2012ApJ...749..114Z

The Sunyaev-Zel'dovich (SZ) effect spectrum crosses through a null where ΔT_CMB= 0 near ν₀= 217 GHz. 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 J1347.5-1145 with Z-Spec, an R ∼ 300 grating spectrometer sensitive between 185 and 305 GHz. 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±5.3 keV 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= 0 km/s, we measure the best-fitting SZ model to be y₀= 4.6^+0.6_- 0.9_x10^–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±810 km/s is found.