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

TL;DR

This study uses gravitational lensing by Abell 2218 to achieve ultra-deep far-infrared observations, resolving a significant portion of the cosmic infrared background and constraining faint galaxy counts below previous sensitivity limits.

Contribution

It provides the deepest far-infrared source counts through gravitational lensing, approaching confusion limits, and estimates the resolved fraction of the cosmic infrared background.

Findings

Resolved 55-88% of the CIB at 100 and 160 microns.

Derived source counts down to 1-2 mJy flux densities.

Constrained the slope of faint galaxy counts consistent with evolutionary models.

Abstract

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 microns 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 microns and 2 mJy at 160 microns, 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 microns. 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 microns and 1.47-35 mJy at 160 microns, we retrieve a cosmic infrared background (CIB) surface brightness of ~8.0 and ~9.9 nW m^-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 microns, disregarding the high uncertainties of DIRBE absolute values.