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 H160 < 28.6 (AB mag), consider only image regions with an rms noise σ160 > 30 mag (within a 0.5-arcsec diameter aperture), and insist on detections in both H160 and J140. The result is a survey covering an effective area (after accounting for magnification) of 10.9 arcmin2, 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., but recover only one of the four z > 8.4 sources reported by Zheng et al. 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 MACSJ0416.1−240 parallel field.
The locations of the 12 z ≃ 9 galaxies uncovered in this study from the four HFF pointings completed to date, with the positions of the objects superimposed on the depth maps of the H160 imaging. The colour coding indicates the 5σ detection depth corresponding to 85 per cent of point-source flux density. As described in the text, these ‘local’ depth measurements were established from the rms obtained from a grid of ≃150 nearby apertures after exclusion of any pixels containing significant source flux density. The effective area of our survey corresponds to the sum of the regions which have depths of 28 mag or deeper in this format, although the impact of gravitational lensing in the two cluster fields also further reduces the effective final area of our survey at z ≃ 9.
Based on the published magnification maps, we find that only one of these 12 galaxies is likely boosted by more than a factor of 2 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 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.
The full paper can be found in McLeod et al. 2015, MNRAS, 450, 3032