Galaxies at [FORMULA][F]z~7-8[/F][/FORMULA]: [FORMULA][F]z[INF]850[/INF][/F][/FORMULA]-Dropouts in the Hubble Ultra Deep Field

R. J. Bouwens, R. I. Thompson, G. D. Illingworth, M. Franx, P. G. van Dokkum, X. Fan, M. E. Dickinson, D. J. Eisenstein, M. J. Rieke
2004 Astrophysical Journal  
We have detected likely z 7-8 galaxies in the 144"x144" NICMOS observations of the Hubble Ultra Deep Field. Objects are required to be >3 sigma detections in both NICMOS bands, J and H. The selection criteria for this sample are (z-J)>0.8, (z-J) > 0.66 (J-H)+0.8, (J-H)<1.2, and no detection at <8500 Angstroms. The 5 selected sources have total magnitudes H 27. Four of the five sources are quite blue compared to typical lower-redshift dropout galaxies and are clustered within a 1 arcmin**2
more » ... . Because all 5 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 3 of the 5 candidates and weakly detected a 4th candidate, suggesting 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 datasets, 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 Angstroms) luminosity density at z 7.5 (integrated down to 0.3L*) is just 0.20_-0.08^+0.12x that found at z 3.8. Effectively this sets an upper limit on the luminosity density down to 0.3 L*. This result is consistent with significant evolution at the bright end of the luminosity function from z 7.5 to z 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, though definitive measurements remain to be made. [abridged]
doi:10.1086/426503 fatcat:3qxnltcmunfwngl2zu4fue2hfq