Expanding the Search for Galaxies at z ~ 7-10 with New NICMOS Parallel Fields

Abstract

We have carried out a search for galaxies at z ~ 7-10 in ~14.4 arcmin^2 of new NICMOS parallel imaging taken in the Great Observatories Origins Deep Survey (5.9 arcmin^2), the Cosmic Origins Survey (7.2 arcmin^2), and SSA22 (1.3 arcmin^2). These images reach 5σ sensitivities of J_(110) = 26.0-27.5 (AB), and combined they increase the amount of deep near-infrared data by more than 60% in fields where the investment in deep optical data has already been made. We find no z > 7 candidates in our survey area, consistent with the Bouwens et al. measurements at z ~ 7 and 9 (over 23 arcmin^2), which predict 0.7 galaxies at z ~ 7 and <0.03 galaxies at z ~ 9. We estimate that 10%-20% of z > 7 galaxies are missed by this survey, due to incompleteness from foreground contamination by faint sources. For the case of luminosity evolution, assuming a Schecter parameterization with a typical Φ_* = 10^(–3) Mpc^(–3), we find M^*> – 20.0 for z ~ 7 and M^*> – 20.7 for z ~ 9 (68% confidence). This suggests that the downward luminosity evolution of Lyman break galaxies continues to z ~ 7, although our result is marginally consistent with the z ~ 6 luminosity function of Bouwens et al. In addition, we present newly acquired deep MMT/Megacam imaging of the z ~ 9 candidate JD2325+1433, first presented in Henry et al. The resulting weak but significant detection at i' indicates that this galaxy is most likely an interloper at z ~ 2.7.