DISCOVERY OF A VERY BRIGHT STRONGLY - LENSED GALAXY CANDIDATE AT z ≈ 7

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 magnitudes. The source is completely undetected (< 1σ) in HST Advanced Camera for Surveys (ACS) g475, r625, i775, and z850 data as well as a 20-orbit ACS “detection” image constructed by combining all of the ACS data. These properties, combined with the very blue J110 −H160 and H160 − [4.5 μm] 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 (χν = 1.1). Attempts to reproduce these properties with a model galaxy at z < 4 yield particularly poor fits (χ ν ≥ 25). A1689-zD1 has an observed (lensed) magnitude of 25.3 AB (8 σ) in the NICMOS J110 band and is ∼1.2 magnitudes brighter than the brightest-known z850-dropout galaxy. By employing a detailed strong-lensing model of Abell 1689, we deproject A1689-zD1 to its source plane and examine its intrinsic properties. When corrected for the cluster magnification of ∼ 9.3 at z ∼ 7.6, the candidate has an intrinsic magnitude of J110 = 27.7 AB, or about an 0.3 L∗ galaxy at z ∼ 7.6. The high detection significance (> 8σ) of A1689-zD1 and the remarkable consistency of the morphology in both the J110 and H160 bands provide strong evidence that the source is real and not a spurious detection. The source plane deprojection shows that the star formation is occurring in compact knots of size . 300 pc. We fit the intrinsic broadband HST and Spitzer IRAC fluxes with stellar population synthesis models to constrain the stellar mass, stellar age, star-formation rate, and dust content of the galaxy. The best-fit values yield a median redshift of 7.6, stellar masses (1.6 − 3.9) × 10 M⊙, stellar ages 45 − 320 Myr, star-formation rates . 7.6 M⊙ yr, 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. Subject headings: cosmology: observations — galaxies: evolution — galaxies: formation — galaxies: high-redshift 1 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. 2 Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218. 3 UCO/Lick Observatory, Department of Astronomy and Astrophysics, University of California Santa Cruz, Santa Cruz, CA 95064. 4 Department of Physics, 1 Shields Avenue, University of California, Davis, CA 95616. 5 Instituto de Matemáticas y F́ısica Fundamental (CSIC), C/Serrano 113-bis, 28006, Madrid, Spain 6 School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel. 7 Leiden Observatory, Leiden University, Postbus 9513, 2300 RA Leiden, Netherlands. 8 Department of Physical Sciences, Dublin City University, Dublin 9, Ireland. 9 Departmento de Astronomı́a y Astrof́ısica, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile. 10 Departamento de F́ısica y Astronomı́a, Universidad de Valparáıso, Av. Gran Bretaña 1111, Valparáıso, Chile. 11 European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany. 12 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218.