Discovery of a Very Bright Strongly Lensed Galaxy Candidate at z ≈ 7.6

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 (<1 σ) in 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 (χ²_ν = 1.1). Attempts to reproduce these properties with a model galaxy at z < 4 yield particularly poor fits (χ²_ν >= 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 lesssim300 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 lesssim7.6 M_⊙ yr^-1, and low reddening with A_V <= 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 (tlesssim 0.63 Gyr). A1689-zD1 is the brightest observed, highly reliable z > 7.0 galaxy candidate found to date.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy under NASA contract NAS5-26555. Based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407.