Y ₁₀₅ − J ₁₂₅ vs. J ₁₂₅ − H ₁₆₀ two-color diagram used to identify z ~ 8 Y ₁₀₅-dropout galaxies over the three ultra-deep WFC3/IR HUDF09 fields (14 arcmin ²). The Y ₁₀₅ − J ₁₂₅/ J ₁₂₅ − H ₁₆₀ colors required for our Y ₁₀₅-dropout selection are indicated by the gray region. The other symbols and lines are as in Figure 2. The overlap of our color selection region with that occupied by L and T dwarfs is not a concern—given that an exceeding small fraction of the sources (≲2%) in our selection appear to be unresolved (see Section 3.5). 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). As noted in Figure 2, our extensive simulations show that these two color criteria—combined with strict optical non-detection requirements—allow for the robust selection of z ~ 8 galaxies.