HIGH-PRECISION PHOTOMETRIC REDSHIFTS FROM SPITZER/IRAC: EXTREME [3.6] - [4.5] COLORS IDENTIFY GALAXIES IN THE REDSHIFT RANGE z ∼ 6.6-6.9

One of the most challenging aspects of studying galaxies in the z greater than or similar to 7 universe is the infrequent confirmation of their redshifts through spectroscopy, a phenomenon thought to occur from the increasing opacity of the intergalactic medium to Ly alpha photons at z > 6.5. The resulting redshift uncertainties inhibit the efficient search for [C II] in z similar to 7 galaxies with sub-millimeter instruments such as ALMA, given their limited scan speed for faint lines. One means by which to improve the precision of the inferred redshifts is to exploit the potential impact of strong nebular emission lines on the colors of z similar to 4 - 8 galaxies as observed by Spitzer/IRAC. At z similar to 6.8, galaxies exhibit IRAC colors as blue as [3.6] - [4.5] similar to-1, likely due to the contribution of [O (III)]+ H beta to the 3.6 mu m flux combined with the absence of line contamination in the 4.5 mu m band. In this paper we explore the use of extremely blue [3.6] - [4.5] colors to identify galaxies in the narrow redshift window z similar to 6.6 - 6.9. When combined with an /-dropout criterion, we demonstrate that we can plausibly select a relatively clean sample of z similar to 6.8 galaxies. Through a systematic application of this selection technique to our catalogs from all five CANDELS fields, we identify 20 probable z similar to 6.6 - 6.9 galaxies. We estimate that our criteria select the similar to 50% strongest line emitters at z similar to 6.8 and from the IRAC colors we estimate a typical [O (III)] + H beta rest-frame equivalent width of 1085 angstrom for this sample. The small redshift uncertainties on our sample make it particularly well suited for follow-up studies with facilities such as ALMA.