The Northern Sky Optical Cluster Survey

Project Summary
The Northern Sky Optical Cluster Survey is a project to utilize the STScI digitized scans of the
Second Palomar Sky Survey (DPOSS) to generate a catalog of galaxy clusters, selected in an objective manner, over the high galactic latitude northern sky. We expect to cover approximately 10,000 square degrees in the final catalog, which will contain about 15,000 clusters, with redshifts in the range 0.1<z<0.3, with a median redshift of ~0.2. This is a significantly larger and deeper sample than the one generated by Abell in 1958. Currently, my thesis covers ~5,000 square degrees, limited only by the availability of good photometric calibration.

The DPOSS has fields taken in 3 filters, converted to Gunn-Thuan g,r, and i. Using an automated classification technique, a catalog of all objects is produced, including information on whether they are stars or galaxies. With color information added, we are able to generate a catalog of early-type galaxies, i.e. ellipticals, which are preferentially found in clusters. The adaptive kernel method (see Silverman, Density Estimation, for details) is then used to find overdensities of such galaxies, indicating clusters. At right is a sample density map, with Abell clusters marked in red, and new candidates marked in blue. The image covers 6 degrees on a side. We have applied this method to two fields, each 36 square degrees, with one near the north Galactic pole, and one towards the southern Galactic cap. Follow up photometry of all original candidates in these two fields is done at the Palomar 60" telescope, and spectroscopy at the 200". Scientific studies from this survey include:

  1. Photometric redshifts. With accurate photometry and spectroscopy, we can calibrate the redshift-color relation for the DPOSS, and thus assign approximate redshifts to all galaxies and clusters, at least, in the survey. We have demonstrated photometric redshifts accurate to an rms dz=0.015. Detailed description in Chapter 7 of thesis.
  2. Large Scale Structure. An important ingredient for testing cosmological models is the cluster-cluster correlation function. Past work has used the Abell catalog of clusters; the work of others shows this catalog to be incomplete by a significant factor. Clearly, a complete, systematic, unbiased catalog is a necessity in this area. Such catalogs have been generated on the basis of x-ray emission, but those surveys are biased towards relaxed systems with the necessary potential wells to contain the hot gas. See some early work in Chapter 9 of my thesis.
  3. Cluster populations: The Butcher-Oemler effect and morphology-density/radius relations as observed in several hundred or even thousands of clusters. See also the Palomar Abell Cluster Optical Survey (PACOS).
  4. Cluster substructure. With precision spectroscopy and spatial information, we can investigate the substructure in clusters, and better define what a cluster truly is. Is it necessarily a relaxed system, or not? Chapter 10 of my thesis includes an example of this type of study.
A cluster at z=0.18 not found by Abell.
At right are some Palomar 200"/COSMIC spectra of galaxies in this cluster, showing them to be at the same redshift.

Thesis - Roy R. Gal

My thesis is available as gzipped Postscript files, either as one huge document , or chapter by chapter. Note that much of this has been superceded by the papers above.

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Updated 25 November 2005