The Anisotropy of the Microwave Background tol= 3500: Mosaic Observations with the Cosmic Background Imager

Using the Cosmic Background Imager, a 13-element interferometer arrayoperating in the 26-36 GHz frequency band, we have observed 40 sq deg of sky inthree pairs of fields, each ~ 145 x 165 arcmin, using overlapping pointings(mosaicing). We present images and power spectra of the cosmic microwavebackground radiation in these mosaic fields. We remove ground radiation andother low-level contaminating signals by differencing matched observations ofthe fields in each pair. The primary foreground contamination is due to pointsources (radio galaxies and quasars). We have subtracted the strongest sourcesfrom the data using higher-resolution measurements, and we have projected outthe response to other sources of known position in the power-spectrum analysis.The images show features on scales ~ 6 - 15 arcmin, corresponding to masses ~(5 - 80)*10^{14} Msun at the surface of last scattering, which are likely to bethe seeds of clusters of galaxies. The power spectrum estimates have aresolution Delta-l = 200 and are consistent with earlier results in themultipole range l <~ 1000. The power spectrum is detected with highsignal-to-noise ratio in the range 300 <~ l <~ 1700. For 1700 <~ l <~ 3000 theobservations are consistent with the results from more sensitive CBI deep-fieldobservations. The results agree with the extrapolation of cosmological modelsfitted to observations at lower l, and show the predicted drop at high l (the"damping tail").