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

We report the first likely spectroscopic confirmation of a z similar to 10.0 galaxy from our ongoing search for distant galaxies with ISAAC/VLT. Galaxy candidates at z greater than or similar to 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 Lyalpha, 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 mu ranges from 25 to 100. For this object we estimate SFR(Ly alpha) similar to (0.8-2.2) M. yr(-1) and SFR(UV) similar to (47-75) M. yr(-1), both uncorrected 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 mu = 25) a lensing corrected stellar mass of M-* similar to 8 X 10(6) M., and luminosities of 2 x 10(10) L., corresponding to a dark matter halo of a mass of typically M-lot greater than or similar to 5 x 10(8) M.. In general ou r 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.