A possible detection of gas rotation in galaxy groups via the kinetic Sunyaev-Zeldovich effect

arXiv:2603.14494 · astro-ph.CO, astro-ph.GA · Submitted 2026-03-15 · Read on arXiv

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Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: I'm Vera, and with me are Jocelyn and Subrahmanyan, guest researcher.

Jocelyn: Today's paper: "A possible detection of gas rotation in galaxy groups via the kinetic Sunyaev-Zeldovich effect".

Vera: The gist: The kinetic Sunyaev-Zeldovich effect (kSZ) is utilized to measure gas rotation in galaxy groups by detecting its dipolar imprint on the cosmic microwave background (CMB),

Jocelyn: First, who's behind it and why it matters.

Paper summary: Vera: So, looking at this paper now, "A possible detection of gas rotation in galaxy groups via the kinetic Sunyaev-Zeldovich effect," the core idea is testing general relativity by looking for a frame-dragging effect using the kinetic Sunyaev-Zeldovich effect >

Jocelyn: The thesis is that non-zero gas rotational velocities will lead to incorrect cluster mass estimates because we usually assume everything is in hydrodynamic equilibrium, and this paper tries to find that rotation signal >

Subrahmanyan: They set up a comparison using the ELUCID constrained realisation simulation against the SDSS-DR7 group sample catalogue, which is key because they use both observational data and simulations together >

Vera: The methodology involves first figuring out the direction of angular momentum by maximizing the redshift dipole around each group center, then they perform oriented stacking on the Planck CMB temperature map >

Jocelyn: They are using a method similar to what Manolopoulou and Plionis did before to determine that initial direction of angular momentum >

Subrahmanyan: The ELUCID simulation itself is quite detailed, evolving thirty thousand seven hundred twenty-three dark matter particles with masses of three point zero nine times one hundred eight h-1M in a volume of five hundred h-1Mpc on a side > <ref:2603.14494#pg3>

Vera: They also used the Hamiltonian Markov Chain Monte Carlo method to reconstruct the initial density field from that simulation, starting from an initial redshift of z ini = one hundred under the WMAP5 cosmology > <ref:2603.14494#pg3,the Hamiltonian Markov Chain Monte Carlo method>

Jocelyn: The result they found is a clear redshift dipole in their stacked data for the selected SDSS groups, and this leads to their tentative detection of the dipolar rotational kSZ effect, or rkSZ >

Subrahmanyan: The specific number they report for that signal is two point three sigma, which they say corresponds to a null-hypothesis probability of zero point nine six percent, which is pretty solid for a tentative finding >

Vera: They also estimate the averaged rotational velocity of the sample to be roughly one hundred to two hundred km per second, with a peak value of one hundred fifty-eight point eight plus or minus13 point 6 km s-one >

Jocelyn: It’s important that they acknowledge their limitations too; they found that the predicted amplitude of the rkSZ signal in the simulation is slightly lower near the center than what they actually measured >

Conclusion: Vera: So, to wrap up this paper, "A possible detection of gas rotation in galaxy groups via the kinetic Sunyaev-Zeldovich effect," it’s a study that uses observational data and simulation to probe the physics of structure formation >

Jocelyn: It's essentially saying that if we can measure this gas rotation, we get a direct way to check predictions about angular momentum transfer within these gravitationally bound systems >

Subrahmanyan: And for the bigger picture, it helps us understand the spin that dark matter haloes acquire through tidal torques, connecting it back to earlier work on structure formation models >

Vera: The implication is that measuring this rotational effect could serve as a test of general relativity through the gravitomagnetic effect >

Jocelyn: It's a tentative detection, so we need more data to confirm if that two point three sigma signal holds up when we collect more observations from groups like these > <ref:2603.14494#pg3>

Subrahmanyan: The paper shows how important it is to build a clear understanding of the physical picture for the rotation of clusters and groups because non-zero gas rotational velocities would otherwise lead to incorrect cluster mass estimates that affect cosmological constraints >

Vera: So, in simple terms, they’ve found a hint of spin in galaxy groups using CMB data, suggesting there's motion we need to account for when we calculate their masses >

