Sunyaev–Zeldovich fluctuations from the first stars?

WMAP's detection of high electron-scattering optical depth τ_e suggests substantial star formation at high redshift z∼ 17 ± 5. On the other hand, the recovered σ₈∼ 0.84 ± 0.04 argues against a cluster Sunyaev–Zeldovich (SZ) origin for the observed small-scale cosmic microwave background (CMB) fluctuation excess, which generally requires σ₈∼ 1.1. Here we consider the effects of high-redshift star formation on the CMB. We derive a fairly model-independent relation between τ_e and the number of ionizing photons emitted per baryon N_γ, and use this to calibrate the amount of high-redshift supernova activity. The resulting supernova remnants Compton cool against the CMB creating a Compton y distortion y∼ few × 10⁻⁶ within observational bounds. However they also create small-scale SZ fluctuations, which could be comparable with SZ fluctuations from unresolved galaxy clusters. This raises the exciting possibility that we have already detected signatures of the first stars not just once, but twice, in the CMB.