| Abstract
| - Aims. We present a weak gravitational lensing analysis of supergroup SG1120 −1202, consisting of four distinct X-ray-luminous groups that will merge to form a cluster comparable in mass to Coma at z = 0 . These groups lie within a projected separation of 1 to 4 Mpc and within Δ v = 550 km s -1 and form a unique protocluster to study the matter distribution in a coalescing system. Methods. Using high-resolution HST/ACS imaging, combined with an extensive spectroscopic and imaging data set, we studied the weak gravitational distortion of background galaxy images by the matter distribution in the supergroup. We compared the reconstructed projected density field with the distribution of galaxies and hot X-ray emitting gas in the system and derived halo parameters for the individual density peaks. Results. We show that the projected mass distribution closely follows the locations of the X-ray peaks and associated brightest group galaxies. One of the groups that lies at slightly lower redshift ( z ≈ 0.35 ) than the other three groups ( z ≈ 0.37 ) is X-ray luminous, but is barely detected in the gravitational lensing signal. The other three groups show a significant detection (up to 5 σ in mass), with velocity dispersions between 355 + 55-70 and 530 + 45-55 km s -1 and masses between 0.8 + 0.4-0.3 × 10 14 and 1.6 + 0.5-0.4 × 10 14h-1M⊙, consistent with independent measurements. These groups are associated with peaks in the galaxy and gas density in a relatively straightforward manner. Since the groups show no visible signs of interaction, this supports the hypothesis that we observe the groups before they merge into a cluster.
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