Hubble Space Telescope ACS weak-lensing analysis of the galaxy cluster RDCS 1252.9-2927 at z=1.24

We present a weak-lensing analysis of one of the most distant massive galaxy clusters known, RDCS 1252.9- 2927, at z = 1.24, using deep images from the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST). By taking advantage of the depth and angular resolution of the ACS images, we detect for the first time at z> 1 a clear weak-lensing signal in both the i (F775W) and z (F850LP) filters. We measure a 5 sigma signal in the i band and a 3 sigma signal in the shallower z-band image. The two radial mass profiles are found to be in very good agreement with each other, and they provide a measurement of the total mass of the cluster inside a 1 Mpc radius of M (< 1 Mpc) = (8.0 +/- 1.3) x 10(14) M-. in the current cosmological concordance model, h = 0.70, Omega(m) = 0.3, and Omega(Lambda) = 0.7, assuming a redshift distribution of background galaxies as inferred from the Hubble Deep Field surveys. A weak-lensing signal is detected out to the boundary of our field ( 30 radius, corresponding to 1.5 Mpc at the cluster redshift). We detect a small offset between the centroid of the weak-lensing mass map and the brightest cluster galaxy, and we discuss the possible origin of this discrepancy. The cumulative weak-lensing radial mass profile is found to be in good agreement with the X-ray mass estimate based on Chandra and XMM-Newton observations, at least out to R-500 similar or equal to 0.5 Mpc.