Constraints on z~10 Galaxies from the Deepest Hubble Space Telescope NICMOS Fields

Abstract

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+H₁₆₀ NICMOS 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+H₁₆₀ NICMOS frames available over the Hubble Deep Field (HDF) North, the HDF-South NICMOS parallel, and the ACS UDF (with 5 σ 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 σ) 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 Å) luminosity density at z~10 (integrated down to 0.3L^*_z=3) is just 0.19^+0.13_-0.09 times 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.

Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.