Constraints on z ≈ 10 galaxies from the deepest Hubble Space Telescope NICMOS fields

We use all available fields with deep NICMOS imaging to search for J ₁₁₀-dropouts (H_160,AB ≲ 28) at z ≈ 10. Our primary data set for this search is the two J₁₁₀ + H₁₆₀ ICMOS fields taken in parallel with the Advanced Camera for Surveys (ACS) Hubble Ultra Deep Field (UDF). The 5 σ limiting magnitudes were ∼28.6 in J ₁₁₀ and ∼28.5 in H₁₆₀ (0″.6 apertures). Several shallower fields were also used: J₁₁₀ + H₁₆₀ NICMOS frames available over the Hubble Deep Field (HDF) North, the HDF-South NICMOS parallel, and the ACS UDF (with 5 a limiting magnitudes in J₁₁₀ and H₁₆₀ ranging from 27.0 to 28.2). The primary selection criterion was (J₁₁₀-H₁₆₀)AB > 1-8. Eleven such sources were found in all search fields using this criterion. Eight of these are clearly ruled out as credible z ≈ 10 sources, either as a result of detections (>2 a) blueward of J₁₁₀ or their colors redward of the break (H₁₆₀-K ∼ 1.5) (redder than ≳98% of lower redshift dropouts). The nature of the three remaining sources could not be determined from the data. This number appears consistent with the expected contamination from low-redshift interlopers. Analysis of the stacked images for the three candidates also suggests some contamination. Regardless of their true redshifts, the actual number of z ≈ 10 sources must be three or fewer. To assess the significance of these results, two lower redshift samples (a z ∼ 3.8 B-dropout and z ∼ 6 i-dropout sample) were projected to z ∼ 7-13 using a (1 + z) ^-1 size scaling (for fixed luminosity). They were added to the image frames and the selection was repeated, giving 15.6 and 4.8 J₁₁₀- dropouts, respectively. This suggests that to the limit of this probe (≈0.3L_z=3*), there has been evolution from z ∼ 3.8 and possibly from z ∼ 6. This is consistent with the strong evolution already noted at z ∼ 6 and z ∼ 7.5 relative to z ∼ 3-4. Even assuming that three sources from this probe are at z ≈ 10, the rest-frame continuum UV (∼1500 ̊) luminosity density at z ∼ 10 (integrated down to 0.3L_z=3*) is just 0.19_-0.09^+0.13 times that at z ∼ 3.8 (or 0.19_-0.10^+0.15 times, including the small effect from cosmic variance). However, if none of our sources are at z ≈ 10, this ratio has a 1 σ upper limit of 0.07.