Detection of Dark Matter Concentrations in the Field of Cl 1604+4304 from Weak Lensing Analysis

Abstract

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^-1₅₀ 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 1.0×10¹⁵ h^-1₅₀ M_solar for <z>=1.0. The estimated mass within a circular aperture of radius 280 h^-1₅₀ kpc centered on the dark clump is 2.4×10¹⁴ h^-1₅₀ M_solar. We have confirmed the existence of the ``dark'' mass concentration from another deep HST observation with a slightly different (~20") pointing.