We report the first robust evidence for the rotational kinematic Sunyaev-Zel’dovich (rkSZ) effect: the dipolar signal produced by the Thomson scattering of cosmic microwave background (CMB) photons off intracluster gas in dynamically active systems (including rotations and mergers). By combining CMB intensity and polarization measurements from the Planck satellite with spectroscopic member-galaxy redshifts from the Sloan Digital Sky Survey in a sample of 25 x-ray cross-matched, low-redshift ($0.02<z<0.09$), massive (${10}^{13.9}\lesssim M_{500\mathrm{c}}/M_{\odot }\lesssim {10}^{14.6}$) galaxy clusters, we detect a dipolar rkSZ signature aligned with the estimated angular momentum of each cluster, ruling out a chance fluctuation at 99.98% confidence ($3.6\sigma$). The significance of this measurement is enhanced by several new methodological improvements for isolating the rkSZ signal from primary CMB fluctuations and noise. The amplitude and shape of the signal are qualitatively consistent with predictions from state-of-the-art hydrodynamical simulations. These results establish a new tool with which to probe the dynamical state of galaxy clusters using CMB data.

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• Cosmic microwave background
• Cosmology
• Large scale structure of the Universe
• Galaxy clusters