31 January 2002 CfA Colloquium

The ROSAT Optical X-ray Survey: A Comparison of Optical and X-ray Cluster Discovery

Speaker: Megan Donahue


Are the mass-to-light ratios in clusters constant? Do all clusters have hot X-ray atmospheres and relatively consistent baryon fractions? How does method used to discover the clusters and define the cluster sample affect our conclusions about galaxy evolution in clusters? To address such questions as well as to find high-redshift clusters of galaxies, we applied optical and X-ray techniques to the areas of sky surveyed by deep ROSAT PSPC images. We directly compare X-ray techniques for cluster discovery (wavelet, VTP) in X-ray images with a current and popular optical technique for defining cluster candidates in galaxy catalogs from I-band CCD photometry (matched filter). We find that while both techniques frequently find the same systems, they do not always find the same systems. Furthermore, the scatter between the optical and X-ray properties is large. We also find that not all X-ray clusters have prominent red sequences, a lack which may result from the ROXS clusters being relatively low X-ray luminosity or low mass. We are in the process of following up that finding with a formal red sequence search courtesy of Mike Gladders. If we have results from that search, we will include them in this talk. We will discuss our methods and the results and implications of our survey.

References for students:

Donahue et al. astro-ph/0112530

Donahue et al. astro-ph/0112529

Postman, M. et al. 1996, AJ, 111, 615. "The Palomar Distant Clusters survey I. The Cluster Catalog"