🔭 astrophysics

XRISM reveals sloshing-driven gas motions in the core of Abell 2029

Using XRISM Resolve spectroscopy, this study reveals that the relaxed cool-core cluster Abell 2029 exhibits ordered gas sloshing motions with a bulk velocity gradient of approximately 280 km/s, while its low turbulent velocity dispersion implies that such motions provide negligible pressure support and insufficient heating to offset radiative cooling in the core.

Original authors: Yuusuke Uchida, Yuna Saito, Naomi Ota, Erwin T. Lau, Eric D. Miller, Tommaso Bartalesi, Stefano Ettori, Kotaro Fukushima, Caroline Kilbourne, Lorenzo Lovisari, Kyoko Matsushita, Brian R. McNamara, Arn

Published 2026-08-17

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CC BY 4.0

Original authors: Yuusuke Uchida, Yuna Saito, Naomi Ota, Erwin T. Lau, Eric D. Miller, Tommaso Bartalesi, Stefano Ettori, Kotaro Fukushima, Caroline Kilbourne, Lorenzo Lovisari, Kyoko Matsushita, Brian R. McNamara, Arnab Sarkar, Kazunori Suda, Irina Zhuravleva

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 ocean. But instead of water, this ocean is filled with super-hot gas called the "intracluster medium" (ICM). This gas is trapped inside massive groups of galaxies known as "clusters," held together by the invisible hand of gravity. For a long time, astronomers thought this cosmic gas was mostly calm, sitting still like a frozen lake, slowly cooling down and glowing in X-rays. However, they suspected that if you looked closely enough, you might find it churning, sloshing, or swirling, much like water in a bathtub after you jump out.

The big question is: Is this gas just sitting there, or is it moving? If it's moving, how fast? And does that movement create enough energy to keep the gas from cooling down too fast? To answer this, scientists need a special kind of telescope that can act like a high-speed camera for sound. They need to measure the speed of the gas atoms by looking at how their light shifts—a bit like how a siren's pitch changes as an ambulance zooms past you. This is the job of a new, incredibly sharp instrument called XRISM, which is designed to listen to the "voice" of the universe with unprecedented clarity.

The Story of the Sloshing Galaxy Cluster

In this new study, a team of astronomers used the XRISM telescope to take a close-up look at a galaxy cluster called Abell 2029. Think of Abell 2029 as a giant, cosmic bowl filled with hot gas. Previous pictures from other telescopes showed that this gas wasn't perfectly still; it had a giant, spiral swirl in it, looking a bit like the pattern you'd see if you stirred a cup of coffee and then stopped. Scientists guessed this swirl was caused by a "sloshing" motion—like when you carry a bowl of soup and accidentally bump it, causing the liquid to rock back and forth. But until now, no one had actually measured the speed of this rocking motion to prove it was real.

The researchers decided to split the center of this cosmic bowl into six different slices, like cutting a pizza. They wanted to see if the gas in the northern slices was moving differently than the gas in the southern slices. Because the telescope's view is a little fuzzy (like a slightly out-of-focus camera), they had to use some clever math to untangle the light from one slice mixing with the light from its neighbors.

What They Found

The results were exciting. The team found that the gas really was moving in an organized way, just like a gentle wave. The gas in the northern part of the cluster was rushing toward us (blueshifted) at speeds up to about 190 kilometers per second, while the gas in the southern part was either standing still or moving slightly away (redshifted) at about 90 kilometers per second. When you compare the fastest northern gas to the fastest southern gas, there is a speed difference of about 280 km s⁻¹.

This confirms that the spiral swirl seen in earlier pictures is indeed a giant "sloshing" motion, likely caused by a smaller galaxy cluster bumping into Abell 2029 billions of years ago. It's like seeing the ripples in a pond and finally measuring the speed of the water to prove a stone was dropped there.

The "Quiet" Surprise

Here is the twist: even though the gas is sloshing, it isn't crashing or churning violently. The team measured how much the gas was jiggling randomly (turbulence) and found it was surprisingly calm. The random jiggling speed was less than 150 km s⁻¹. To put that in perspective, the gas is moving much slower than the speed of sound in that hot environment. It's like a gentle breeze rather than a hurricane.

Because the gas isn't moving wildly, it doesn't create much extra pressure to hold up the cluster. The "non-thermal pressure" (the extra push from movement) is tiny, less than 2.5% of the total pressure. This means the cluster is still mostly in a state of calm balance, known as hydrostatic equilibrium, despite the sloshing.

Does the Movement Heat the Gas?

Finally, the team asked: Does this sloshing create enough heat to stop the gas from cooling down? They calculated how much energy the turbulence could release and compared it to how much energy the gas is losing by glowing in X-rays. They found that the heating from the sloshing is too weak to stop the cooling. In the center of the cluster, the heating rate is only about 0.2 times the cooling rate. It's like trying to keep a campfire going by blowing on it gently with a straw; it just isn't enough to stop the fire from dying out.

