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We present the first detailed X-ray observations of optically-selectedclusters at high redshift. Two clusters, Cl 1324+3011 at z = 0.76 and Cl1604+4304 at z = 0.90, were observed with XMM-Newton. The optical center ofeach cluster is coincident with an extended X-ray source whose emission isdetected out to a radius of 0.5 Mpc. The emission from each cluster appearsreasonably circular, with some indication of asymmetries and more complexmorphologies. Similarly to other optically-selected clusters at redshifts of z> 0.4, both clusters are modest X-ray emitters with bolometric luminosities ofonly Lx = 1.4 - 2.0 x 10^(44) erg/s. We measure gas temperatures of T = 2.88(+0.71/-0.49) keV for Cl 1324+3011 and 2.51 (+1.05/-0.69) keV for Cl 1604+4304.The X-ray properties of both clusters are consistent with the high-redshiftLx-T relation measured from X-ray-selected samples at z > 0.5. However, basedon the local relations, their X-ray luminosities and temperatures are low fortheir measured velocity dispersions (sigma). The clusters are cooler by afactor of 2 - 9 compared to the local sigma-T relation. We briefly discuss thepossible explanations for these results.

astrophysical journal/the astrophysical journal

The First Detailed X-Ray Observations of High-Redshift, Optically Selected Clusters: XMM-Newton Results for Cl 1324+3011 at z = 0.76 and Cl 1604+4304 at z = 0.90