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

Abstract

We study the stellar population properties of the IRAC-detected $6\lesssim z\lesssim 10$ 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 $6\lesssim z\lesssim 10$ with $S/N\ge 3$ in at least one of the IRAC $3.6\hat{I}¼$m and $4.5\hat{I}¼$m bands. According to the best mass models available for the surveyed galaxy clusters, these IRAC-detected galaxy candidates are magnified by factors of $\sim 1.2$--$5.5$. We find that the IRAC-detected $6\lesssim z\lesssim 10$ sample is likely not a homogeneous galaxy population: some are relatively massive (stellar mass as high as $4\times 10^9{M}_{\odot }$) and evolved (age $\lesssim 500$ Myr) galaxies, while others are less massive ($M_{\text{stellar}}\sim 10^8{M}_{\odot }$) and very young ($\sim 10$ Myr) galaxies with strong nebular emission lines that boost their rest-frame optical fluxes. We identify two Ly$\hat{I}\pm$ emitters in our sample from the Keck DEIMOS spectra, one at $z_{\text{Ly}\hat{I}\pm }=6.76$ (in RXJ1347) and one at $z_{\text{Ly}\hat{I}\pm }=6.32$ (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.

ApJ in press