Wide-field Weak Lensing Mass Reconstructions of Merging Galaxy Clusters Brett Ragozzine, Douglas Clowe Ohio University, Department of Physics & Astronomy, Athens, OH Introduction  Current cosmological models show that baryonic matter comprises 4.5% of the total mass distribution of the universe and that dark matter comprises 23%. Thus the dark matter component is approximately 5 times more abundant than baryonic matter.  Alternative gravity models have been proposed which replace the need for dark matter with gravity being stronger than Newtonian on Mpc scales.  Dark matter follows the distribution of baryonic matter in the universe. In order to test its existence, the dark matter component must be separated from the baryonic component.  Galaxy clusters are permeated by x-ray gas, which outweighs the remaining baryonic matter by a factor of 10. By showing the offset of the cluster’s gravitational potential from the x-ray plasma, one can infer the existence of dark matter.  Merging clusters provide a unique view where the dark matter component is separated from x-ray gas. X-ray gas is slowed through ram pressure while the galaxies themselves move nearly uninterrupted.  Weak lensing reveals the total mass distribution. comparing the x-ray gas distribution with the mass reconstruction of weak lensing reveals an x-ray gas offset from the dominant mass doing the lensing.  It has been previously shown that merging clusters have a baryonic mass distribution that is offset from the majority of the mass in the lens.  Results are needed from additional merging clusters to avoid conspiracy models. Abell 1758 Figure 1. Bullet Cluster (Clowe et al, 2006). Colors follow x-ray gas. Green contours follow mass component.  Data taken with Subaru Suprime Cam in April Images included exposures in B (30 minutes) and V (80 minutes).  Mass contours are aligned with the Brightest Cluster Galaxies of the merging components. Figure 2. Abell 1758, the northeastern cluster’s dark matter component follows the optical component more closely than does the x-ray gas in Figure 3. The gray- scale image is the 80-minute V band Subaru exposure. The blue contours are the  reconstruction coming from 24,774 galaxies (33.2 galaxies per sq arcmin), outer contour is 0.03 and increasing in steps of Figure 3. Abell 1758, contours follow the x-ray gas distribution. The gray-scale image is from the DSS. The x-ray contours are from a 58 ksec XMM exposure. The northern cluster is an ongoing merger between two 7 keV clusters, the southern cluster is an ongoing merger between two 5 keV clusters (from David and Kempner, 2004). Both figures cover an area of 16’ x 15’ (4.07 x 3.82 Mpc at z=0.279).     This work is funded in part by Space Telescope Science Institute, AURA, grant HST-GO A.  This weak lensing analysis on Abell 1758 was performed using the KSB method. The preliminary results are as follows.  X-ray contours are offset from one of the merging cluster components.