A SEARCH FOR ARCMIN-SCALE ANISOTROPY IN THE COSMIC MICROWAVE BACKGROUND

The Australia Telescope antenna array has been used in an ultracompact configuration to image an almost-equal-to 8 arcmin diameter field with an angular resolution of about 2 arcmin and an rms brightness sensitivity of about 36 muK. Interferometer spacings which responded to an independent and higher range of spatial frequencies were used to model and subtract the confusion due to the discrete radio sources in the field with smaller angular sizes. As a result, the sensitivity in the imaging was reduced well below the telescope confusion limit. The residual image was examined for fluctuations above the instrument thermal noise that could be due to any arcmin-scale anisotropy in the cosmic microwave background. No excess fluctuations were detected, and we derive a 95 per cent confidence upper limit of 22 muJy beam-1 on the rms flux density fluctuations in the background sky at 8.7 GHz with an angular scale of 2 arcmin. Assuming a Gaussian form for the autocorrelation function of the CMB fluctuations, we obtain a 95 per cent confidence limit of DELTAT/T < 2 x 10(-5) on the rms fractional temperature fluctuations in a background with a coherence scale of 1 arcmin.