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Session 53 - Surveys of Galaxies & Clusters.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center
We report on the preliminary results of an effort to create an unbiased catalog of groups and clusters of galaxies, from the galaxy catalogs derived from the digitized POSS-II (DPOSS). We investigated several correlations of galaxian properties which can be used to select early-type galaxies, which should better delineate high-density regions (e.g., rich clusters and compact groups), or correlations which would group together a set of galaxies at a common distance from us. We used the correlations of the mean surface brightness vs. the concentration index, color vs. magnitude, and color vs. color. We found that the (g-r) vs. (r-i) color-color diagram is the most effective one for this purpose. We used several methods to generate the galaxy surface density maps, including: galaxy counts in cells of a given size; the nearest neighbors algorithm; and an adaptive kernel mapping technique. The major advantage of the adaptive kernel method is that it uses a two-step process which significantly smooths the low density regions, and at the same time keeps the high density peaks almost untouched, resulting in a nearly constant S/N ratio over the entire field. Using the counts in cells of 2 arcmin size, we were successful in recovering the known Abell clusters in the field, and have found new cluster candidates which were apparently missed by Abell. We used these surface density maps to detect all significant structures above a given threshold, which was established in such way to recover Abell clusters of richness class 0 or higher. Eventually, we hope to generate an objectively and automatically catalogs of compact groups and clusters, with well defined selection criteria, over the entire high-latitude northern sky. We anticipate that this catalog will contain > 10,000 rich clusters, reaching on the average a factor of 2 or 3 deeper then the Abell catalog. These algorithms can be also applied to star catalogs from DPOSS, to search for previously unknown sparse globular clusters, nearby dwarf galaxies, or other stellar aggregates in our Galaxy.
