/-0.1 in the central region (extending to half a virial radius) to -1.8+/-0.1 in the Southern envelope of the cluster. Cosmic variance and uncertain star-galaxy separation are our main limiting factors in measuring the faint-end of the LFs. The large-scale environment of Abell 496, probed with the fairly complete 6dFGS catalogue, shows a statistically significant 36 Mpc long filament at PA=137 deg, but we do not find an enhanced LF along this axis. Our LFs do not display the large number of dwarf galaxies (alpha ~ -2) inferred by several authors, whose analyses may suffer from field contamination caused by inexistent or inadequate colour cuts. Alternatively, different clusters may have different faint-end slopes, but this is hard to reconcile with the wide range of slopes found for given clusters and for wide sets of clusters.
This study follows a recent analysis of the galaxy luminosity functions and colour-magnitude red sequences in the Coma cluster (Adami et al. 2007). We analyze here the distribution of very faint galaxies and globular clusters in an east-west strip of $sim 42 times 7$ arcmin$^2$ crossing the Coma cluster center (hereafter the CS strip) down to the unprecedented faint absolute magnitude of M$_R sim -9.5$. This work is based on deep images obtained at the CFHT with the CFH12K camera in the B, R, and I bands. The analysis shows that the observed properties strongly depend on the environment, and thus on the cluster history. When the CS is divided into four regions, the westernmost region appears poorly populated, while the regions around the brightest galaxies NGC 4874 and NGC 4889 (NGC 4874 and NGC 4889 being masked) are dominated by faint blue galaxies. They show a faint luminosity function slope of -2, very significantly different from the field estimates. Results are discussed in the framework of galaxy destruction (which can explain part of the very faint galaxy population) and of structures infalling on to Coma.
