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Register a new userSpectroscopy of clusters in the ESO Distant Cluster Survey (EDisCS)
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Authors
C. Halliday
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We present spectroscopic observations of galaxies in 4 clusters at z = 0.7-0.8 and in one cluster at z~0.5 obtained with the FORS2 spectrograph on the VLT as part of the ESO Distant Cluster Survey (EDisCS), a photometric and spectroscopic survey of 20 intermediate to high redshift clusters. We describe our target selection, mask design, observation and data reduction procedures, using these first 5 clusters to demonstrate how our strategies maximise the number of cluster members for which we obtain spectroscopy. We present catalogues containing positions, I-band magnitudes and spectroscopic redshifts for galaxies in the fields of our 5 clusters. These contain 236 cluster members, with the number of members per cluster ranging from 30 to 67. Our spectroscopic success rate, i.e. the fraction of spectroscopic targets which are cluster members, averages 50% and ranges from 30% to 75%. We use a robust biweight estimator to measure cluster velocity dispersions from our spectroscopic redshift samples. We also make a first assessment of substructure within our clusters. The velocity dispersions range from 400 to 1100 km s-1. Some of the redshift distributions are significantly non-Gaussian and we find evidence for significant substructure in two clusters, one at z~0.79 and the other at z~0.54. Both have velocity dispersions exceeding 1000 km s-1 but are clearly not fully virialised; their velocity dispersions may thus be a poor indicator of their masses. The properties of these first 5 EDisCS clusters span a wide range in redshift, velocity dispersion, richness and substructure, but are representative of the sample as a whole. Spectroscopy for the full dataset will allow a comprehensive study of galaxy evolution as a function of cluster environment and redshift.
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We find a significant number of massive and compact galaxies in clusters from the ESO Distant Clusters Survey (EDisCS) at 0.4<z<1. They have similar stellar masses, ages, sizes and axial ratios to local z~0.04 compact galaxies in WINGS clusters, and to z=1.4-2 massive and passive galaxies found in the general field. If non-BCG cluster galaxies of all densities, morphologies and spectral types are considered, the median size of EDisCS galaxies is only a factor 1.18 smaller than in WINGS. We show that for morphologically selected samples, the morphological evolution taking place in a significant fraction of galaxies during the last Gyrs may introduce an apparent, spurious evolution of size with redshift, which is actually due to intrinsic differences in the selected samples. We conclude that the median mass-size relation of cluster galaxies does not evolve significantly from z~0.7 to z~0.04. In contrast, the masses and sizes of BCGs and galaxies with M*>4x10^11 Msun have significantly increased by a factor of 2 and 4, respectively, confirming the results of a number of recent works on the subject. Our findings show that progenitor bias effects play an important role in the size-growth paradigm of massive and passive galaxies.
[Abridged] We present K-band data for the brightest cluster galaxies (BCGs) from the ESO Distant Cluster Survey. These data are combined with photometry from Aragon-Salamanca et al. (1998) and a low-redshift comparison sample from von der Linden et al. (2007). The K-band Hubble diagram for BCGs exhibits very low scatter (~0.35mag) since z=1. The colour and $K$-band luminosity evolution of the BCGs are in good agreement with passively-evolving stellar populations formed at z>2. We do not detect any significant change in the stellar mass of the BCG since z~1. These results do not seem to depend on the velocity dispersion of the parent cluster. There is a correlation between the 1D velocity dispersion of the clusters and the K-band luminosity of the BCGs (after correcting for passive evolution). The clusters with large velocity dispersions tend to have brighter BCGs, i.e., BCGs with larger stellar masses. This dependency, although significant, is relatively weak: the stellar mass of the BCGs changes only by ~70% over a two-order-of-magnitude range in cluster mass. This dependency doesnt change significantly with redshift. The models of De Lucia & Blaizot (2007) predict colours which are in reasonable agreement with the observations because the growth in stellar mass is dominated by the accretion of old stars. However, the stellar mass in the model BCGs grows by a factor of 3-4 since z=1, a growth rate which seems to be ruled out by the observations. The models predict a dependency between the BCGs stellar mass and the velocity dispersion of the parent cluster in the same sense as the data, but the dependency is significantly stronger than observed. However, one major difficulty in this comparison is that we have measured fixed metric aperture magnitudes while the models compute total luminosities.
The results of a search for distant clusters of galaxies performed using the I-band data obtained by the ESO Imaging Survey (EIS) are presented. Cluster candidates are identified using a matched filter algorithm, that provides not only an objective detection criterion, but also the means to estimate the cluster redshift and richness. A preliminary sample of distant clusters has been obtained, containing 252 cluster candidates with estimated redshift in the interval 0.2 < z < 1.3 (median redshift z_med ~ 0.4) over an area of approximately 14 square degrees. The adopted selection criteria for the inclusion of cluster candidates in this sample has been in general conservative, as the primary concern has been the reliability of the candidates rather than the completeness of the sample.
We present XMM-Newton observations of three optically-selected z > 0.6 clusters from the ESO Distant Cluster Survey (EDisCS), comprising the first results of a planned X-ray survey of the full EDisCS high-redshift sample. The EDisCS clusters were identified in the Las Campanas Distant Cluster Survey as surface brightness fluctuations in the optical sky and their masses and galaxy populations are well described by extensive photometric and spectroscopic observations. We detect two of the three clusters in the X-ray and place a firm upper limit on diffuse emission in the third cluster field. We are able to constrain the X-ray luminosity and temperature of the detected clusters and estimate their masses. We find the X-ray properties of the detected EDisCS clusters are similar to those of X-ray-selected clusters of comparable mass and -- unlike other high-redshift, optically-selected clusters -- are consistent with the T - sigma and L_x - sigma relations determined from X-ray selected clusters at low redshift. The X-ray determined mass estimates are generally consistent with those derived from weak lensing and spectroscopic analyses. These preliminary results suggest that the novel method of optical selection used to construct the EDisCS catalog may, like selection by X-ray luminosity, be well-suited for identification of relaxed, high-redshift clusters whose intracluster medium is in place and stable by z ~ 0.8.
We summarize several results based on the velocity data-set for cluster galaxies provided by ENACS (the ESO Nearby Abell Cluster Survey). We describe the distribution of velocity dispersions of a complete sample of rich galaxy clusters, and compare it to the distribution of cluster X-ray temperatures, and with predictions of theoretical models. We then address the issue of the existence of a Fundamental Plane (FP) for rich clusters, first suggested by Schaeffer et al. We confirm the existence of this FP with the ENACS data-set. The cluster FP is different from the FP of elliptical galaxies, and from the virial prediction. Some implications of the cluster FP are discussed. Finally, we describe the phase-space distributions of different populations of cluster galaxies. Different galaxy classes are defined according to their morphological or spectral type, and the presence of emission-lines. Star-forming (late-type) galaxies have a velocity dispersion profile that (in combination with their wider spatial distribution) is suggestive of first infall into the cluster. On the contrary, quiescent (early-type) galaxies show evidence of a dynamically relaxed distribution.
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