Clusters of Galaxies in the SDSS

1. Clusters selected in 3D. The magnitude limit of the spectroscopic survey (approximately g'=18.5) corresponds to M* at z=0.1 for rich Abell clusters. We can therefore expect redshifts for tens of cluster members per cluster out to this redshift, thus allowing a full 3D selection of clusters. Moreover, for nearby rich clusters we can expect hundreds of cluster member redshifts, allowing detailed studies of cluster kinematics as a function distance from the cluster centroid.

2. Clusters selected in 2D. The photometric SDSS survey is estimated to reach a limiting magnitude of r'=22 for faint galaxies (depending on their surface brightness profiles). Furthermore, the SDSS will provide 5 colour information on all sources. Clearly, this database will be a goldmine for finding distant and/or poorer clusters (in 5 colours!). Using the surface density of overdensities presented in the Edinburgh/Durham Cluster Catalogue, it is estimated that over 10,000 systems will be found out to a redshift of 0.3-0.4.

3. Clusters with a redshift measurement. In addition to the nearby sample of clusters that will have many redshift measurements, a large fraction of the detected 2D clusters will have one, or a few, redshifts measurements each. Moreover, since the surface density of clusters is relatively low, it is planned to place a fibre on the Brightest Cluster Member (BCM) for clusters with a BCM magnitudes of less than g'=19.5 (one magnitude fainter than the main galaxy magnitude cut). The combined effect will be to have a redshift determination for between 3000 and 4000 clusters out to redshifts of 0.3-0.4.

4. Distant Clusters. One of the prime goals of the SDSS is to obtain repeat scans of 300 square degrees of sky centered at the South Galactic Pole. It is estimated that this photometric sample will reach a limiting magnitude of r'=24, two magnitudes deeper than the northern SDSS photometric survey. This deep survey will then be used as a basis for a deep spectroscopic survey to r'=20 for galaxies. Clearly, this database will be ideal for studying and understanding distant cluster population.

Scientific Motivation

The main scientific motivation is to construct robust samples of clusters as a smooth function of redshift. This will allow confident investigations of cluster evolution as well as studies of large-scale structure evolution using the clusters as tracers. The main strength of the SDSS for cluster research is the coherent manner in which both distant and nearby clusters will be selected from the same raw data.

Here is a short list of cluster research areas where the SDSS will make a significant impact:

• Cluster dynamics: mapped out into the field population
• Cluster luminosity function: nature or nurture?
• Cluster mass function: direct measurement of mass perturbations on cluster scales
• Clustering of clusters: superclusters
• Evolution: colours, morphologies, masses
• Galaxy density-morphology relationship: colours and velocity dispersions for galaxies in clusters
• Combine with information from other wavelengths: x-ray and radio
• Gravitational lensing: weak and strong
• Cluster-QSO correlations: dust in clusters, lensing?
• QSO in clusters
• Large-scale flows using clusters

Cluster Selection

Work has already begun on formulating a scheme for finding and classifying clusters of galaxies in the SDSS databases. At present, the favoured methods are based on matched-filter and/or a wavelet smoothing. People who have worked on these are; Bob Nichol , Brad Holden (UC), Marc Postman (STScI) & Neta Bahcall (Princeton). (Note that these files are very large)

Cluster Test Year Plans