Spectroscopy of clusters in the ESO Distant Cluster Survey (EDisCS)

Abstract

Abstract. 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.

The galaxy stellar mass function and its evolution with time show no dependence on global environment

“…The level of contamination, even if reasonably low (see e.g. Halliday et al 2004;Milvang-Jensen et al 2008), may conceivably still have some effect. Larger spectroscopic surveys would therefore help to confirm (or otherwise) our results.…”

Frequency and properties of bars in cluster and field galaxies at intermediate redshifts

“…The ESO Distant Cluster Survey is a study of 26 optically selected and spectroscopically confirmed galaxy systems, from rich groups to massive clusters, smoothly distributed between redshifts 0.39 and 0.96 (Halliday et al 2004;Milvang-Jensen et al 2008). For all systems, we have assembled three-band optical VLT deep photometry, deep NTT/SOFI near-infrared imaging, and optical VLT/FORS2 spectroscopy (White et al 2005).…”

“…The coordinates of the clusters/groups correspond to the positions of the brightest cluster/group galaxies. The redshifts (z) and line-of-sight cluster/group velocity dispersions (σ) are taken from Halliday et al (2004) and Milvang-Jensen et al (2008). The virial radius (R 200 ) has been determined using Eq.…”

“…These limits are based on our spectroscopy and range from 0.1 to 0.35 depending on the filter set available for each particular field (White et al 2005;Pelló et al 2008). Cluster/group membership for objects with spectroscopic observations is defined as being within ±3σ cl of z cl (Halliday et al 2004;Milvang-Jensen et al 2008). Photometric redshifts were determined using the methods described in Rudnick et al (2001) and Pelló et al (2008), and were based on the optical+near-infrared photometry.…”

“…Spectroscopic redshifts are available for a subsample of 459 galaxies. For the low redshift cluster fields (z < 0.6) spectroscopic observations were restricted to objects with I < 22 mag and for the high redshift cluster fields (z > 0.6) to I < 23 mag (Halliday et al 2004;Milvang-Jensen et al 2008). This ensures that the distribution in absolute magnitude for galaxies with spectroscopic redshifts remains roughly the same over the entire redshift range (z = 0.4−0.8).…”