High - Redshift , Optically - Selected Clusters : XMM - Newton Results for Cl 1324 + 3011 at z = 0 . 76 and Cl 1604 + 4304 at z = 0 . 90

We present the first detailed X-ray observations of optically-selected clusters at high redshift. Two clusters, Cl 1324+3011 at z = 0.76 and Cl 1604+4304 at z = 0.90, were observed with XMM-Newton. The optical center of each cluster is coincident with an extended X-ray source whose emission is detected out to a radius of ∼ 0.5 h 70 Mpc. The emission from each cluster appears reasonably circular, with some indication of asymmetries and more complex morphologies. Similarly to other optically-selected clusters at redshifts of z & 0.4, both clusters are modest X-ray emitters with bolometric luminosities of only L x = 1.4 − 2.0 × 10 h 70 erg s . We measure gas temperatures of T = 2.88 −0.49 keV for Cl 1324+3011 and 2.51 −0.69 keV for Cl 1604+4304. The X-ray properties of both clusters are consistent with the high-redshift Lx − T relation measured from X-ray–selected samples at z ≥ 0.5. However, based on the local relations, their X-ray luminosities and temperatures are low for their measured velocity dispersions (σ). The clusters are cooler by a factor of 2–9 compared to the local σ − T relation. We briefly discuss the possible explanations for these results. Subject headings: cosmology: observations – galaxies: clusters: individual (Cl 1324+3011 and Cl 1604+4304) – X-rays: galaxies Department of Physics, University of California, One Shields Avenue, Davis, CA 95616; [email protected] The Observatories of the Carnegie Institution of Washington, 813 Santa Barbara St., Pasadena, CA 91101; [email protected] Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218; [email protected]