High spectral resolution measurement of the Sunyaev-Zel'dovich effect null with Z-Spec

The Sunyaev-Zel'dovich (SZ) effect spectrum crosses through a null where ΔT_CMB = 0 near ν₀ = 217GHz. In a cluster of galaxies, ν₀ can be shifted from the canonical thermal SZ effect value by corrections to the SZ effect scattering due to the properties of the inter-cluster medium. We have measured the SZ effect in the hot galaxy cluster RX J 1347.5 - 1145 with Z-Spec, an R ∼ 300 grating spectrometer sensitive between 185 and 305GHz. These data comprise a high spectral resolution measurement around the null of the SZ effect and clearly exhibit the transition from negative to positive ΔT_CMB over the Z-Spec band. The SZ null position is measured to be ν₀ = 225.8 ± 2.5(stat.) ± 1.2(sys.)GHz, which differs from the canonical null frequency by 3.0σ and is evidence for modifications to the canonical thermal SZ effect shape. Assuming the measured shift in ν₀ is due only to relativistic corrections to the SZ spectrum, we place the limit kT_e = 17.1 ± 5.3keV from the zero-point measurement alone. By simulating the response of the instrument to the sky, we are able to generate likelihood functions in {y₀, T_e, v_pec} space. For v _pec = 0kms^-1, we measure the best-fitting SZ model to be y₀ = 4.6^+0.6_{- 0.9} × 10^-4, T_{e, 0} = 15.2⁺¹²_{- 7.4}keV. When v_pec is allowed to vary, a most probable value of v_pec = + 450 ± 810kms^-1 is found.