Searching for the highest redshift sources in 250-500 micron
  submillimeter surveys
  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 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>S350; this sets an upper limit of 17 +/- 4 deg-2 if we assume all are at
z>4. This is <35% of all 500 micron-selected galaxies down to S500>45 mJy
(LIR>2e13Lsun for z>4). Modeling with conventional temperature and redshift
distributions estimates the percentage of these 500 micron peak galaxies at z>4
to be between 10-85%. Our results are consistent with other estimates of the
number density of very high redshift submillimeter galaxies and follows the
decline in the star formation rate density at z>4.
