SPITZER 70μm SOURCE COUNTS IN GOODS-NORTH

We present ultra-deep Spitzer 70μm observations of GOODS-North (Great Observatories Origins Deep Survey). For the first time, the turn-over in the 70μm Euclidean-normalized differential source counts is observed. We derive source counts down to a flux density of 1.2mJy. From the measured source counts and fluctuation analysis, we estimate a power-law approximation of the faint 70μm source counts of dN/dS ∝ S, consistent with that observed for the faint 24μm sources. An extrapolation of the 70μm source counts to zero flux density implies a total extragalactic background light (EBL) of 7.4± 1.9 nW msr−1. The source counts above 1.2mJy account for about 60% of the estimated EBL. From fluctuation analysis, we derive a photometric confusion level of σc = 0.30 ± 0.15mJy (q = 5) for the Spitzer 70μm band. Subject headings: cosmology: observations — galaxies: evolution — galaxies: high-redshift — infrared: galaxies