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

summary

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

In short

Researchers used the kinetic Sunyaev-Zeldovich (kSZ) effect to measure gas rotation in galaxy groups using CMB data from SDSS groups. They found a tentative 2.3$\sigma$ detection of the dipolar rotational kSZ effect (rkSZ), estimating an average rotational velocity of about 100–200 km s$^{-1}$ within the group virial radii. This result is consistent with simulations, suggesting gas rotation in these structures.

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.

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This episode discusses

The paper

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

Tianyi Yang, Yan-Chuan Cai, Weiguang Cui, John A. Peacock, Romeel Davé, Houjun Mo, Huiyuan Wang, Xiaohu Yang

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

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.

Transcript

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 >

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