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 - J110 break of at least 2.2 mag and is completely undetected (<1 σ) in HST Advanced Camera for Surveys (ACS) g 475, r625, i775, and Z850 data. These properties, combined with the very blue J110 - H160 and H160 - [4.5 μ] 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 (χv 2 = 1.1). Attempts to reproduce these properties with a model galaxy at z < 4 yield particularly poor fits (χv 2 ≥ 25). A1689-zD1 has an observed (lensed) magnitude of 24.7 AB (8 σ) in the NICMOS H160 band and is ∼1.3 mag brighter than the brightest known z850-dropout galaxy. When corrected for the cluster magnification of ∼9.3 at z ∼ 7.6, the candidate has an intrinsic magnitude of H160 = 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) × 109 M⊙, stellar ages 45-320 Myr, star formation rates ≲7.6 M⊙ yr_1, and low reddening with Av ≤; 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.