Left: z ₈₅₀ − J ₁₂₅ vs. J ₁₂₅ − H ₁₆₀ two-color diagram we use to identify z ~ 7 z ₈₅₀ dropouts over the ~40 arcmin ² ERS observations (first part of our two-part z ₈₅₀-dropout criterion). Symbols and lines are the same as Figure 2. Right: z ₈₅₀ − J ₁₂₅ vs. Y ₀₉₈ − J ₁₂₅ two-color diagram used to identify z ~ 7 z ₈₅₀-dropout galaxies (second part of our two-part z ₈₅₀-dropout criterion). Lines and symbols are as in the left panel. The colors required for our z ₈₅₀-dropout selection are indicated in gray. We base our z ~ 7 z ₈₅₀-dropout selection on a z ₈₅₀ − J ₁₂₅-dropout criterion rather than a z ₈₅₀ − Y ₀₉₈-dropout criterion (e.g., as used by Wilkins et al. 2010) to extend our selection over a wider redshift range (see Section 3.4). In addition to the two-color Lyman-break selection presented, we also enforce a very strict optical non-detection criterion involving a χ ² _opt quantity (Section 3.3; Appendix D).