ISAAC/VLT observations of a lensed galaxy at z = 10.0

Abstract

We report the first likely spectroscopic confirmation of a z ∼ 10.0 galaxy from our ongoing search for distant galaxies with ISAAC/VLT. Galaxy candidates at ≳ 7 are selected from ultra-deep JHKs images in the core of gravitational lensing clusters for which deep optical imaging is also available, including HST data. The object reported here, found behind Abell 1835, exhibits a faint emission line detected in the J band, leading to z = 10.0 when identified as Lyα, in excellent agreement with the photometric redshift determination. Redshifts z < 7 are very unlikely for various reasons we discuss. The object is located on the critical lines corresponding to z = 9 to 11. The magnification factor μ ranges from 25 to 100. For this object we estimate SFR(Lyα) ∼ (0.8-2.2) M⊙yr-1 and SFR(UV) ∼ (47-75) M⊙yr-1, both unconnected for lensing. The steep UV slope indicates a young object with negligible dust extinction. SED fits with young low-metallicity stellar population models yield (adopting μ = 25) a lensing corrected stellar mass of M*∼ 8 × 106 M⊙, and luminosities of 2 × 10 10 L⊙, corresponding to a dark matter halo of a mass of typically Mtot ≳ 5 × 108 M ⊙. In general our observations show that under excellent conditions and using strong gravitational lensing direct observations of galaxies close to the "dark ages" are feasible with ground-based 8-10 m class telescopes.