New redshift z ≃ 9 galaxies in the Hubble Frontier Fields: implications for early evolution of the UV luminosity density

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 andACS imaging. To ensure robust photometric redshift solutions, and tominimize incompleteness, we confine our search to objects withH160 <28.6 (AB mag), consider only image regions with anrms noise σ160 > 30 mag (within a 0.5-arcsecdiameter aperture), and insist on detections in both H160 andJ140. The result is a survey covering an effective area(after accounting for magnification) of 10.9 arcmin2, whichyields 12 galaxies at 8.4 <z <9.5. Within the Abell-2744 clusterand parallel fields, we confirm the three brightest objects reported byIshigaki et al., but recover only one of the four z > 8.4 sourcesreported by Zheng et al. In the MACSJ0416.1-240 cluster field, we reportfive objects, and explain why each of these eluded detection orclassification as z ≃ 9 galaxies in the published searches of theshallower CLASH data. Finally, we uncover four z ≃ 9 galaxies fromthe MACSJ0416.1-240 parallel field. Based on the published magnificationmaps, we find that only one of these 12 galaxies is likely boosted bymore than a factor of 2 by gravitational lensing. Consequently, we areable to perform a fairly straightforward reanalysis of the normalizationof the z ≃ 9 UV galaxy luminosity function as explored previouslyin the HUDF12 programme. We conclude that the new data strengthen theevidence for a continued smooth decline in UV luminosity density (andhence star formation rate density) from z ≃ 8 to 9, contrary torecent reports of a marked drop-off at these redshifts. This providesfurther support for the scenario in which early galaxy evolution issufficiently extended to explain cosmic reionization.