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 Hubble Space Telescope ( HST )/Wide Field Planetary Camera 2 (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 ratio (S/N) within the field. The resulting S/N map reveals a clear pronounced peak located about 1 7 (850 h 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 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 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 × 10 15 h M ☉ for ⟨ z ⟩ = 1.0. The estimated mass within a circular aperture of radius 280 h kpc centered on the dark clump is 2.4 × 10 14 h M ☉ . We have confirmed the existence of the "dark" mass concentration from another deep HST observation with a slightly different (~20'') pointing.