A Census of Star-Forming Galaxies in Thez∼ 9-10 Universe Based Onhst+spitzerobservations Over 19 Clash Clusters: Three Candidatez∼ 9-10 Galaxies and Improved Constraints on the Star Formation Rate Density Atz∼ 9.2

We utilize a two-color Lyman-break selection criterion to search for z ∼ 9–10 galaxies over the first 19 clusters in the CLASH program. A systematic search yields three z ∼ 9–10 candidates. While we have already reported the most robust of these candidates, MACS1149-JD, two additional z ∼ 9 candidates are also found and have H 160 -band magnitudes of ∼26.2–26.9. A careful assessment of various sources of contamination suggests ≲1 contaminants for our z ∼ 9–10 selection. To determine the implications of these search results for the luminosity function (LF) and star formation rate density at z ∼ 9, we introduce a new differential approach to deriving these quantities in lensing fields. Our procedure is to derive the evolution by comparing the number of z ∼ 9–10 galaxy candidates found in CLASH with the number of galaxies in a slightly lower-redshift sample (after correcting for the differences in selection volumes), here taken to be z ∼ 8. This procedure takes advantage of the fact that the relative volumes available for the z ∼ 8 and z ∼ 9–10 selections behind lensing clusters are not greatly dependent on the details of the lensing models. We find that the normalization of the UV LF at z ∼ 9 is just that at z ∼ 8, which is ∼1.4 × lower than extrapolating z ∼ 4–8 LF results. While consistent with the evolution in the UV LF seen at z ∼ 4–8, these results marginally favor a more rapid evolution at z > 8. Compared to similar evolutionary findings from the HUDF, our result is less insensitive to large-scale structure uncertainties, given our many independent sightlines on the high-redshift universe.