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We present the first detailed analysis of the rest-frame UV spectrum of thegravitationally lensed Lyman break galaxy (LBG), the `8 o'clock arc'. Thespectrum of the 8 o'clock arc is rich in stellar and interstellar medium (ISM)features, and presents several similarities to the well-known MS1512-cB58 LBG.The stellar photospheric absorption lines allowed us to constrain the systemicredshift, z_sys = 2.7350+/-0.0003, of the galaxy, and derive its stellarmetallicity, Z=0.82 Z_sol. With a total stellar mass of ~4.2x10^{11} M_sol, the8 o'clock arc agrees with the mass-metallicity relation found for z>2star-forming galaxies. The 31 ISM absorption lines detected led to theabundance measurements of 9 elements. The metallicity of the ISM, Z=0.65 Z_sol(Si), is very comparable to the metallicity of stars and ionized gas, andsuggests that the ISM of the 8 o'clock arc has been rapidly polluted andenriched by ejecta of OB stars. The ISM lines extend over ~1000 km/s and havetheir peak optical depth blueshifted relative to the stars, implying gasoutflows of about -120 km/s. The Ly-alpha line is dominated by a dampedabsorption profile on top of which is superposed a weak emission, redshiftedrelative to the ISM lines by about +690 km/s and resulting from multiplybackscattered Ly-alpha photons emitted in the HII region surrounded by thecold, expanding ISM shell. A homogeneous spherical radiation transfer shellmodel with a constant outflow velocity, determined by the observations, is ableto reproduce the observed Ly-alpha line profile and dust content. These resultsfully support the scenario proposed earlier, where the diversity of Ly-alphaline profiles in LBGs and Ly-alpha emitters, from absorption to emission, ismostly due to variations of HI column density and dust content (abridged).

Rest-frame ultraviolet spectrum of the gravitationally lensed galaxy `the 8 o'clock arc': stellar and interstellar medium properties