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

We present a Hubble Space Telescope Advanced Camera for Surveys (ACS) weak-lensing study of RX J0849+4452 and RX J0848+4453, the two most distant (at z = 1.26 and z = 1.27, respectively) clusters yet measured with weak-lensing. The two clusters are separated by ∼ 4 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 ∼ 4σ levels. The mass distribution indicated by the reconstruction map 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 projected masses of (2.0± 0.6)× 10M⊙ and (2.1± 0.7)× 10 M⊙, respectively, within a 0.5 Mpc (∼ 60) 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 re-analyzed the archival Chandra data and obtained T = 3.8 −0.7 and 1.7 +0.7 −0.4 keV for RX J0849+4452 and RX J0848+4453, respectively. Combined with the X-ray surface brightness profile measurement under the assumption of isothermal β model, the temperature of RX J0849+4452 predicts that the projected mass of the cluster within r = 0.5 Mpc is 2.3 −0.4 × 10 M⊙, consistent with the weak-lensing analysis. On the other hand, for RX J0848+4453 we find that the mass derived from this X-ray analysis is much smaller (6.3 −1.5×10 M⊙) than the weak-lensing measurement. One possibility for this observed discrepancy is that the intracluster medium (ICM) of RX J0848+4453 has not yet fully thermalized. Although Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218. University of California Observatories/Lick Observatory, University of California, Santa Cruz, CA 95064. Department of Physics and Astronomy, Washington State University, WA 99164.