Mapping cluster angular momentum with the rotational kinetic Sunyaev-Zel'dovich effect

Abstract

Galaxy clusters carry angular momentum. If the intracluster gas rotates coherently, the kinetic Sunyaev-Zel'dovich (kSZ) effect imprints a dipole on the microwave sky centered on the cluster. We show that microwave maps alone can be used to reconstruct a noisy vector field tracing the projected angular momentum of the ionized gas. We present an optimal multifrequency matched filter (MMF) that isolates the rotational kSZ (rkSZ) signal from the much larger miscentered thermal Sunyaev-Zel'dovich (tSZ) effect, cosmic microwave, and infrared background contributions, improving on both real-space oriented stacking and matched filtering of maps cleaned with full-sky multifrequency weights. For the rkSZ profile, we introduce a simple model for coherent intracluster gas rotation and validate it against TNG-Cluster and TNG300-1. The model consists of differentially rotating concentric shells with constant equatorial tangential velocity, characterized by a mass- and redshift-indepedent tangential-to-circular velocity ratio of $v_{\rm tan}/V_{c,200c}=0.18$. Using this model, we forecast single-cluster signal-to-noise (SNR) as a function of mass and redshift for Simons Observatory; cross-correlation with an external catalog of rotation proxies yields a cumulative detection SNR. For an idealized mass-limited catalog with perfectly known spin directions, the cumulative SNR of our optimal MMF reaches $\approx 53$, $25$, and $6$ when including all clusters above $\log_{10}(M_{200c}/M_\odot)>14$, 14.5, and 15, respectively. This work provides a practical framework for CMB measurements of intracluster gas rotation, with applications to the rotational contribution to non-thermal pressure in cluster mass calibration, as well as spin alignments and clustering biases.