Predicting Herschel Extragalactic Confusion Limits Using Fluctuations and Source Counts Analysis

We discuss the confusion limits due to extragalactic sources to be expected in Herschel (but also Spitzer MIPS and SCUBA) broad-band photometric channels on the basis of models of extragalactic source populations by Franceschini et al. 2007 (in prep) fitting all available data (number counts, z -distributions, L -functions and CIRB levels) from ISO, SCUBA and Spitzer. Models allow to predict source number counts and redshift distributions of sources detected down to different flux limits as well as, crucially, confusion limits as a function of wavelength and beam size. We present and compare predictions of the confusion limits based both on a complete and self-consistent analysis of the cell-to-cell fluctuations due to unresolved sources and on the complementary criterion of a maximum number of resolved sources per beam. We finally discuss implications for the planning of Herschel surveys.