The Takeaway

This paper tells us that Abell 2029 is a fascinating mix of motion and calm. It has a giant, organized sloshing motion that creates a beautiful spiral pattern, proving that the cluster is not completely frozen in time. However, this motion is gentle and subsonic, meaning it doesn't create enough chaos or heat to fully explain why the gas hasn't cooled down completely. The universe, it seems, is full of gentle waves that are just as interesting as the violent storms.

Technical Summary: XRISM Reveals Sloshing-Driven Gas Motions in the Core of Abell 2029

Problem and Motivation
The dynamical state of the intracluster medium (ICM) in galaxy clusters provides critical insights into cluster assembly, energy transport, and the balance between heating and cooling. While recent high-resolution X-ray spectroscopy (e.g., Hitomi, XRISM) has established that the hot gas in many clusters, including the relaxed cool-core cluster Abell 2029 (A2029), exhibits small velocity dispersions and is largely in hydrostatic equilibrium, deep Chandra observations reveal a prominent, large-scale spiral surface brightness structure in A2029. This spiral is widely interpreted as the signature of gas sloshing induced by a past off-axis minor merger. However, previous X-ray observatories lacked the spectral resolution to directly measure the associated velocity fields. A key question remains: does the presence of such large-scale sloshing structures imply significant non-thermal pressure support or bulk motions that challenge the hydrostatic equilibrium assumption, or can these features coexist with a quiescent, subsonic ICM?

Methodology
This study utilizes spatially resolved spectroscopy from the XRISM Resolve microcalorimeter to map the gas kinematics in the core of A2029. The analysis combines performance verification (PV) and General Observer (GO) observations, totaling significant exposure time on the central region and a northern offset pointing.

To address the challenge of photon mixing caused by the XRISM point spread function (PSF), which is comparable to the field of view of the Resolve instrument, the authors employ a spatial-spectral mixing (SSM) analysis. The cluster core is subdivided into six distinct regions: Center, North East (NE), North West (NW), South East (SE), South West (SW), and a Far North East (FNE) region derived from the offset pointing.

Spectral fitting is performed using XSPEC with the bapec model (thermal broadening with velocity) and TBabs for Galactic absorption. The SSM procedure involves generating Ancillary Response Files (ARFs) for all combinations of the six source regions and extraction regions to account for the fraction of photons detected in one region originating from another. This allows for the simultaneous fitting of all spectra to disentangle the true kinematic properties of each subregion. The analysis focuses on the energy band above 2 keV to minimize calibration uncertainties.

Key Results

Ordered Bulk Velocity Gradient: The study detects a coherent line-of-sight bulk velocity gradient across the cluster core. Northern regions (NE, NW, and FNE) are blueshifted relative to the Brightest Cluster Galaxy (BCG), while southern regions (SE and SW) are consistent with zero velocity or slightly redshifted.
- The FNE region shows the largest blueshift ( $\approx -190$ km s $^{-1}$ ), and the SE region shows a redshift of $\approx +90$ km s $^{-1}$ .
- The maximum velocity difference between opposite sides of the core is approximately $280$ km s $^{-1}$ .
- The north-south velocity gradient is robust, with a significance of $11.5\sigma$ ( $8.8\sigma$ when including systematic energy-scale uncertainties).
Low Velocity Dispersion: In contrast to the bulk motions, the turbulent velocity dispersion ( $\sigma_v$ ) is small across all regions. Measured values and upper limits are generally below $\sim 150$ km s $^{-1}$ .
- This corresponds to a three-dimensional turbulent Mach number ( $M_{3D}$ ) of $\sim 0.1-0.2$ .
- The non-thermal pressure fraction derived from turbulence alone ( $\alpha_{turb}$ ) is below $\sim 2.5\%$ in all regions. Even when including the ordered bulk velocity in the effective pressure calculation, the total non-thermal pressure fraction remains below $\sim 2.5\%$ .
Heating vs. Cooling: The authors estimate the turbulent heating rate ( $Q_{heat}$ ) using the measured velocity dispersion and compare it to the radiative cooling rate ( $Q_{cool}$ ) derived from the X-COP electron density profile.
- The estimated turbulent heating rate is lower than the radiative cooling rate in all regions (ratios $Q_{heat}/Q_{cool} \approx 0.1-0.2$ ).
- This suggests that turbulent dissipation alone is insufficient to offset radiative cooling in the entire core, although gas motions may still contribute to thermal redistribution via mixing.

Significance and Conclusions
The paper claims that these results demonstrate that A2029 is not kinematically featureless, despite its reputation as a relaxed cluster. The detection of a coherent, subsonic bulk velocity gradient provides direct spectroscopic evidence supporting the interpretation that the large-scale spiral structure seen in Chandra images is driven by gas sloshing.

Crucially, the study establishes that sloshing-induced bulk motions can exist without generating significant turbulence or non-thermal pressure support. The low velocity dispersion implies that the hydrostatic mass bias in the core of A2029 due to gas motions is small (a few percent), reinforcing the validity of the hydrostatic equilibrium approximation for this system. The findings highlight that while sloshing drives ordered flows, it does not necessarily compromise the dynamical quiescence of the ICM, offering a nuanced view of energy transport in cool-core clusters.

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