2010ApJ...715L.171P

We explore a technique for identifying the highest redshift (z>4) sources in Herschel/SPIRE and BLAST submillimeter surveys by localizing the position of the far-infrared dust peak. Just as Spitzer/IRAC was used to identify stellar "bump" sources, the far-IR peak is also a redshift indicator; although the latter also depends on the average dust temperature. We demonstrate the wide range of allowable redshifts for a reasonable range of dust temperatures and show that it is impossible to constraint the redshift of individual objects using solely the position of the far-IR peak. By fitting spectral energy distribution models to simulated Herschel/SPIRE photometry we show the utility of radio and/or far-infrared data in breaking this degeneracy. With prior knowledge of the dust temperature distribution it is possible to obtain statistical samples of high redshift submillimeter galaxy (SMG) candidates. We apply this technique to the BLAST survey of ECDFS to constrain the number of dusty galaxies at z>4. We find 8±2 galaxies with flux density ratios of S₅₀₀>S ₃₅₀; this sets an upper limit of 17±4/deg2 if we assume all are at z>4. This is <35 % of all 500 µm-selected galaxies down to S₅₀₀>45 mJy (L_IR>2x10¹³ L_☉ for z>4). Modeling with conventional temperature and redshift distributions estimates the percentage of these 500 µm peak galaxies at z>4 to be between 10% and 85%. Our results are consistent with other estimates of the number density of very high redshift SMGs and follow the decline in the star formation rate density at z>4.