Nebular and global properties of the gravitationally lense d galaxy “ the 8 o ’ clock arc ” ⋆ , ⋆ ⋆

We present the analysis of new near-infrared, intermediate -resolution spectra of the gravitationally lensed galaxy “ the 8 o’clock arc” at zsys= 2.7350 obtained with the X-shooter spectrograph on the Very La ge Telescope. These rest-frame optical data, combined wit h Hubble and Spitzer Space Telescopes images, provide very va luable information, which nicely complement our previous d etailed restframe UV spectral analysis, and make the 8 o’clock arc one of t he better understood “normal” star-forming galaxies at thi s early epoch of the history of the Universe. From high-resolution HST ima ges, we reconstruct the morphology of the arc in the source pl ane, and identify that the source is formed of two majors parts, the ma in g laxy component and a smaller blob separated by 1.2 kpc in rojected distance. The blob, with a twice larger magnification factor , is esolved in the X-shooter spectra. The multi-Gaussian fi tti g of detected nebular emission lines and the spectral energy distributio n m deling of the available multi-wavelength photometry pr ovide the census of gaseous and stellar dust extinctions, gas-phase metalli cities, star-formation rates (SFRs), and stellar, gas, and dy amical masses for both the main galaxy and the blob. As a result, the 8 o’cloc k arc shows a marginal trend for a more attenuated ionized gas than stars, and supports a dependence of the dust properties on th e SFR. With a high specific star-formation rate, SSFR = 33± 19 Gyr, this lensed Lyman-break galaxy deviates from the mass-SFR r elation, and is characterized by a young age of 40 +25 −20 Myr and a high gas fraction of about 72%. The 8 o’clock arc satisfies the fund amental mass, SFR, and metallicity relation, and favors tha t it olds up beyondz ≃ 2.5. We believe that the blob, with a gas mass Mgas= (2.2±0.9)×10 M⊙ (one order of magnitude lower than the mass of the galaxy), a half-light radius r1/2 = 0.53± 0.05 kpc, a star-formation rate SFR Hα = 33± 19 M⊙ yr −1, and in rotation around the main core of the galaxy, is one of these star-forming clumps commo nly bserved inz > 1 star-forming galaxies, because it is characterized by very similar physical properties. The knowledge of detai l d physical properties of these clumps is a very useful inpu t to models that aim to predict the formation and evolution of these clum ps within high-redshift objects.