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

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 z ⪆ 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 s,yr-1 and SFR(UV) ˜ (47-75) M s,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 μ=25) a lensing corrected stellar mass of Mstar ˜ 8 × 106 M s, and luminosities of 2 × 1010 L s, corresponding to a dark matter halo of a mass of typically M_tot ⪆ 5 × 108 M s. 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. Based on observations collected with the ESO VLT-UT1 Antu Telescope (70.A-0355, 271.A-5013), the Hubble Space Telescope (HST) and the CFHT.

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