A preliminary detection of arcminute-scale cosmic microwave background anisotropy with the BIMA array

We have used the Berkeley-Illinois-Maryland Association (BIMA) array outfit ted with low-noise millimeterwave receivers to expand our search for arcmin ute-scale anisotropy of the cosmic microwave background (CMB). The interfer ometer was placed in a compact configuration for sensitivity on arcminute s cales over its 6.'6 FWHM held of view. This experiment is most sensitive to hat band power described by an average multipole of l = 5530, correspondin g to an angular scale of similar to2'. We present the analysis of 11 indepe ndent fields that were selected based on their low contrast in infrared dus t emission and lack of bright radio sources. Applying a Bayesian analysis t o the visibility data, we find CMB anisotropy flat band power Q(flat) = 6.1 (-4.8)(+2.8), muK at 68% confidence. The measured signal exceeds that expec ted from instrument noise with 76% confidence, and we find an upper limit o f Q(flat) < 12.4 <mu>K at 95% confidence. We have supplemented our BIMA obs ervations with concurrent observations at 4.8 GHz with the Very Large Array to search for and remove point sources. We find that point sources make an insignificant contribution to the observed anisotropy.