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Bulk and turbulent gas motions in the interacting galaxy cluster Abell 3395 South observed with XRISM

Using XRISM high-resolution X-ray spectroscopy, this study reveals that the Abell 3395 South subcluster exhibits significant bulk motion but low turbulence, indicating a dynamically unrelaxed state and inefficient turbulent particle reacceleration consistent with the absence of a radio halo.

Original authors: Naomi Ota, Angie Veronica, Jakob Dietl, Anri Yanagawa, Thomas H. Reiprich, Veronica Biffi, Klaus Dolag, Marcus Brüggen, Esra Bulbul, Florian Pacaud, Yoshiki Toba

Published 2026-06-01

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CC BY 4.0

Original authors: Naomi Ota, Angie Veronica, Jakob Dietl, Anri Yanagawa, Thomas H. Reiprich, Veronica Biffi, Klaus Dolag, Marcus Brüggen, Esra Bulbul, Florian Pacaud, Yoshiki Toba

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, cosmic construction site. In this site, massive "cities" of galaxies, called galaxy clusters, are constantly being built and merged together. The space between these galaxies isn't empty; it's filled with a super-hot, invisible fog called the Intracluster Medium (ICM).

This paper is like a high-tech weather report for one specific construction site: a cluster called Abell 3395 South (A3395S). Scientists used a new, incredibly sharp X-ray telescope called XRISM (specifically its "Resolve" instrument) to take the temperature and measure the wind speed of this cosmic fog.

Here is what they found, explained simply:

1. The "Wind" vs. The "Storm"

To understand the gas in this cluster, the scientists looked at two types of motion:

Bulk Motion (The Wind): This is the gas moving in one big, organized direction, like a steady breeze blowing across a field.
Turbulence (The Storm): This is the gas churning, swirling, and mixing up in all directions, like a chaotic storm or white water rapids.

The Discovery:
The scientists found that the gas in the center of this cluster has a strong, steady wind blowing through it at about 263 km/s (roughly 590,000 mph). However, the "storm" (turbulence) is surprisingly weak. The gas is only churning a little bit, at about 124 km/s.

The Analogy:
Think of a river. Usually, when two rivers crash into each other (which is what happens when galaxy clusters merge), you expect a massive, churning whirlpool (high turbulence). But in A3395S, it's more like two streams of water merging where the water is still flowing fast in one direction, but the surface is relatively calm and smooth. It hasn't turned into a chaotic mess yet.

2. The System is "Not Ready"

Because the gas is still moving in a big, organized wind rather than just churning randomly, the scientists conclude that this galaxy cluster is not yet "relaxed."

The Analogy:
Imagine a group of dancers. If they are all moving in a synchronized line, they are in a "bulk motion." If they are all bumping into each other, spinning, and stopping randomly, they are "turbulent."
A3395S is like a dance troupe that has just started to merge with another troupe. They are still moving in their original lines (bulk motion) and haven't yet started the chaotic, energetic dance of a full merger (turbulence). They are in the middle of the process, not at the end.

3. Why No "Radio Halo"?

In many merging clusters, scientists see a "radio halo"—a giant, fuzzy glow of radio waves. This glow is created when the chaotic turbulence acts like a giant particle accelerator, speeding up electrons to create radio signals.

The Finding:
A3395S has no radio halo.
The Connection:
This makes perfect sense with their findings. Because the "storm" (turbulence) is so weak, there isn't enough energy to act as a particle accelerator. The gas is too calm to create the radio glow. It's like trying to make a fire with wet wood; the wind is blowing, but the wood isn't rubbing together hard enough to spark a flame.

4. The "Big Picture" Context

This cluster is part of a larger family (Abell 3391, 3395 North, and 3395 South) that is embedded in a giant cosmic "filament" (a long bridge of matter connecting galaxies).

The Theory: Scientists predicted that clusters in these filaments would have strong winds (bulk motion) because they are constantly being fed gas from the bridge.
The Proof: This paper confirms that theory. The gas is indeed rushing along the filament, but it hasn't had enough time to slow down and turn into a chaotic storm yet.

Summary

Using a super-powerful X-ray camera, the scientists looked at the "wind" inside a galaxy cluster. They found that the gas is moving fast in a straight line but isn't very chaotic. This tells us the cluster is still in the early stages of a cosmic collision, hasn't settled down yet, and is too calm to create the radio signals seen in older, more violent collisions. It's a snapshot of a cosmic event in progress, captured with unprecedented clarity.

