XRISM–Subaru views of Abell 754: An off-axis, near-line-of-sight merging cluster

Abstract We report a weak-lensing (WL) mass measurement for the merging cluster Abell 754 and impose constraints on the merger trajectory. The trajectory analysis adopts a two-body model with a point-mass approximation and dynamical friction, refined using numerical simulations of major mergers and characterized by Euler angles. We first conduct WL analysis using the two-dimensional (2D) shear pattern from the Subaru Hyper Suprime-Cam in combination with Suprime-Cam images to assist in color selection. The WL mass map shows a distinct double-peak structure located around the western and eastern brightest cluster galaxies, as reported in the literature. The two-halo component analysis, which utilizes the 2D shear pattern over the cluster’s entire region and considers the lensing covariance matrix from uncorrelated large-scale structures, indicates mass values of $M_{200}^{\rm W}=3.13_{-1.00}^{+1.53}\times 10^{14}\, h_{70}^{-1}\, {M}_{\odot }$ and $M_{200}^{\rm E}=6.41_{-1.97}^{+2.92}\times 10^{14}\, h_{70}^{-1}\, {M}_{\odot }$. Thus, the eastern mass component associated with the X-ray tadpole-shaped gas is the main cluster. No substantial structural components are detected in the line-of-sight velocities of the member galaxies. By combining the WL parameters, line-of-sight velocities, X-ray morphology, and priors informed by X-ray kinematics, we estimate an impact parameter of approximately 0.77 Mpc at an initial separation of 2 Mpc from the main cluster. The merger plane is inclined at about 20° relative to the line of sight. Interestingly, this system is an off-axis, near-line-of-sight merger. This characteristic arises because the trajectory within the merger plane is altered during the pericenter passage, causing the apparent motion to transition from predominantly along the line of sight before the core passage to mainly within the plane of the sky afterward. This study will assist in conducting numerical simulations to understand the XRISM observations.