Astrophysics Research Institute, Liverpool John Moores University of Liverpool, Institute for Astronomy, University of Edinburgh, Departamento de Física Teórica, Universidad Autónoma de Madrid, Centro de Investigación Avanzada en Física Fundamental (CIAFF), Department of Astronomy, University of Massachusetts

astro-ph.CO, astro-ph.GA

Submitted: 2026-03-15

Updated: 2026-10-09

Comments: 12 pages, 5 figures. Accepted by MNRAS

License: http://creativecommons.org/licenses/by/4.0/

Importance score: 71/100

The gist: The gist: The kinetic Sunyaev-Zeldovich effect (kSZ) is utilized to measure gas rotation in galaxy groups by detecting its dipolar imprint on the cosmic microwave background (CMB), yielding an

Key concepts

Kinetic Sunyaev-Zeldovich Effect (kSZ)
The kSZ effect measures the Doppler shift caused by the bulk motion of ionized gas as it moves relative to the Cosmic Microwave Background (CMB). This motion creates a specific temperature distortion on the CMB photons, which scientists use to probe the kinetic energy and velocity of gas within galaxy groups.
Rotational kSZ Effect (rkSZ)
The rkSZ effect is a specific dipolar signature imprinted on the CMB caused by the coherent rotation of ionized gas within a structure like a galaxy group. By stacking CMB maps oriented according to the estimated rotation, researchers look for this unique pattern to confirm gas rotation.
Redshift Dipole
A redshift dipole is a statistical pattern observed in galaxy data where the measured redshifts of satellite galaxies around a group center show an asymmetry across different directions on the sky. This asymmetry helps determine the direction of angular momentum, which is crucial for orienting the CMB maps correctly.
ELUCID Simulation
The ELUCID constrained realization simulation is a computational model used to predict how dark matter haloes and their associated gas motions should manifest in observable signals like the rkSZ effect. Comparing the observed signal to these simulations helps validate the physical interpretation of the measurements.

Terminology

Summary

The gist: The kinetic Sunyaev-Zeldovich effect (kSZ) is utilized to measure gas rotation in galaxy groups by detecting its dipolar imprint on the cosmic microwave background (CMB), yielding an estimated rotational velocity of approximately 100–200 km s−1 within the virial radii of these groups.

Introduction and Motivation

Galaxy clusters and groups are thought to be the largest gravitationally bound structures in the Universe, formed through hierarchical merging over cosmic time <ref:2603.14494#pg2>. Understanding the rotational motions of clusters and groups provides unique constraints on models for their formation, as well as the transfer of angular momentum between baryonic and dark matter <ref:2603.14494#pg3>. The rotational motions can serve as a test of general relativity through the prediction of a frame-dragging effect called the gravitomagnetic effect <ref:2603.14494#pg3>. Non-zero gas rotational velocities will also lead to incorrect cluster mass estimates, as these commonly assume hydrodynamic equilibrium <ref:2603.14494#pg3>.

Methodology and Data

The measurement requires three pieces of information: a catalogue of groups with ionised gas, the projected angular momentum on the sky for each group, and a CMB temperature map <ref:2603.14494#pg4>. The study employs the catalogue of galaxy clusters and groups derived from the SDSS-DR7 galaxy sample <ref:2603.14494#pg4>. A key step involves determining the direction of angular momentum using a method where the axis of rotation is determined by the direction that maximises the amplitude of the redshift dipole <ref:2603.14494#pg6>. This is done by measuring the observed redshifts of satellite galaxies around each centre of a group <ref:2603.14494#pg6>.

Sample Selection and Matching

The selection process involves matching observed groups to simulated haloes from the ELUCID constrained realisation simulation <ref:2603.14494#pg4>. To ensure accurate matching, a mass threshold of Mth of 1013.5 h−1M⊙ is applied to the observed list of groups <ref:2603.14494#pg8>. The orientation of angular momentum for each object is determined by fitting a model sinusoidal rotation curve to the measured velocity pattern, and the axis that maximizes this amplitude is taken as the orientation <ref:2603.14494#pg6>. A sample of 134 matched fast-rotating systems was ultimately selected from both simulations and observations <ref:2603.14494#pg8>.

