WEAK-LENSING DETECTION AT z 1.3: MEASUREMENT OF THE TWO LYNX CLUSTERS WITH THE ADVANCED CAMERA FOR SURVEYS

We present aHubble Space TelescopeAdvanced Camera for Surveys (ACS)weak-lensing study of RX J0849+ 4452 andRX J0848+4453, the twomost distant (at z 1⁄4 1:26 and 1.27, respectively) clusters yet measuredwithweak lensing. The two clusters are separated by 40 from each other and appear to form a supercluster in the Lynx field. Using our deep ACS i775 and z850 imaging, we detected weak-lensing signals around both clusters at 4 levels. The mass distribution indicated by the reconstructionmap is in good spatial agreement with the cluster galaxies. From the singular isothermal sphere (SIS) fitting, we determined that RX J0849+4452 and RX J0848+4453 have similar projectedmasses of (2:0 0:6) ; 10 and (2:1 0:7) ; 10 M , respectively, within a 0.5 Mpc ( 6000) aperture radius. In order to compare the weak-lensing measurements with the X-ray results calibrated by the most recent low-energy quantum efficiency determination and time-dependent gain correction, we also reanalyzed the archival Chandra data and obtained TX 1⁄4 3:8þ1:3 0:7 and 1:7þ0:7 0:4 keV for RX J0849+4452 and RX J0848+4453, respectively. Combinedwith the X-ray surface brightness profile measurement under the assumption of an isothermal -model, the temperature of RX J0849+ 4452 predicts that the projected mass of the cluster within r 1⁄4 0:5Mpc is 2:3þ0:8 0:4 ; 10 M , consistent with the weaklensing analysis. On the other hand, for RX J0848+4453 we find that the mass derived from this X-ray analysis is much smaller (6:3þ2:6 1:5 ; 10 13 M ) than the weak-lensing measurement. Subject headinggs: cosmology: observations — dark matter — galaxies: clusters: individual (RX J0849+4452, RX J0848+4453) — galaxies: high-redshift — gravitational lensing — X-rays: galaxies: clusters Online material: color figure