Dissecting the cosmic infra-red background with Herschel/PEP

The constituents of the cosmic IR background (CIB) are studied at its peak wavelengths (100 and 160 μm) by exploiting Herschel/PACS observations of the GOODS-N, Lockman Hole, and COSMOS fields in the PACS evolutionary probe (PEP) guaranteed-time survey. The GOODS-N data reach 3σ depths of ∼3.0 mJy at 100 μ ma nd∼5.7 mJy at 160 μm. At these levels, source densities are 40 and 18 beams/source, respectively, thus hitting the confusion limit at 160 μm. Differential number counts extend from a few mJy up to 100-200 mJy, and are approximated as a double power law, with the break lying between 5 and 10 mJy. The available ancillary information allows us to split number counts into redshift bins. At z ≤ 0.5 we isolate a class of luminous sources (LIR ∼ 10 11 L� ), whose SEDs resemble late-spiral galaxies, peaking at ∼130 μm restframe and significantly colder than what is expected on the basis of pre-Herschel models. By integrating number counts over the whole covered flux range, we obtain a surface brightness of 6.36± 1.67 and 6.58± 1.62 [nW m −2 sr −1 ] at 100 and 160 μm, resolving ∼45% and ∼52% of the CIB, respectively. When stacking 24 μm sources, the inferred CIB lies within 1.1σ and 0.5σ from direct measurements in the two bands, and fractions increase to 50% and 75%. Most of this resolved CIB fraction was radiated at z ≤ 1.0, with 160 μm sources found at higher redshift than 100 μm ones.