Weak gravitational lensing by a sample of X-ray-luminous clusters of galaxies. III. Serendipitous weak lensing detections of dark and luminous mass concentrations

In the course of a weak gravitational lensing survey of 39 clusters of galaxies, covering a total sky area of similar to1 deg(2), we have serendipitously discovered mass concentrations in the fields of A1705 and A1722 that are most probably not associated with the main cluster target. By combining weak lensing information with two-color galaxy photometry in fields centered on our sample clusters, we identify a new cluster candidate at z similar to 0.5 in the field of A1705. This cluster candidate also displays strong lensing in the form of a giant luminous arc. The new mass concentration in the field of A1722 also seems to be associated with an optically luminous cluster of galaxies at z similar to 0.5, but in this case there is some evidence for additional structures along the line of sight that may contribute to the lensing signal. A third cluster, A959, has a dark subclump that shows interesting morphological evidence in the mass map for being associated with the main cluster. This is the first case where there is any significant evidence for a physical association between a dark subclump ( discovered from weak lensing) and a normal cluster. Analysis of archival X-ray data shows that the three new mass concentrations are not firmly detected in X-rays and that they are X-ray - underluminous.