Spitzer UltRa Faint SUrvey Program (SURFS UP). II. IRAC-Detected Lyman-Break Galaxies at 6 < z < 10 Behind Strong-Lensing Clusters

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We study the stellar population properties of the IRAC-detected 6 z 10 galaxy candidates from the Spitzer UltRa Faint SUrvey Program (SURFS UP). Using the Lyman Break selection technique, we find a total of 16 new galaxy candidates at 6 z 10 with S/N ≥ 3 in at least one of the IRAC 3.6μm and 4.5μm bands. According to the best mass models available for the surveyed galaxy clusters, these IRAC-detected galaxy candidates are magnified by factors of 1.2--5.5. We find that the IRAC-detected 6 z 10 sample is likely not a homogeneous galaxy population: some are relatively massive (stellar mass as high as 4 × 10⁹M) and evolved (age 500 Myr) galaxies, while others are less massive (Mstellar 10⁸M) and very young (10 Myr) galaxies with strong nebular emission lines that boost their rest-frame optical fluxes. We identify two Lyα emitters in our sample from the Keck DEIMOS spectra, one at zLyα=6.76 (in RXJ1347) and one at zLyα=6.32 (in MACS0454). We show that IRAC [3.6]-[4.5] color, when combined with photometric redshift, can be used to identify galaxies likely with strong nebular emission lines within certain redshift windows.