http://arxiv.org/abs/1412.1472
We present the results of a new search for galaxies at redshift z ~ 9 in the first two Hubble Frontier Fields with completed HST WFC3/IR and ACS imaging. To ensure robust photometric redshift solutions, and to minimize incompleteness, we confine our search to objects with H_{160} < 28.6 (AB mag), consider only image regions with an rms noise sigma_{160} > 30 mag (within a 0.5-arcsec diameter aperture), and insist on detections in both H_{160} and J_{140}. The result is a survey covering an effective area (after accounting for magnification) of 10.9 sq. arcmin, which yields 12 galaxies at 8.4 < z < 9.5. Within the Abell-2744 cluster and parallel fields we confirm the three brightest objects reported by Ishigaki et al. (2014), but recover only one of the four z > 8.4 sources reported by Zheng et al. (2014). In the MACSJ0416.1-240 cluster field we report five objects, and explain why each of these eluded detection or classification as z ~ 9 galaxies in the published searches of the shallower CLASH data. Finally, we uncover four z ~ 9 galaxies from the previously unsearched MACSJ0416.1-240 parallel field. Based on the published magnification maps we find that only one of these 12 galaxies is likely boosted by more than a factor of two by gravitational lensing. Consequently we are able to perform a fairly straightforward reanalysis of the normalization of the z ~ 9 UV galaxy luminosity function as explored previously in the HUDF12 programme. We conclude that the new data strengthen the evidence for a continued smooth decline in UV luminosity density (and hence star-formation rate density) from z ~ 8 to z ~ 9, contrary to recent reports of a marked drop-off at these redshifts. This provides further support for the scenario in which early galaxy evolution is sufficiently extended to explain cosmic reionization.
D. McLeod, R. McLure, J. Dunlop, et. al.
Fri, 5 Dec 14
35/56
Comments: 14 pages, 7 figures, submitted to MNRAS