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

摘要

Gravitational lensing by massive galaxy clusters allows study ofthe population of intrinsically faint infrared galaxies that liebelow the sensitivity and confusion limits of current infrared andsubmillimeter telescopes. We present ultra-deep PACS100 and160mobservations toward the cluster lens Abell 2218 to penetrate theHerschel confusion limit. We derive source countsdown to a flux density of 1mJy at 100m and2mJy at 160m, aided by strong gravitational lensing. Atthese levels, source densities are 20 and 10beams/source inthe two bands, approaching source density confusion at 160m. The slopeof the counts below the turnover of the Euclidean-normalizeddifferential curve is constrained in both bands and is consistent withmost of the recent backwardsevolutionary models. By integrating number counts over the flux rangeaccessed by Abell2218 lensing (0.94-35mJy at100mand 1.47-35mJy at 160m), we retrieve a cosmic infrared backgroundsurface brightness of 8.0and 9.9nWm-2sr-1,in the respective bands. These values correspond to % and % of DIRBEdirect measurements. Combining Abell2218 results withwider/shallower fields, these figures increase to % and % CIB totalfractions, resolved at 100 and 160m, disregarding the high uncertainties of DIRBEabsolute values.Key words: surveys - infrared: galaxies -Galaxy: evolution - galaxies: high-redshift - gravitational lensing:strong - galaxies: clusters: general