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

We have detected likely zsimilar to7-8 galaxies in the 144x144 Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) observations of the Hubble Ultra Deep Field. Objects are required to be greater than or equal to3 sigma detections in both NICMOS bands, J(100) and H-160. The selection criteria for this sample are (z(850)-J(110))(AB)>0.8, (z(850)-J(110))(AB)>0.66(J(110)-H-160)(AB)+0.8, (J(110)-H-160)(AB)<1.2 and no detection at less than 8500 angstrom. The five selected sources have total magnitudes H-160,H-AB similar to 27. Four of the five sources are quite blue compared to typical lower redshift dropout galaxies and are clustered within a 1 arcmin(2) 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 similar to 7-8 galaxies as our sample. With the same detection criteria on simulated data sets, assuming no evolution from z similar to 3.8, we predict 10 sources at z similar to 7-8, or 14 if we use a more realistic (1+z)(-1) size scaling. We estimate that the rest-frame continuum UV (similar to 1800 angstrom) luminosity density at z similar to 7.5 (integrated down to 0.3L(z=3)(*)) is just times 0.20(-0.08)(+0.12) times that found at zsimilar to3.8 (or 0.20(-0.12)(+0.23) times this quantity including cosmic variance). Effectively this sets an upper limit on the luminosity density down to 0.3L(z=3)(*) and is consistent with significant evolution at the bright end of the luminosity function from zsimilar to7.5 to 3.8. Even with the lower UV luminosity density at zsimilar to7.5, it appears that galaxies could still play an important role in reionization at these redshifts, although definitive measurements remain to be made.