Through the looking glass: Bright, highly magnified galaxy candidates at z ∼ 7 behind A1703

We report the discovery of seven strongly lensed Lyman-break galaxy (LBG) candidates at z ∼ 7 detected in Hubble Space Telescope Wide Field Camera 3 (WFC3) imaging of A1703. The brightest candidate, called A1703-zD1, has an observed (lensed) magnitude of 24.0AB (26σ) in the WFC3/IR F160W band, making it 0.2mag brighter than the z₈₅₀-dropout candidate recently reported behind the Bullet Cluster and 0.7mag brighter than the previously brightest known z ∼ 7.6 galaxy, A1689-zD1. With a cluster magnification of ∼9, this source has an intrinsic magnitude of H₁₆₀ = 26.4AB, a strong z₈₅₀ - J₁₂₅ break of 1.7mag, and a photometric redshift of z 6.7. Additionally, we find six other bright LBG candidates with H₁₆₀-band magnitudes of 24.9-26.4, photometric redshifts z 6.4 - 8.8, and magnifications μ 3-40. Stellar population fits to the Advanced Camera for Surveys, WFC3/IR, and Spitzer/Infrared Array Camera data for A1703-zD1 and A1703-zD4 yield stellar masses (0.7 - 3.0) × 10⁹ M _·, stellar ages 5-180Myr, and star formation rates ∼7.8 M_·yr^-1, and low reddening with A_V ≤ 0.7. The source-plane reconstruction of the exceptionally bright candidate A1703-zD1 exhibits an extended structure, spanning ∼4 kpc in the z ∼ 6.7 source plane, and shows three resolved star-forming knots of radius r ∼ 0.4kpc.