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>~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 Lya photons at z>6.5. The resulting redshift uncertainties inhibit
the efficient search for [C II] in z~7 galaxies with sub-mm 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~4-8 galaxies as
observed by Spitzer/IRAC. At z~6.8, galaxies exhibit IRAC colors as blue as
[3.6]-[4.5] ~-1, likely due to the contribution of [O III]+Hb to the 3.6 mum
flux combined with the absence of line contamination in the 4.5 mum 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~6.6-6.9. When combined with an
I-dropout criterion, we demonstrate that we can plausibly select a relatively
clean sample of z~6.8 galaxies. Through a systematic application of this
selection technique to our catalogs from all five CANDELS fields, we identify
20 probable z~6.6-6.9 galaxies. We estimate that our criteria select the ~50%
strongest line emitters at z~6.8 and from the IRAC colors we estimate a typical
[O III]+Hb rest-frame equivalent width of 1085A for this sample. The small
redshift uncertainties on our sample make it particularly well suited for
follow-up studies with facilities such as ALMA.