Technical Summary: Bulk and Turbulent Gas Motions in Abell 3395 South Observed with XRISM

Problem and Motivation
Galaxy clusters are the most massive gravitationally bound structures in the Universe, with the majority of their baryonic mass residing in the intracluster medium (ICM). Understanding gas motions within the ICM is critical for elucidating cluster assembly processes, including mergers, active galactic nucleus (AGN) feedback, and gas accretion along large-scale filaments. Historically, direct measurements of ICM bulk and turbulent velocities were limited by the spectral resolution of X-ray instruments, relying on indirect methods or lower-resolution emission-line measurements. The advent of microcalorimeter spectroscopy, specifically with the X-ray Imaging and Spectroscopy Mission (XRISM), has enabled precise Doppler measurements of emission-line centroids and widths. This study addresses the need to characterize the dynamical state of the interacting Abell 3391–3395 system, a nearby ( $z \simeq 0.05$ ) cluster complex embedded in a prominent large-scale filament, by directly measuring gas kinematics in the Abell 3395 South (A3395S) subcluster.

Methodology
The authors utilized high-resolution X-ray spectroscopy from the XRISM/Resolve microcalorimeter to observe A3395S. The observation was conducted as a single pointing with the cluster center near the center of the Resolve field of view (FOV), providing a net exposure of approximately 266 ks.

Data Reduction: Spectral analysis was performed using the standard XRISM pipeline (HEASoft 6.36) and calibration database. Non-X-ray background (NXB) was estimated using the dedicated NXB database, while the cosmic X-ray background (CXB) was modeled using complementary Xtend imaging data from regions outside the cluster's $R_{500}$ .
Spectral Fitting: Spectra were extracted from the entire Resolve FOV (excluding specific calibration pixels and pixel 27) and fitted in the 2–10 keV energy band using an absorbed thermal plasma model (tbabs × bapec). The analysis focused on the Fe XXV He $\alpha$ and Fe XXVI Ly $\alpha$ emission lines to constrain temperature, redshift, and velocity broadening.
Kinematic Derivation:
- Bulk Velocity: Derived from the shift in the emission-line centroid relative to the systemic redshift of the cluster, determined via optical spectroscopic redshifts of member galaxies and the Brightest Cluster Galaxy (BCG).
- Turbulent Velocity: Derived from the broadening of the Fe XXV He $\alpha$ line after accounting for instrumental line spread function (LSF) and thermal broadening.
Systematic Checks: The authors evaluated potential systematic uncertainties, including resonant scattering, energy band dependence, multi-temperature structures, and instrumental energy scale calibration. They confirmed that these factors do not significantly bias the velocity measurements.

Key Results

Turbulent Velocity: The one-dimensional turbulent velocity dispersion in the core of A3395S is measured to be $\sigma_v = 124 \pm 21$ km s $^{-1}$ . Assuming isotropic turbulence, this corresponds to a three-dimensional turbulent velocity of $\sim 215$ km s $^{-1}$ .
Mach Number: Based on the best-fit temperature ( $kT \approx 5$ keV), the turbulent Mach number is calculated as $M_{\text{turb}} = 0.19 \pm 0.03$ , indicating subsonic turbulence.
Bulk Velocity: A finite line-of-sight bulk velocity of $v_{\text{bulk}} = 263 \pm 23$ km s $^{-1}$ (relative to the BCG) is detected. Even when referenced to the median redshift of the galaxy population, a significant bulk motion of $117 \pm 23$ km s $^{-1}$ remains.
Non-Thermal Pressure: The ratio of turbulent pressure to thermal pressure is $P_{\text{turb}}/P_{\text{th}} \approx 0.02$ . Including the bulk motion, the total non-thermal pressure support is $P_{\text{nth,tot}}/P_{\text{th}} \approx 0.05$ , indicating that gas motions are dynamically subdominant and the core remains close to hydrostatic equilibrium.
Optical Comparison: Optical spectroscopy confirms distinct redshift distributions for A3395S, A3395N, and A3391, with A3395S and A3395N showing a line-of-sight velocity difference of $\sim 830$ km s $^{-1}$ .

Significance and Interpretation
The paper claims that the coexistence of low turbulence ( $M_{\text{turb}} \sim 0.2$ ) and significant bulk motion suggests that A3395S has not yet reached a dynamically relaxed state. The results imply that the system is in an early-to-intermediate merger stage where large-scale coherent gas motions (driven by accretion along the filament and the interaction with A3395N) are present, but the turbulent cascade has not yet fully developed to convert this kinetic energy into small-scale, volume-filling turbulence.

This interpretation is supported by the non-detection of a radio halo in A3395S, which is typically associated with turbulent particle reacceleration during major mergers. The authors note that while a central FR I radio galaxy (S1) exists, its feedback appears relatively gentle, with no clear evidence of strong shocks or cavities that would drive the high turbulence levels seen in other merging cores.

The study demonstrates that XRISM/Resolve provides a powerful means to directly constrain ICM dynamics in merging clusters. The authors emphasize that the measured velocity dispersion is an emissivity-weighted estimate on scales of $\sim 100$ kpc; thus, the low turbulence may reflect the specific spatial scale probed rather than the absence of all gas motions. The paper concludes that these findings provide a reference for future comparative studies of the A3395N and A3391 components within the same large-scale structure to better understand the roles of mergers and AGN feedback in driving gas motions.