Mass calibration
A crucial step in going from clusters in the survey data to estimates of cosmological parameters is understanding how our observable, cluster richness (Bgc, Longair & Seldner 1979), relates to the mass of a cluster. We have invested large amounts of effort into studying this issue via several different mass tracers including spectroscopic velocity dispersions, X-ray temperatures and weak gravitational lensing.
Bgc -- mass relation
Core sample
In order to best understand how the richness we measure relates to the cluster mass and, furthermore, how well different mass estimators agree with each other, we have undertaken an ambitious programme targeting a uniformily selected core subsample of clusters. The aim is to concentrate multi-wavelength observations on a well-defined sample, selected in bins of richness and redshift. In this way, we hope to understand how the the properties of clusters and the galaxies in them are affected primarily by a). cluster mass and b). cosmic time.
The 36 clusters are divided into redshift and richness bins as below:| 0.3
< z < 0.5 |
0.5
< z < 0.7 |
0.7
< z < 0.9 |
|
| 1000 < Bgc <
2000 |
6 |
6 |
6 |
| 500 < Bgc <
1000 |
6 |
6 |
6 |
Photometric redshifts
Each cluster will have mutli-colour imaging (BVRI or BVRz) in order to produce photometric redshift catalogues. The photometric redshifts alone will allow us to study the properties of cluster galaxies. However, the main purpose of the photo-zs is to allow us to pre-select the most probable cluster galaxies for wide-field spectroscopy using IMACS on Magellan. This is important in tracing structure in the outskirts of clusters, where the background density of galaxies is so high that simple magnitude selection of all galaxies would otherwise result in a very low success rate in selecting cluster members.
Spectroscopy
Wide-field spectroscopy is being carried out using three custom band-limiting filters (one for each redshift bin, above) to obtain extremely high multiplex spectroscopy (~800 slits per mask). An example mask layout is shown to the right. Note that this was observed prior to the delivery of our custom filters and comprises a "mere" 400 slits. With two masks per cluster (one for bright and one for faint targets), we expect ~40 000 redshifts from ~60 000 slits for the core sample survey. This should result in up to 20 000 cluster galaxy redshifts (c.f. 1300 in CNOC1).
High redshift sample
The core sample is being extended at the high-redshift end (z>~0.9) using GMOS on Gemini, FORS2 on the VLT and now LDSS-3 on Magellan.
Our aim is to obtain 20-30 members per cluster, at these redshifts, in order to obtain reliable mass estimates via velocity dispersions. Preliminary spectroscopy shows that from a handful of galaxies per cluster, the RCS technique is still efficient and accurate at these redshifts.
Weak-lensing masses for clusters at these redshifts will be obtained from ACS images obtained in collaboration with the Perlmutter Supernova group in Berkeley.