We have investigated the implications of the recent SCUBA number counts for the Planck Surveyor. Since it observes at the same frequency as one of the main science channels on Planck, SCUBA can provide constraints on the point-source contribution to the CMB angular power spectrum which require no extrapolation in frequency. While previous authors have investigated mainly one-point statistics of several SCUBA fields, it is the two-point function which has the most impact on the CMB science which will be done with Planck. We have calculated the two-point function of point-sources, using a Poisson model normalized to the observed counts. While the current data are uncertain, under reasonable assumptions the point-source contribution to the anisotropy is comparable to the instrumental noise in the 353 GHz channel. We have emphasized that if the instrumental noise power spectrum can be accurately estimated (as is expected to be the case for Planck, since each pixel is observed multiple times), then even a white noise contribution from point sources could still be detected. The clustering of these sources is extremely uncertain, however if they cluster like the Lyman-break galaxies their signal would be larger than the primary anisotropy signal on scales smaller than about 10 arcminutes. We expect the intensity of these sources to decrease for wavelengths longward of m. At the next lowest frequency channel, 217 GHz, the contribution from both the clustered and Poisson terms is dramatically reduced.
The bottom-line is that the sub-mm sources revealed by SCUBA will not have a strong impact on the most important goal of the Planck mission, that of precisely characterising the CMB anisotropy. For the entire Low Frequency Instrument, and the three lowest frequency channels of the HFI there will be no significant contribution. And certainly the signals in the higher frequency channels can be used to remove point sources and recover most of the CMB information even at 353 GHz. Moreover, the possibility of actually measuring the Poisson and clustering signals over most of the sky for these galaxies provides Planck with yet another way of tackling fundamental cosmological issues.
© European Southern Observatory (ESO) 1999
Online publication: May 6, 1999