The Infrared Properties of Submillimeter Galaxies: Clues from Ultradeep 70 μm Imaging

Abstract

We present 70 μm properties of submillimeter galaxies (SMGs) in the Great Observatories Origins Deep Survey (GOODS) North field. Out of 30 submillimeter galaxies (S850 > 2 mJy) in the central GOODS-N region, we find two with secure 70 μm detections. These are the first 70 μm detections of SMGs. One of the matched SMGs is at z~0.5, and has S70/S850 and S70/S24 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 μm properties of the SMGs by performing a stacking analysis, which also allows us to estimate that S850 > 2 mJy SMGs contribute 9% ± 3% of the 70 μm background light. The S850/S70 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 (zmedian = 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 μm = 8:0 x 10^11 L☉. We estimate the AGN contribution to the total infrared luminosity of low-z SMGs is less than 23%.