Herschel deep far-infrared counts through Abell 2218 cluster-lens

Gravitational lensing by massive galaxy clusters allows study of the population of intrinsically faint infrared galaxies that lie below the sensitivity and confusion limits of current infrared and submillimeter telescopes. We present ultra-deep PACS 100 and 160 mu m observations toward the cluster lens Abell 2218 to penetrate the Herschel confusion limit. We derive source counts down to a flux density of 1 mJy at 100 mu m and 2 mJy at 160 mu m, aided by strong gravitational lensing. At these levels, source densities are 20 and 10 beams/source in the two bands, approaching source density confusion at 160 mu m. The slope of the counts below the turnover of the Euclidean-normalized differential curve is constrained in both bands and is consistent with most of the recent backwards evolutionary models. By integrating number counts over the flux range accessed by Abell 2218 lensing (0.94-35 mJy at 100 mu m and 1.47-35 mJy at 160 mu m), we retrieve a cosmic infrared background surface brightness of similar to 8.0 and similar to 9.9 nWm(-2) sr(-1), in the respective bands. These values correspond to 55 +/- 24% and 77 +/- 31% of DIRBE direct measurements. Combining Abell 2218 results with wider/shallower fields, these figures increase to 62 +/- 25% and 88 +/- 32% CIB total fractions, resolved at 100 and 160 mu m, disregarding the high uncertainties of DIRBE absolute values.