2011A&A...533A..15D

We present the analysis of new near-infrared, intermediate-resolution spectra of the gravitationally lensed galaxy ``the 8 o'clock arc'' at z<sub>sys</sub>=2.7350 obtained with the X-shooter spectrograph on the Very Large Telescope. These rest-frame optical data, combined with Hubble and Spitzer Space Telescopes images, provide very valuable information, which nicely complement our previous detailed rest-frame UV spectral analysis, and make the 8 o'clock arc one of the better understood ``normal'' star-forming galaxies at this early epoch of the history of the Universe. From high-resolution HST images, we reconstruct the morphology of the arc in the source plane, and identify that the source is formed of two majors parts, the main galaxy component and a smaller blob separated by 1.2kpc in projected distance. The blob, with a twice larger magnification factor, is resolved in the X-shooter spectra. The multi-Gaussian fitting of detected nebular emission lines and the spectral energy distribution modeling of the available multi-wavelength photometry provide the census of gaseous and stellar dust extinctions, gas-phase metallicities, star-formation rates (SFRs), and stellar, gas, and dynamical masses for both the main galaxy and the blob. As a result, the 8 o'clock arc shows a marginal trend for a more attenuated ionized gas than stars, and supports a dependence of the dust properties on the SFR. With a high specific star-formation rate, SSFR=33±19G/yr, this lensed Lyman-break galaxy deviates from the mass-SFR relation, and is characterized by a young age of 40⁺²⁵_–20Myr and a high gas fraction of about 72%. The 8 o'clock arc satisfies the fundamental mass, SFR, and metallicity relation, and favors that it holds up beyond z≃2.5. We believe that the blob, with a gas mass M_gas=(2.2±0.9)x10⁹M_☉ (one order of magnitude lower than the mass of the galaxy), a half-light radius r_1/2=0.53±0.05kpc, a star-formation rate SFR_Hα=33±19M_☉/yr, and in rotation around the main core of the galaxy, is one of these star-forming clumps commonly observed in z>1 star-forming galaxies, because it is characterized by very similar physical properties. The knowledge of detailed physical properties of these clumps is a very useful input to models that aim to predict the formation and evolution of these clumps within high-redshift objects.