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Published September 1, 2006 | Published
Journal Article Open

X-Ray Galaxy Clusters in NoSOCS: Substructure and the Correlation of Optical and X-Ray Properties


We present a comparison of optical and X-ray properties of galaxy clusters in the northern sky, using literature data from BAX and optically selected clusters in DPOSS. We determine the recovery rate of X-ray-detected clusters in the optical as a function of richness, redshift, and X-ray luminosity, showing that the missed clusters are typically low-contrast systems when observed optically (either poor or at high redshifts). We employ four different statistical tests to test for the presence of substructure using optical two-dimensional data. We find that approximately 35% of the clusters show strong signs of substructure in the optical. However, the results are test-dependent, with variations also due to the magnitude range and radius utilized. We have also performed a comparison of X-ray luminosity and temperature with optical galaxy counts (richness). We find that the slope and scatter of the relations between richness and the X-ray properties are heavily dependent on the density contrast of the clusters. The selection of substructure-free systems does not improve the correlation between X-ray luminosity and richness, but this comparison also shows much larger scatter than one obtained using the X-ray temperature. In the latter case, the sample is significantly reduced because temperature measurements are available only for the most massive (and thus high-contrast) systems. However, the comparison between temperature and richness is very sensitive to the exclusion of clusters showing signs of substructure. The correlation of X-ray luminosity and richness is based on the largest sample to date (~750 clusters), while tests involving temperature use a similar number of objects as previous works (≾100). The results presented here are in good agreement with existing literature.

Additional Information

© 2006 American Astronomical Society. Received 2006 February 14, accepted for publication 2006 May 5. The processing of DPOSS and the production of the Palomar- Norris Sky Catalog (PNSC) on which this work was based was supported by generous grants from the Norris Foundation, and other private donors. Some of the software development was supported by the NASA AISRP program. We also thank the staff of Palomar Observatory for their expert assistance in the course of many observing runs. We would like to thank the anonymous referee for useful comments. Finally, we acknowledge the efforts of the POSS-II team, and the plate scanning team at STScI. P. A. A. L. was supported by the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP, process 03/04110-3). S. G. D. and A. A. M. were supported in part by the NSF grant AST 04-07448. Several undergraduates participated in the data acquisition and processing toward the photometric calibration of DPOSS. P. A. A. L. would like to thank Jason Pinkney for sharing the code used for estimating substructure and for helpful discussions on this code's implementation. This research has made use of the X-Rays Clusters Database (BAX) which is operated by the Laboratoire d'Astrophysique de Tarbes-Toulouse (LATT), under contract with the Centre National d'Etudes Spatiales (CNES).

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