Columbia Team Finds Faint Imprint of Spinning Gas Inside Galaxy Clusters in Big Bang Light
Stacking Planck satellite maps of 25 clusters reveals a lopsided hot-and-cold pattern at 3.6 sigma, evidence that gas inside the clusters is rotating or sloshing.
Astronomers at Columbia University say they have found a faint but measurable fingerprint of gas moving inside galaxy clusters, written into the oldest light in the universe.
The work, led by Ph.D. student Samuel Goldstein and his adviser J. Colin Hill, was published in Physical Review Letters and is also posted on arXiv. It builds on earlier hints from Eric Baxter at the University of Pennsylvania and collaborators at other institutions.
The light in question is the cosmic microwave background, or CMB, which was released about 380,000 years after the Big Bang. On its way to Earth, some of those photons scatter off hot, ionized gas inside galaxy clusters, which are groups of galaxies bound together by gravity. If the gas is rotating, one side of the cluster should appear slightly hotter against the CMB and the other side slightly colder. The reason is the Doppler effect: gas moving toward Earth makes the background light look a little hotter, and gas moving away makes it look a little colder. A cluster spinning perpendicular to our line of sight should therefore show a lopsided, two-sided temperature pattern. Physicists call this the rotational kinematic Sunyaev-Zel'dovich effect, a variant of a better-known effect tied to the motion of whole clusters.
The signal from any single cluster is far too weak to see. So the team studied 25 clusters that show signs of internal motion, such as rotation or mergers. Each contained at least 50 member galaxies cataloged by the Sloan Digital Sky Survey. The researchers estimated each cluster's rotation axis from the motions of those galaxies, then cut out the surrounding patch of temperature maps from the European Space Agency's Planck satellite. They rotated the cutouts so the axes lined up and averaged them. To sharpen the result, they weighted each cluster optimally, used polarization data to reduce noise and processed the maps to suppress unrelated fluctuations.
The stacked image showed the expected dipole, with one half warmer than the other. The signal reached 3.6 sigma in statistical significance. Only about 0.02 percent of stacks placed at random positions on the sky produced a dipole as large as the measured one. The authors describe the result as evidence for a signal "consistent with gas motions" linked to cluster rotation and mergers. It falls short of the 5 sigma that physicists usually require to call a discovery.
The team is open about the limits. The measurement cannot yet separate coherent rotation from other small-scale gas motions, especially those produced when clusters collide and merge. Settling that will take better data.
Goldstein plans to repeat the search using new galaxy measurements from the Dark Energy Spectroscopic Instrument and CMB maps from the Simons Observatory. The team also hopes to apply the same techniques to other faint effects in the CMB.
Goldstein noted that most work on the kinematic effect has focused on where gas sits around galaxies and clusters. This study instead probes how the gas moves inside individual clusters and merging systems, which is tied to the momentum of ionized gas across the universe.
