Limits on the peculiar velocities of two distant clusters using the kinematic Sunyaev-Zeldovich effect

We report millimeter-wavelength observations of the Sunyaev-Zeldovich (S-Z) effect in two distant galaxy clusters. A relativistically correct analysis of the S-Z data is combined with the results of X-ray observations to determine the radial peculiar velocities (v_r) of the clusters. We observed Abell 2163 (z = 0.201) in three millimeter-wavelength bands centered at 2.1, 1.4, and 1.1 mm. We report a significant detection of the thermal component of the S-Z effect seen as both a decrement in the brightness of the CMB at 2.1 mm and as an increment at 1.1 mm. Including uncertainties due to the calibration of the instrument, distribution and temperature of the intracluster gas, and astrophysical confusion, a simultaneous fit to the data in all three bands gives v_r = +490_-880⁺¹³⁷⁰ km s^-1 at 68% confidence. We observed Abell 1689 (z = 0.181) in the 2.1 and 1.4 mm bands. Including the same detailed accounting of uncertainty, a simultaneous fit to the data in both bands gives v_r = +170_-630⁺⁸¹⁵ km s^-1. The limits on the peculiar velocities of A2163 and A1689 correspond to deviations from the uniform Hubble flow of ≲2%-3%.