Columbia researchers trace gas motion in galaxy clusters using CMB light

Researchers at Columbia University are using the cosmic microwave background as a flashlight to map gas moving inside massive galaxy clusters. The faint afterglow dates to roughly 380,000 years after the Big Bang and serves as a key tool for studying cosmic structure and evolution. As electrons in moving gas scatter the photons, the background’s temperature shifts due to the Doppler effect. Those shifts reveal where ionized gas resides and how fast it moves within a cluster.

The kinematic Sunyaev-Zel’dovich effect occurs when cosmic microwave background photons scatter off electrons in moving gas inside galaxy clusters and merging systems. It reveals the momentum of ionized gas, which depends on its location and velocity. That gives researchers insight into cluster dynamics beyond the gas’s distribution alone. The signal appears as a temperature shift imprinted on the background radiation by the moving gas itself.

The Columbia team detected a dipolar temperature pattern in the cosmic microwave background, providing 3.6σ evidence for the rotational kinematic Sunyaev-Zel’dovich effect. The pattern had one side of a cluster appearing slightly hotter and the other cooler, consistent with internal gas motion driven by rotation. The researchers analyzed CMB data from 25 galaxy clusters, each containing at least 50 member galaxies recorded by the Sloan Digital Sky Survey. They sought a temperature shift pattern indicative of internal gas motion within those systems, using the member galaxies to define the sample.

Samuel Goldstein and J. Colin Hill studied cosmic microwave background observations from the European Space Agency’s Planck satellite, building on earlier work by Eric Baxter and collaborators. Their research built on prior investigations of similar signals in the cosmic microwave background. The pair developed techniques to improve sensitivity, such as optimally weighting clusters and using polarization data, which they could adapt for future studies examining faint effects.

Samuel Goldstein said the project began after a meeting with his Ph.D. adviser, J. Colin Hill, about cosmic microwave background signals not yet observed. “Due to the Doppler effect, gas moving toward us makes the CMB appear slightly hotter, while gas moving away makes it slightly colder,” Samuel Goldstein said. Goldstein described the origin of the project in a conversation with his adviser. That discussion led the team to look for unseen signals in the cosmic microwave background.

Goldstein introduced a new method for analyzing cosmic microwave background data to extract signals of gas motion within galaxy clusters. The method is potentially applicable to other faint effects in the background. Future studies using data from the Dark Energy Spectroscopic Instrument and the Simons Observatory aim to further investigate gas motions in galaxy clusters using cosmic microwave background observations. Those upcoming datasets promise to improve the distinction between coherent rotation and other gas motions.

One example of other gas motion is movement caused by mergers, which future data may help separate from coherent rotation. Dark Energy Spectroscopic Instrument and Simons Observatory observations will support those investigations. The Simons Observatory will provide new data for these inquiries. These facilities will examine how gas behaves in clusters undergoing collisions.

Atacama Cosmology Telescope data tested gravitational accelerations in galaxy clusters. Atacama Cosmology Telescope data refuted Modified Newtonian Dynamics with better than 3σ certainty by testing gravitational accelerations in galaxy clusters. The study analyzed the kinematic Sunyaev-Zel’dovich effect in thousands of galaxy clusters. It concluded that their gravitational behavior aligns with general relativity and dark matter rather than MOND, based on the kinematic signal across the large cluster sample.

The External Field Effect, which accounts for external gravitational influences on internal accelerations, may challenge that refutation. Researchers acknowledged that ignoring the External Field Effect could potentially invalidate their conclusions. That leaves room for debate about the strength of the MOND test and the validity of the assumptions behind it, particularly regarding external gravitational influences.

The Sunyaev-Zel’dovich effect was first theorized in the late 1960s and observed in 1983. It involves cosmic microwave background photons colliding with high-energy electrons in hot plasma, distorting the blackbody spectrum of background radiation. Both thermal and kinematic forms arise from interactions between the photons and hot plasma in galaxy clusters. Researchers use the effect to study galaxy clusters and test gravitational theories, including Modified Newtonian Dynamics and general relativity, through distortions of the background radiation that reveal cluster properties.