Galaxies at z ∼ 7-8: z850-Dropouts in the Hubble Ultra Deep Field

We have detected likely z ~ 7-8 galaxies in the 144′′ × 144′′ Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) observations of the Hubble Ultra Deep Field. Objects are required to be ≥3 σ detections in both NICMOS bands, J110 and H160. The selection criteria for this sample are (z850 - J110)AB > 0.8, (z850 - J110)AB > 0.66(J110 - H160)AB + 0.8, (J110 - H160)AB < 1.2 and no detection at less than 8500 Å. The five selected sources have total magnitudes H160, AB ~ 27. Four of the five sources are quite blue compared to typical lower redshift dropout galaxies and are clustered within a 1 arcmin2 region. Because all five sources are near the limit of the NICMOS data, we have carefully evaluated their reality. Each of the candidates is visible in different splits of the data and a median stack. We analyzed several noise images and estimate the number of spurious sources to be 1 ± 1. A search using an independent reduction of this same data set clearly revealed three of the five candidates and weakly detected a fourth candidate, suggesting that the contamination could be higher. For comparison with predictions from lower redshift samples, we take a conservative approach and adopt four z ~ 7-8 galaxies as our sample. With the same detection criteria on simulated data sets, assuming no evolution from z ~ 3.8, we predict 10 sources at z ~ 7-8, or 14 if we use a more realistic (1 + z)-1 size scaling. We estimate that the rest-frame continuum UV (~1800 Å) luminosity density at z ~ 7.5 (integrated down to 0.3L) is just 0.20 times that found at z ~ 3.8 (or 0.20 times this quantity including cosmic variance). Effectively this sets an upper limit on the luminosity density down to 0.3L and is consistent with significant evolution at the bright end of the luminosity function from z ~ 7.5 to 3.8. Even with the lower UV luminosity density at z ~ 7.5, it appears that galaxies could still play an important role in reionization at these redshifts, although definitive measurements remain to be made.