XRISM-Subaru views of Abell 754: Energetic ICM Motions Revealed by XRISM/Resolve

TL;DR

XRISM/Resolve delivers spatially resolved ICM velocity fields for Abell 754, revealing a large LOS bulk-velocity difference of $656\pm35$ km s$^{-1}$ between the eastern main core and the middle filament, corresponding to a bulk Mach number of $\mathcal{M}_{\rm bulk}\simeq0.45$. The eastern core exhibits pronounced regional variation in velocity dispersion, including a southern region with $\sim$500 km s$^{-1}$, while narrow-band Fe K diagnostics indicate multi-phase gas with a cool $kT$\,$\sim$\,5 keV and a hot $kT$\,$\sim$\,16 keV component, whose distinct velocities imply a mixing interface rather than extreme turbulence. Weak-lensing confirms a ~2:1 mass ratio between eastern and western components and supports an off-axis, post-core-passage merger that injects angular momentum and drives rotational and fallback gas flows. Together, these results quantify the kinetic energy budget in a major cluster merger and illustrate how multi-phase structure and projection effects shape the interpretation of ICM motions.

Abstract

We present high-resolution X-ray spectroscopy of the merging cluster Abell~754 using \textit{XRISM}/Resolve. In GO1 phase, \textit{XRISM}/Resolve observed Abell 754 in two deep pointings, targeting the eastern primary core (114~ks) and the middle of the X-ray filamentary structure (190~ks). Spectral fits to full field-of-view data reveal a line-of-sight velocity difference of $656 \pm 35$~km~s$^{-1}$ between the two pointing, corresponding to a bulk Mach number of 0.45$\pm$0.03. Velocity dispersions are measured to be $220^{+26}_{-29}$~km~s$^{-1}$ and $279^{+24}_{-23}$~km~s$^{-1}$ in the eastern and middle pointing, respectively. Within the eastern core, the velocity dispersion shows spatial variation, reaching $497^{+144}_{-117}$~km~s$^{-1}$ in the southern core with high temperature -- among the largest values yet reported in galaxy clusters to date. Narrow-band analysis of the Fe-K complex in this region reveals systematically higher temperatures derived from He-like and H-like Fe line ratio compared to those obtained via broadband fits, indicating multi-phase structures. Two-temperature modeling further separates a cooler core phase from a hotter, shock or turbulence-heated phase whose velocity is blueshifted, similar to that of the middle pointing. These results point to a mixing interface where post-shock gas from the south overlaps, in projection, with cooler core gas, inflating the observed line widths in this region. Weak-lensing analysis with Subaru/HSC and Suprime-cam confirms that the eastern component is about twice as massive as the western one, consistent with disruption and gas stripping of the latter. The curved morphology of the eastern X-ray core, together with the measured kinematics, is naturally explained by an off-axis, post--core-passage merger that imparts angular momentum and drives large-scale rotational and fallback flows.

Figures (6)

• Figure 1: XMM-Newton/EPIC mosaic image of A754 in the 1.6–4.0 keV band (constructed following the procedure described in 2024AA...689A.173O), overlaid with color regions used for XRISM spatially resolved spectroscopy. The white and green boxes indicate the Resolve FoV for the MAIN and SUB observations, respectively. White rectangles within the white box represent the subgrid areas designated for spectral extraction. White dashed contours indicate the projected mass distribution from the weak-lensing mass map of Okabe2025b, shown at significance levels of 1, 2, 3, 4, and 5$\sigma$. Cyan crosses mark the positions of the E-BCG ($z_{\rm EB} = 0.054291$) and W-BCG ($z_{\rm WB} = 0.054841$). The dashed orange regions represent the sky regions for which the ICM parameters are derived. Alt text: X-ray Image of Abell 754, with right ascension on the horizontal axis and declination on the vertical axis.
• Figure 2: (Top) Resolve spectra in the 3.0--9.5 keV energy band for MAIN (black) and SUB (red), respectively. The best-fit models are overlaid on each spectrum with NXB models, illustrating the goodness of fit between the observational data and the corresponding model. Data are binned by a factor of 4 eV for display purposes. (Bottom) Zoom-in of the Resolve spectra around the He-like Fe lines (left) and H-like Fe lines (right). Data are binned by a factor of 2 eV for display purposes. Alt text: Three line graphs. In the upper panel, the x axis shows the energy from 3.0 to 9.5 kilo electron volt. The y axis shows the count from 0.0002 to 0.2 counts per second and per kilo electron volt, and the residuals of minus 5 to 5 in lower part. In the lower left panel, the x axis shows the energy from 6.25 to 6.40 kilo electron volt. In the lower right panel, the x axis shows the energy from 6.55 to 6.65 kilo electron volt. The y axis shows the count from 0.0 to 0.2 counts per second and per kilo electron volt, and the residuals of minus 5 to 5 in lower part.
• Figure 3: Maps of temperature (top left), abundance (top right), bulk velocity (bottom left), and velocity dispersion (bottom right) for MAIN pointing, based on the spatial regions defined in figure \ref{['fig1:image']}. The black star indicates the position of the E-BCG. All maps are derived from broad-band fits in the 3.0–9.5 keV range; values in parentheses indicate the corresponding results from the narrow-band fits (6.2–6.7 keV). Contours represent the X-ray surface brightness, highlighting the correspondence with spatial structures. The cyan box indicates the field of view of the MAIN observation. The units of the color bars are, in order: keV, solar, km s$^{-1}$, and km s$^{-1}$. Alt text: Four color maps arranged in two rows and two columns, each with right ascension on the horizontal axis and declination on the vertical axis. The upper left map shows temperature in kilo–electronvolts ranging from 6.0 to 13.0. The upper right map shows abundance in solar units ranging from 0.1 to 0.6. The lower left map shows bulk velocity in kilometers per second ranging from minus 250 to plus 250. The lower right map shows velocity dispersion in kilometers per second ranging from 0 to 500.
• Figure 4: Resolved spectra around the Fe K$\alpha$ lines of detector regions a (upper left), b (upper right), c (lower left), and d (lower right) for the SSM analysis. The solid colored lines represent the modeled contributions from individual regions: A (red), B (orange), C (blue), and D (green). The black lines represent the sum of models. Alt text: Two line graphs. In the two panels, the x axis shows the energy from 6.2 to 6.7 kilo electron volt. The y axis shows the count from 0.00002 to 0.8 counts per second and per kilo electron volt, and the residuals of minus 5 to 5 in lower part.
• Figure 5: Resolved spectrum around the Fe K$\alpha$ lines of detector region d for the SSM analysis with 2T models. The solid colored lines represent the modeled contributions from individual regions: A (red), B (orange), C (blue), cold component of D (green), and hot component of D (magenta). The black lines represent the sum of models. Alt text: Two line graphs. In the two panels, the x axis shows the energy from 6.2 to 6.7 kilo electron volt. The y axis shows the count from 0.0009 to 0.7 counts per second and per kilo electron volt, and the residuals of minus 5 to 5 in lower part.