A Census of Star-forming Galaxies in the Z ~ 9-10 Universe based on HST+Spitzer Observations over 19 Clash Clusters: Three Candidate Z ~ 9-10 Galaxies and Improved Constraints on the Star Formation Rate Density at Z ~ 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 lsim1 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 0.28_-0.20^+0.39× that at z ~ 8, which is ~1.4_-0.8^+3.0× 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.

Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.