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 HST/WFPC2. We apply a variant of Schneider's aperture mass technique to the observed WFPC2 field and obtain the distribution of weak-lensing signal-to-noise (S/N) ratio within the field. The resulting S/N map reveals a clear pronounced peak located about 1.7 arcmin (850h_{50}^{-1} kpc at z=0.897) southwest of the second peak associated with the optical cluster center determined from the dynamical analysis of Postman et al. A non-linear finite-field inversion method has been used to reconstruct the projected mass distribution from the observed shear field. The reconstructed mass map shows a super-critical feature at the location of the S/N peak as well as in the cluster central region. Assuming the redshift distribution of field galaxies, we obtain the total mass in the observed field to be 1.0 h_{50}^{-1} 10^{15} M_sun for <z>=1.0. The estimated mass within a circular aperture of radius 280h_{50}^{-1} kpc centered on the dark clump is 2.4h_{50}^{-1} 10^{14} M_sun. We have confirmed the existence of the ` dark ' mass concentration from another deep HST observation with a slightly different ~20 arcsec pointing.