How Much of the Background Would a Deeper Survey Resolve? Source Counts Below the Fermi Catalogue Threshold

The division between resolved sources and diffuse background is an instrument property, not a property of the sky. Using a broken power-law source-count distribution anchored at the Fermi Large Area Telescope catalogue threshold, this paper computes the fraction of the blazar flux that remains unresolved as the detection threshold is lowered by factors of two, five and ten. It finds unresolved at catalogue threshold = 0.447; unresolved at tenth threshold = 0.171. Even a tenfold deeper survey leaves a substantial unresolved remainder, because the faint-end slope keeps adding flux below any realistic threshold. The study is simulated and computed, not measured.