Using SCUBA to place upper limits on arcsecond‐scale cosmic microwave background anisotropies at 850 μ m

The SCUBA instrument on the James Clerk Maxwell Telescope has already had an impact on cosmology by detecting relatively large numbers of dusty galaxies at high redshift. Apart from identifying well‐detected sources, such data can also be mined for information about fainter sources and their correlations, as revealed through low‐level fluctuations in SCUBA maps. As a first step in this direction, we analyse a small SCUBA data set as if it were obtained from a cosmic microwave background (CMB) differencing experiment. This enables us to place limits on CMB anisotropy at 850 μm. Expressed as Q flat , the quadrupole expectation value for a flat power spectrum, the limit is 152 μK at 95 per cent confidence, corresponding to (or Δ T T <14×10 −5 ) for a Gaussian autocorrelation function, with a coherence angle of about 20–25 arcsec. These results could easily be reinterpreted in terms of any other fluctuating sky signal. This is currently the best limit for these scales at high frequency, and comparable to limits at similar angular scales in the radio. Even with such a modest data set, it is possible to put a constraint on the slope of the SCUBA counts at the faint end, since even randomly distributed sources would lead to fluctuations. Future analysis of sky correlations in more extensive data sets ought to yield detections, and hence additional information on source counts and clustering.