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 6-element bolometer array (the Sunyaev-Zel'dovich Infrared Experiment) at the Caltech Submillimeter Observatory. Two patches of sky, each approximately 36'x4' 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 uK 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 Delta T/T <= 2.1 x 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.