An upper limit to arcminute-scale anisotropy in the cosmic microwave background radiation at 142 GHz

We present limits to anisotropies in the cosmic microwave background radiation (CMB) at angular scales of a few arcminutes. The observations were made at a frequency of 142 GHz using a six-element bolometer array (the Sunyaev-Zel'dovich Infrared Experiment) at the Caltech Submillimeter Observatory. Two patches of sky, each approximately 36′ × 4′ and free of known sources, were observed for a total of 6-8 hours each, resulting in approximately 80 independent 1′.7 full width half-maximum pixels. Each pixel is observed with both a dual-beam and a triple-beam chop, with a sensitivity per pixel of 90-150 μK in each chop. These data have been analyzed using maximum-likelihood techniques by assuming a Gaussian autocorrelation function for the distribution of CMB fluctuations on the sky. We set an upper limit of ΔT/T ≤ 2.1 × 10^-5 (95% confidence) for a coherence angle to the fluctuations of 1′.1. These limits are comparable to the best limits obtained from centimeter-wavelength observations on similar angular scales but have the advantage that the contribution from known point sources is negligible at these frequencies. They are the most sensitive millimeter-wavelength limits for coherence angles ≤ 3′. The results are also considered in the context of secondary sources of anisotropy, specifically the Sunyaev-Zel'dovich effect from galaxy clusters.