Signal Modeling and Measurement

The expected rkSZ signal is modeled using the dark matter particle field around the selected haloes in the simulation, with the amplitude given by an integral of electron momentum along the line of sight <ref:2603.14494#pg7>. The observed signal is quantified through oriented stacking, where the stacked map of rkSZ is weighted by an estimate of the angular momentum as follows <ref:2603.14494#pg6>. This procedure involves translating and rotating each temperature map according to the estimated direction of the projected angular momentum <ref:2603.14494#pg6>.

Results and Conclusion

The analysis detects a redshift dipole with a high significance for the selected SDSS groups, and an associated rkSZ dipole at the level of ∼ 2.3sigma <ref:2603.14494#pg7>. The tentative rkSZ signal peaks at approximately half of the average virial radius at ∼ 25 μK <ref:2603.14494#pg7>. The rotational velocity of the sample is estimated to be approximately 100–200 km s−1 <ref:2603.14494#pg8>. The discrepancy between the simulated and observed rkSZ signal suggests that the slight discrepancy between the simulated and observed rkSZ signal probably reflects an imperfect match between the simulated haloes and the observed groups <ref:2603.14494#pg9>. The results are consistent with predictions based on ELUCID constrained realisation simulation, with the predicted amplitude of the rkSZ signal being slightly lower near the centre <ref:2603.14494#pg7>.

Improvements for AI systems

  1. Bold header: Improved Gas Rotation Measurement Capability. The system can now estimate the averaged rotational velocity of the sample to be ∼ 100 − 200 km s−1 by performing oriented stacking of Planck CMB temperature maps using group centres and directions of angular momenta, enabling a measurement with a 2.3sigma measurement of the coherent rotational kSZ effect (rkSZ).

  2. Bold header: Enhanced Halo Property Matching and Selection. The system can perform one-to-one matching between observed SDSS groups and simulated haloes using a mass threshold of 1013.5 h−1M⊙, allowing for model predictions of the rotational kSZ signal tailored to the observational selection.

  3. Bold header: Robust Rotational Velocity Estimation. The system can estimate the averaged rotational velocity by fitting sinusoidal rotation curves to binned measurements and determining the axis that maximises the velocity amplitude vrot, yielding an estimate of approximately 100-200 km s−1.

  4. Bold header: Noise Characterization and Error Quantification. The system can quantify noise levels by constructing noise matrices from observational data and generating Gaussian realisations, resulting in a statistical significance measurement of ∼ 2.3sigma (or ∼ 2.7sigma) level for the rkSZ signal, consistent with the paper's finding.

  5. Bold header: Discrepancy Analysis for Baryonic Physics Inference. The improved system can compare the observed redshift dipole and simulated version of the rkSZ signal to infer physical processes, such as suggesting that the slight discrepancy between the simulated and observed rkSZ signal probably reflects an imperfect match between the simulated haloes and the observed groups.

Abstract

We utilise the kinetic Sunyaev-Zeldovich effect (kSZ) to measure the rotation of ionised gas within galaxy groups defined in the SDSS-DR7 galaxy sample, via their dipolar imprint on the cosmic microwave background (CMB). We estimate the direction of the projected angular momentum for each of our selected fast-rotating groups by measuring the redshift dipole of satellite galaxies around their group centres. We find a clear redshift dipole in the stacked data for the selected SDSS groups. We then perform oriented stacking of the Planck CMB temperature map using the group centres and directions of angular momenta. We report a 2.3σ tentative detection of the dipolar rotational kSZ effect (rkSZ) within the virial radii of our fast-rotating groups from SDSS with an average mass of 10 14h-1 M (corresponding to a null-hypothesis probability of 0.96%). We estimate the averaged rotational velocity of the sample to be about 100-200 km s-1, reaching a maximum value of 158.8 plus or minus13.6 km s-1. Our results are consistent within the errors with predictions based on the ELUCID constrained realisation simulation, with the predicted amplitude of the rkSZ signal being slightly lower near the centre. We also identify a systematic bias when estimating rotational velocities using the observed redshifts of galaxies, but find it to be subdominant for our analysis.

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