2014ApJ...793L..12Z

The deflection angles of lensed sources increase with their distance behind a given lens. We utilize this geometric effect to corroborate the z_phot≃ 9.8 photometric redshift estimate of a faint near-IR dropout, triply imaged by the massive galaxy cluster A2744 in deep Hubble Frontier Fields images. The multiple images of this source follow the same symmetry as other nearby sets of multiple images that bracket the critical curves and have well-defined redshifts (up to z_spec≃ 3.6), but with larger deflection angles, indicating that this source must lie at a higher redshift. Similarly, our different parametric and non-parametric lens models all require this object be at z ≳ 4, with at least 95% confidence, thoroughly excluding the possibility of lower-redshift interlopers. To study the properties of this source, we correct the two brighter images for their magnifications, leading to a star formation rate of ∼0.3 M_☉/yr, a stellar mass of ∼4x10⁷ M_☉, and an age of ≲ 220 Myr (95% confidence). The intrinsic apparent magnitude is 29.9 AB (F160W), and the rest-frame UV (∼1500 Å) absolute magnitude is M_UV, AB_= -17.6. This corresponds to ∼0.1L_z=8^* (∼0.2 L_z=10^*, adopting dM*/dz ∼ 0.45), making this candidate one of the least luminous galaxies discovered at z ∼ 10.