Young Galaxy Candidates in the Hubble Frontier Fields. IV. MACS J1149.5+2223

We search for high-redshift dropout galaxies behind the Hubble Frontier Fields (HFF) galaxy cluster MACS J1149.5+2223, a powerful cosmic lens that has revealed a number of unique objects in its field. Using the deep images from the Hubble and Spitzer space telescopes, we find 11 galaxies at z > 7 in the MACS J1149.5+2223 cluster field, and 11 in its parallel field. The high-redshift nature of the bright z similar or equal to 9.6 galaxy MACS1149-JD, previously reported by Zheng et al., is further supported by non-detection in the extremely deep optical images from the HFF campaign. With the new photometry, the best photometric redshift solution for MACS1149-JD reduces slightly to z = 9.44 +/- 0.12. The young galaxy has an estimated stellar mass of (7 +/- 2) x 10(8) M-circle dot, and was formed at z = 13.2(-1.6)(+1.9) when the universe was approximate to 300 Myr old. Data available for the first four HFF clusters have already enabled us to find faint galaxies to an intrinsic magnitude of M-UV similar or equal to -15.5, approximately a factor of 10 deeper than the parallel fields.