A SCUBA/Spitzer Investigation of the Far Infra-Red Extragalactic Background

We have measured the contribution of submillimeter and mid-infrared sources to the extragalactic background radiation at 70 and 160μm. Specifically, we have stacked flux in 70 and 160μm Spitzer Space Telescope (SST) observations of the Canada-UK Deep Sub-millimeter Survey 14h field at the positions of 850μm sources detected by SCUBA and also 8 and 24μm sources detected by the SST. We find that per source, the SCUBA galaxies are the strongest and the 8μm sources the weakest contributors to the background flux at both 70 and 160μm. However, the reverse is true of the total contribution, the full 8μm source catalogue accounting for twice the total 24μm source contribution and ∼ 10 times the total SCUBA source contribution. These measurements indicate that the contribution of SCUBA galaxies to the background at its peak is higher than previously estimated. The 8μm sources account for the majority of the background radiation at 160μm with a flux of 0.87±0.16 MJy/sr and at least a third at 70μm with a flux of 0.103±0.019 MJy/sr. These measurements are consistent with current lower limits on the background at 70 and 160μm. Finally, we have investigated the 70 and 160μm emission from the 8 and 24μm sources as a function of redshift. We find that the average 70μm flux per 24μm source and the average 160μm flux per 8 and 24μm source is constant over all redshifts, up to z ∼ 4. In contrast, the low-redshift half (z < 1) of the of 8μm sample contributes approximately four times the total 70μm flux of the high-redshift half.