A Near-Infrared Analysis of the Submillimeter Background and the Cosmic Star-Formation History

We use new deep NIR and MIR observations to analyze the 850 ?m image of the GOODS-N region. We show that much of the submillimeter background is picked out by sources with H(AB) or 3.6 ?m(AB) < 23.25 (1.8 ?Jy). These sources contribute an 850 ?m background of 24 ? 2 Jy deg-2. This is a much higher fraction of the measured background (31-45 Jy deg-2) than is found with current 20 cm or 24 ?m samples. Roughly one-half of these NIR-selected sources have spectroscopic identifications, and we can assign robust photometric redshifts to nearly all of the remaining sources using their UV to MIR SEDs. We use the redshift and spectral type information to show that a large fraction of the 850 ?m background light comes from sources with z = 0-1.5 and that the sources responsible have intermediate spectral types. Neither the elliptical galaxies, which have no star formation, nor the bluest galaxies, which have little dust, contribute a significant amount of 850 ?m light, despite the fact that together they comprise approximately half of the galaxies in the sample. The redshift distribution of the NIR-selected 850 ?m light lies well below that of the much smaller amount of light traced by the more luminous, radio-selected submillimeter sources. We therefore require a revised star formation history with a lower star formation rate at high redshifts. We use a stacking analysis of the 20 cm light in the NIR sample to show that the star formation history is relatively flat down to z ~ 1 and that half of the total star formation occurs at z < 1.4.