Discovery of an X-ray bridge between the comma-shaped gas and the main cluster in MCXC J0157.4-0550

TL;DR

This paper presents the discovery of a faint X-ray bridge between the main cluster and a comma-shaped substructure in MCXC J0157.4-0550, detected in XMM-Newton data with model-independent filament identification methods and a significance of $\sim5.5\sigma$. It combines this X-ray morphology with a 2D weak-lensing analysis from Subaru/HSC-SSP data, using an elliptical NFW model and a stellar-mass based prior to obtain two cluster masses: $M_{200}^{\rm main}=2.68_{-0.93}^{+1.11}\times10^{14}\,h_{70}^{-1}M_\odot$ and $M_{200}^{\rm sub}=0.46_{-0.22}^{+0.38}\times10^{14}\,h_{70}^{-1}M_\odot$, with a sub-to-main ratio $f_{\rm sub,200}=0.15\pm0.11$. A joint likelihood that includes the X-ray filament morphology and 2D WL data yields merger parameters such as an infall velocity of $\sim10^{3}$ km s$^{-1}$ and an impact parameter around $0.9$ Mpc, consistent with hydrostatic mass estimates within uncertainties. The authors interpret the filament as a product of ram-pressure stripping near pericenter together with tidal rotation, aligning the gas filament with the tangential velocity at pericenter and providing a robust framework for constraining cluster merger dynamics from combined X-ray and lensing data. This work advances understanding of gas–dark-matter interplay in mergers and demonstrates how X-ray substructure plus weak-lensing shear jointly constrain merger geometry.

Abstract

We report the discovery of a faint X-ray bridge connecting between the comma-shaped gas and the main cluster in MCXC J0157.4-0550, using {\it XMM-Newton} image. The filamentary structure is found in a model-independent manner in both topological features and Gaussian Gradient Magnitude filtering. The X-ray surface brightness profile perpendicular to the filament is detected at a $5.5σ$ level. Weak-lensing (WL) analysis using the Subaru/HSC-SSP Survey archive data strongly supports the two mass components. Given a prior from the stellar masses, we obtain $M_{200}^{\rm main}=2.68_{-0.92}^{+1.11}\times 10^{14}\,h_{70}^{-1}M_\odot$ and $M_{200}^{\rm sub}=0.46_{-0.22}^{+0.38}\times 10^{14}\,h_{70}^{-1}M_\odot$. The main axis of the projected halo distribution is more likely to align with the direction of the main cluster than to be oriented perpendicularly. Similar X-ray distributions have been identified in the literature on numerical simulations. The filamentary structure forms in the following manner: as the gas is stripped by ram pressure near the pericenter, it gets dragged by tidal rotation. Once free from this rotation, the gas moves inertially in a direction parallel to the tangential velocity at the pericenter. The comma-shaped gas, with tails pointing in the opposite direction to the main cluster, is also formed by the current tidal rotation as it moves away from the main cluster. This warrants us that, although it is sometimes thought based on the X-ray morphology alone that the tail is pointing in the opposite direction to the merger motion, this is not necessarily the case. The information of the X-ray filamentary remnant from the cluster merger, together with the 2D WL shear data, provides constraints on the merger parameters, indicating an infalling velocity of approximately $1000\, {\rm km\, s^{-1}}$ and an impact parameter of $0.9$ Mpc.

Figures (5)

• Figure 1: Top: The XMM-Newton count rate image in 0.4-2.3 keV smoothed with a Gaussian kernel of FWHM$\simeq2\,[{\rm arcmin}]$. The black contours are represented by [10,16,34,64,106,160] in the unit of counts s$^{-1}$ deg$^{-2}$. The red line denotes the filament detected by DisPerSE. Black $+$ and $\times$ represent the brightest galaxies ($z=0.12823$ and $z=0.12869$) associated with each gas halo. The white contours are the modelled mass map with the best-fit parameters derived by 2D WL analysis with a prior of the mass ratio, spaced at square root intervals between [$0.02$,$1$] in $10^{15}\,h_{70}M_\odot{\rm Mpc}^{-2}$. The white circle in the left-bottom corner is the typical uncertainty of the centroid determinations. Bottom: Background color represents an adaptive GGM-filtered image. A weak edge is found at the south of the filament. Alt text: Two stacked images showing the filament.
• Figure 2: The surface brightness profiles along a direction perpendicular to the filament (red diamonds) and to the axisymmetric position of the line connecting the two bright galaxies (blue squares). The PSF convolved and deconvolved Gaussian profiles are shown by the black solid and dashed lines, respectively. Alt text: A graph showing the best-fitting results to the count rate across the filament, overplotted with the count rate at a non-filamentary position.
• Figure 3: Left: Adaptively GGM-filtered image ($8'\times 8'$). A comma-shaped tail is found in the northern-east (NE) direction. The red curve represents a maximum radius (3 arcmin) to compute surface brightness and X-ray temperature. The black curve denotes the best-fit discontinuity in the surface brightness profile. Right: Surface brightness and temperature profiles. From top to bottom, southern-east (SE), southern-west (SW), and the whole regions are represented. Blue points are the surface brightness profile in arbitrary unit. Blue solid and dashed lines represent the best-fit model convolved with and without the PSF, respectively. The red solid and regions are the temperature and its uncertainty, respectively. Alt text: Two line graphs showing the X-ray count rate around the comma-shaped gas.
• Figure 4: Numerical simulation from 2011ApJ...728...54Z and 2018ApJS..234....4Z. Middle: the background colour represents the X-ray surface brightness at $\sim$1.8 Gyr after the beginning. The original image was inverted with respect to the x-axis direction. The spiral structure of the subcluster and the filamentary bridge between the main and sub clusters are found. The white solid and black dashed curves are a trajectory of the simulated subcluster and an analytical solution in the rest-frame of the main cluster, respectively. The white ellipticals represent the projected ellipticity of the DM halo at $1.0$, $1.2$, $1.4$, and $1.8$ Gyr after the beginning of the simulation (from right to left). The inset shows a zoomed view of the comma-shaped gas. The red line is an auxiliary line indicating the direction of the tidal rotation stripping. The red auxiliary line is parallel to the X-ray filament. Top: the surface brightness (blue circles) and temperature (red diamonds) profiles of the subcluster in the cold front (black dashed line) direction. Second-top: the X-ray surface brightness perpendicular to the filament. Bottom: The X-ray images at $1.2, 1.3, 1.4$ and $1.6$ Gyr. Alt text: Four stacked graphs illustrating the results of the numerical simulation.
• Figure 5: The model orbit. The blue curve and elliptical circles represent the best-fit orbit and the $1\sigma$ uncertainty at each time, respectively. The black contours are the projected mass distribution obtained by the joint constraint. The red solid line represents the X-ray filament. The green solid line is the tangential direction at the pericenter. Alt text: A graph representing the results of the joint constraints.