ESA Science & Technology - Discovery of a Very Bright Strongly Lensed Galaxy Candidate at z ~ 7.6

Publication date: 11 May 2008

Authors: Bradley, L.D. et al.

Journal: ApJ.
Volume: 678
Issue: 2
Page: 647-654
Year: 2008

Copyright: The American Astronomical Society

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 sigma) 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 micron 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 (Chi_mu-square=1.1). Attempts to reproduce these properties with a model galaxy at z < 4 yield particularly poor fits (Chi_mu-square=25). A1689-zD1 has an observed (lensed) magnitude of 24.7 AB (8 sigma) 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_solar, stellar ages 45-320 Myr, star formation rates <~7.6 M_solar 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 (t<~0.63Gyr). A1689-zD1 is the brightest observed, highly reliable z>7.0 galaxy candidate found to date.