Abstract

Using Hubble Space Telescope (HST) and Spitzer IRAC imaging, we report the discovery of a very bright strongly lensed Lyman break galaxy (LBG) candidate atz ? 7.6 in the field of the massive galaxy cluster Abell 1689 (z = 0.18). The galaxy candidate, which we refer to as A1689-zD1, shows a strong z 850 - J 110 break of at least 2.2 mag and is completely undetected (<1 ?) in HST Advanced Camera for Surveys (ACS) g 475, r 625, i 775, and Z 850 data. These properties, combined with the very blue J 110 - H 160 and H 160 - [4.5 ?] colors, are exactly the properties of an z ? 7.6 LBG, and can only be reasonably fit by a star-forming galaxy at z = 7.6 ± 0.4 (? v 2 = 1.1). Attempts to reproduce these properties with a model galaxy at z < 4 yield particularly poor fits (? v 2 ? 25). A1689-zD1 has an observed (lensed) magnitude of 24.7 AB (8 ?) in the NICMOS H 160 band and is ?1.3 mag brighter than the brightest known z 850-dropout galaxy. When corrected for the cluster magnification of ?9.3 at z ? 7.6, the candidate has an intrinsic magnitude of H 160 = 27.1 AB, or about an L* galaxy atz ? 7.6. The source-plane deprojection shows that the star formation is occurring in compact knots of size ?300 pc. The best-fit stellar population synthesis models yield a median redshift of 7.6, stellar masses (1.6-3.9) × 10 9 M ?, stellar ages 45-320 Myr, star formation rates ?7.6 M ? yr _1, and low reddening with A v ?; 0.3. These properties are generally similar to those of LBGs found atz ? 5-6. The inferred stellar ages suggest a formation redshift of z ? 8-10 (t ? 0.63 Gyr). A1689-zD1 is the brightest observed, highly reliable z > 7.0 galaxy candidate found to date. © 2008. The American Astronomical Society. All rights reserved.