2005ApJ...624L...5B

We use all available fields with deep NICMOS imaging to search for J₁₁₀-dropouts (H_160,AB≲28) at z~10. Our primary data set for this search is the two J₁₁₀+H₁₆₀NICMOS 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 J₁₁₀ and ∼28.5 in H₁₆₀(0".6 apertures). Several shallower fields were also used: J₁₁₀+H₁₆₀NICMOS frames available over the Hubble Deep Field (HDF) North, the HDF-South NICMOS parallel, and the ACS UDF (with 5 σ limiting magnitudes in J₁₁₀and H₁₆₀ranging 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 σ) blueward of J₁₁₀or 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 Å) luminosity density at z∼10 (integrated down to 0.3L^*_z=3) is just 0.19^+0.13_–0.09times that at z∼3.8 (or 0.19^+0.15_–0.10 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.