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

TL;DR

Abell 754 is a clearly merging cluster whose weak-lensing signal reveals a robust double-peak mass distribution around its two BCGs. The authors perform a 2D weak-lensing analysis with Subaru/HSC and Suprime-Cam data, accounting for uncorrelated large-scale structure, and model the system with a two-halo NFW framework to obtain $M_{200}$ for the western and eastern components. They then adopt a two-body point-mass dynamics with a calibrated dynamical-friction term to constrain the merger trajectory, showing an off-axis, near-line-of-sight configuration with $b\approx0.77\,h_{70}^{-1}{\rm Mpc}$, $v_0\approx1806\,{\rm km\,s^{-1}}$, and inclination $20^{\circ}\pm19^{\circ}$. The results indicate a major merger with $f_{\rm ratio}=0.49\pm0.33$ and provide a practical analytic pathway to generate merger scenarios consistent with XRISM observations of gas motions.

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 shear pattern from the Subaru HSC 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 entire region and considers the lensing covariance matrix from uncorrelated large-scale structures, indicates mass values of $M_{200}^W=3.13_{-1.00}^{+1.53}\times10^{14}h_{70}^{-1}M_\odot$ and $M_{200}^E=6.41_{-1.97}^{+2.92}\times10^{14}h_{70}^{-1}M_\odot$. 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. Utilizing WL parameters, line-of-sight velocities, and X-ray information on morphology and kinematics, we determine 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 degrees 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.

Figures (7)

• Figure 1: Top: XMM-Newton X-ray image ($30'\times20'$) overlaid with the signal-to-noise ratio of the WL mass map, stepped by $1\sigma$ from $1\sigma$. The red $+$ and $\times$ represent the W-BCG and E-BCG positions, respectively. Bottom: Smoothed $i2'$ luminosity of spec-z member galaxies in a unit of $10^{11}L_\odot$. Alt text: Two color images aligned vertically.
• Figure 2: Trajectories of the main and subclusters versus the merger time. The $x$ and $y$ positions obtained from numerical simulations 2011ApJ...728...54Z2018ApJS..234....4Z are represented by the solid red and blue lines, respectively. The dotted red and blue lines correspond to $x$ and $y$ of the analytical model as described by equation (\ref{['eq:trajectory']}), respectively. Results from the main cluster are presented in the first, third, and fifth rows, while those from the sub-cluster appear in the second, fourth, and sixth rows. The inset annotations denote the parameters utilized for merging. Altext: a panel layout consisting of 3 columns and 6 rows.
• Figure 3: The scatter map of the line-of-sight velocity of the member galaxies. The red and blue colors express a positive and negative line-of-sight velocity relative to that of the main cluster, respectively. The circle size is proportional to the square root of the luminosity. The contours are the significance level of the WL mass distribution, which is the same as Figure \ref{['fig:massmap']}. Altext: one scatter map.
• Figure 4: Top: the magenta line represents the merger trajectory in the rest frame of the main cluster, overlaid on the top panel of Figure \ref{['fig:massmap']}. The magenta $+$ and $\times$ are the NFW centers determined by 2D WL analysis. Middle : the relative position of the subcluster versus the merger time. The time is set to be zero at the pericenter. The red solid, blue dotted, and green dashed lines are the ${\tilde{x}}$, ${\tilde{y}}$, and ${\tilde{z}}$ positions, respectively. The shaded areas represent the $1\sigma$ uncertainties. The vertical gray line expresses the current time. The vertical gray dashed line is the pericenter. Second-to-bottom: the relative velocity of the subcluster versus the merger time. Bottom: the successful ratio, $f_{\rm suc,GMM}$, to identify member galaxies in mock spec-z data using the GMM method. The horizontal line is the probability of the successful ratio by chance. Alttext: fourth vertically aligned figures.
• Figure 5: The trajectory in the three-dimensional space. The transparent blue plane denotes the merger plane. The colors along the trajectory curve indicate the progression of time, from blue to red.