Z ∼ 7 galaxy candidates from NICMOS observations over the HDF-South and the CDF-South and HDF-North GOODS fields

Abstract

We use ∼88 arcmin2 of deep (≳26.5 mag at 5σ) NICMOS data over the two GOODS fields and the HDF-South to conduct a search for bright z ≳ 7 galaxy candidates. This search takes advantage of an efficient preselection over 58 arcmin2 of NICMOS H160-band data where only plausible z ≳ 7 candidates are followed up with NICMOS J 110-band observations. ∼248 arcmin2 of deep ground-based near-infrared data (≳25.5 mag, 5σ) are also considered in the search. In total, we report 15z850-dropout candidates over this area - 7 of which are new to these search fields. Two possible z ∼ 9 J110-dropout candidates are also found, but seem unlikely to correspond to z ∼ 9 galaxies (given the estimated contamination levels). The present z ∼ 9 search is used to set upper limits on the prevalence of such sources. Rigorous testing is undertaken to establish the level of contamination of our selections by photometric scatter, low-mass stars, supernovae, and spurious sources. The estimated contamination rate of our z ∼ 7 selection is ∼24%. Through careful simulations, the effective volume available to our z ≳ 7 selections is estimated and used to establish constraints on the volume density of luminous (Lz=3*, or ∼-21 mag) galaxies from these searches.We find that the volume density of luminous star-forming galaxies at z ∼ 7 is 13-5+8 times lower than at z ∼ 4 and >25 times lower (1σ) at z ∼ 9 than at z ∼ 4. This is the most stringent constraint yet available on the volume density of ≳Lz=3*galaxies at z ∼ 9. The present wide-area, multi-field search limits cosmic variance to ≲20%. The evolution we find at the bright end of theUVLF is similar to that found from recent Subaru Suprime-Cam,HAWK-I or ERS WFC3/IR searches. The present paper also includes a complete summary of our final z ∼ 7 z850-dropout sample (18 candidates) identified from all NICMOS observations to date (over the two GOODS fields, the HUDF, galaxy clusters). © 2010. The American Astronomical Society. All rights reserved.