Discovery of a Very Bright Strongly Lensed Galaxy Candidate atz≈ 7.61

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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 at z ~ 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 z850 − J110 break of at least 2.2 mag and is completely undetected (HST Advanced Camera for Surveys (ACS) g475, r625, i775, and z850 data. These properties, combined with the very blue J110 − H160 and H160 − [ 4.5 μ m ] 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 (χ2ν = 1.1). Attempts to reproduce these properties with a model galaxy at z < 4 yield particularly poor fits (χ2ν ≥ 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 at z ~ 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 at z ~ 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.