A Preliminary Detection of Arcminute-Scale Cosmic Microwave Background Anisotropy with the BIMA Array

We have used the Berkeley-Illinois-Maryland Association (BIMA) array outfitted with low-noise millimeter-wave receivers to expand our search for arcminute-scale anisotropy of the cosmic microwave background (CMB). The interferometer was placed in a compact configuration for sensitivity on arcminute scales over its 6.6‧ FWHM field of view. This experiment is most sensitive to flat band power described by an average multipole of l=5530, corresponding to an angular scale of ~2'. We present the analysis of 11 independent fields that were selected based on their low contrast in infrared dust emission and lack of bright radio sources. Applying a Bayesian analysis to the visibility data, we find CMB anisotropy flat band power Q_flat=6.1^+2.8_-4.8 μK at 68% confidence. The measured signal exceeds that expected from instrument noise with 76% confidence, and we find an upper limit of Q_flat<12.4 μK at 95% confidence. We have supplemented our BIMA observations 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.