Searching for the relativistic Sunyaev-Zeldovich effect with the PLANCK experiment

Populations of relativistic electrons in clusters of galaxies should imprint a characteristic spectral distortion signature on the Cosmic Microwave Background (CMB) photons – the relativistic Sunyaev-Zeldovich (SZ) effect. We investigate how sensitive PLANCK and successor experiments will be in detecting or constraining the total optical depth τrel of relativistic electrons. We find an expected sensitivity of PLANCK of στrel ∼ 2 · 10 −5 for an individual cluster, which is too insensitive to detect even optimistic scenarios, which predict τrel ∼ 10 −6 to 10. However, by stacking the PLANCK-signal of all SZ detectable clusters the relativistic SZ effect can be statistically probed to a sensitivity level of στrel ∼ 10 . PLANCK successor experiments will be able to probe even conservative scenarios of relativistic electron populations in clusters of galaxies.