Kinetic Sunyaev-Zel'dovich effect from halo rotation

Abstract

We discuss the kinetic Sunyaev-Zel'dovich (SZ) contribution to cosmic microwave background (CMB) temperature fluctuations due to coherent rotational velocity component of electrons within halos. This effect produces a distinct dipole-like temperature distribution across the the cluster, and provides a promising way to measure the angular momentum distribution of gas inside clusters. Information obtained from such a measurement may provide new insights to the origin and evolution of angular momentum in hierarchical structure formation theory. For typical, well relaxed, clusters of mass a few times 10$^{14}$ M$_{\sun}$, the peak fluctuation is of the order a few $μ$K, depending on the rotational velocity and the inclination angle of the rotational axis. For clusters which had underwent a recent merger, the contribution to temperature fluctuations could be even larger. This dipole signature is similar to the one produced by lensed CMB towards galaxy clusters, though the lensing contribution spans a larger angular extent than the one due to rotational scattering as the former depends on the gradient of the cluster potential. Since the lensing contribution towards clusters are aligned with the large scale CMB gradient, when higher resolution observations towards clusters are combined with a wide field CMB map, these two effects can be separated. An additional, but less important, source of confusion is the dipolar pattern produced by the moving-lens effect involving, again, the gradient of the cluster potential and the transverse velocity. The angular power spectrum of temperature anisotropies produced by the halo rotation is expected to be smaller than those due to the thermal SZ and peculiar velocity kinetic SZ effects.