Use of the Sunyaev-Zeldovich effect as a precise cosmological probe necessitates a realistic assessment of all possible contributions to Comptonization of the cosmic microwave background in clusters of galaxies. We have calculated the additional intensity change due to various possible populations of energetic electrons that have been proposed in order to account for measurements of intracluster radio, nonthermal X-ray, and (possibly also) EUV emission. Our properly normalized estimates of (the highly model-dependent value of) the predicted intensity change due to these electrons are well below ~6% and ~35% of the usual Sunyaev-Zeldovich effect as a result of electrons in the hot gas in Coma and A2199, respectively. These levels constitute high upper limits since they are based on energetic electron populations whose energy densities are comparable to those of the thermal gas. The main impact of nonthermal Comptonization is a shift of the crossover frequency (where the thermal effect vanishes) to higher values. Such a shift would have important consequences for our ability to measure cluster peculiar velocities from the kinematic component of the Sunyaev-Zeldovich effect.
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