The infrared properties of submillimeter galaxies:: Clues from ultradeep 70 μm imaging

We present 70 mu m properties of submillimeter galaxies (SMGs) in the Great Observatories Origins Deep Survey ( GOODS) North field. Out of 30 submillimeter galaxies (S-850 > 2 mJy) in the central GOODS-N region, we find two with secure 70 mu m detections. These are the first 70 mu m detections of SMGs. One of the matched SMGs is at z similar to 0.5, and has S-70/S-850 and S-70/S-24 ratios consistent with a cool galaxy. The second SMG( z 1: 2) has infrared- submillimeter colors that indicate it is more actively forming stars. We examine the average 70 mu m properties of the SMGs by performing a stacking analysis, which also allows us to estimate that S-850 > 2 mJy SMGs contribute 9% +/- 3% of the 70 mu m background light. The S-850/S-70 colors of the SMG population as a whole is best fit by cool galaxies, and because of the redshifting effects these constraints are mainly on the lower z subsample. We fit spectral energy distributions (SEDs) to the far- infrared data points of the two detected SMGs and the average low- redshift SMG (z(median) = 1.4). We find that the average low- z SMG has a cooler dust temperature than local ultraluminous infrared galaxies (ULIRGs) of similar luminosity and an SED that is best fit by scaled- up versions of normal spiral galaxies. The average low- zSMGis found to have a typical dust temperature T = 21-33 K and infrared luminosity L8-1000 mu m 8.0 x 10(11) L-circle dot. We estimate the AGN contribution to the total infrared luminosity of low-z SMGs is less than 23%.