Constraints on z ≈ 10 Galaxies from the Deepest Hubble Sp a ce Telescope NICMOS Fields

Abstract

We use all available fields with deep NICMOS imaging to search for J110-dropouts (H160,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 (06 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), 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 A) luminosity density at z ~ 10 (integrated down to 0.3L) is just 0.19 times that at z ~ 3.8 (or 0.19 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.