We present a weak lensing analysis of one of the most distant massive galaxy cluster known , RDCS 1252.9 - 2927 at z = 1.24 , using deep images from the Advanced Camera for Survey ( ACS ) on board the Hubble Space Telescope ( HST ) .
By taking advantage of the depth and of the 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 provide a measurement of the total mass of the cluster inside a 1 \mbox { Mpc } radius of M ( < 1 \mbox { Mpc } ) = ( 7.3 \pm 1.3 ) \times 10 ^ { 14 } \mbox { M } _ { \odot } in the current cosmological concordance model h = 0.70 , \Omega _ { \mathrm { m } } = 0.3 , \Omega _ { \Lambda } = 0.7 , assuming a redshift distribution of background galaxies as inferred from the Hubble Deep Fields surveys .
A weak lensing signal is detected out to the boundary of our field ( 3 \arcmin radius , corresponding to 1.5 \mbox { 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 } \simeq 0.5 \mbox { Mpc } .