Detection of dark matter concentrations in the field of Cl 1604+4304 from weak lensing analysis

We present a weak lensing analysis of a region around the galaxy cluster Cl 1604+4304 (z = 0.897) on the basis of the deep observations with the Hubbl e Space Telescope (HST)/Wide Field Planetary Camera 2 (WFPC2). We apply a v ariant of Schneider's aperture mass technique to the observed WFPC2 field a nd obtain the distribution of weak lensing signal-to-noise ratio (S/N) with in the field. The resulting S/N map reveals a clear pronounced peak located about 1.'7 (850 h(50)(-1) kpc at z = 0.897) southwest of the second peak a ssociated with the optical cluster center determined from the dynamical ana lysis of Postman et al. A nonlinear finite-field inversion method has been used to reconstruct the projected mass distribution from the observed shear field. The reconstructed mass map shows a supercritical feature at the loc ation of the S/N peak as well as in the cluster central region. Assuming th e redshift distribution of field galaxies, we obtain the total mass in the observed field to be 1.0 x 10(15) h(50)(-1) M. for [z] = 1.0. The estimated mass within a circular aperture of radius 280 h(50)(-1) kpc centered on th e dark clump is 2.4 x 10(14) h(50)(-1) M.. We have confirmed the existence of the "dark" mass concentration from another deep HST observation with a s lightly different (-20 ") pointing.