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For refcode 1992ApJ...397..430U:
Retrieve 13 NED objects in this reference.
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Copyright by American Astronomical Society. Reproduced by permission
1992ApJ...397..430U CLUSTERS IN COLLISION? M. P. ULMER Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208 G. D. WIRTH Board of Studies in Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064 AND M. P. KOWALSKI E. O. Hulburt Center for Space Research, Code 4129, Naval Research Laboratory, Washington, DC 20375 Received 1991 January 31; accepted 1992 April 13 ABSTRACT We have determined the positions of the optical and X-ray centers in 13 rich galaxy clusters studied by Dressler. We tested the accuracy and precision of four methods for determining cluster centers: nearest- neighbor density, mean, median, and density-weighted mean; the last two proved superior. With the use of bootstrap resampling to obtain accurate confidence intervals on the positions of the centers, we have quantified the significance Of the X-ray/optical offsets. Insignificant offsets were found in Abell 119, 154, 376,400, 539, 1656, 1991, 2063, and 2634. The offset previously found by Snyder et al. in the Perseus cluster (Abell 426) was reproduced in our study but found to be only marginally significant, as were the offsets in Abell 754 and 2256. Abell 168 exhibits an apparently real discrepancy between its visible and X-ray centers, which we interpret as evidence that this cluster (and by implication other clusters) resulted from the collision of two approximately equal sized clusters. Subject headings: galaxies: clustering - X-rays: galaxies
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